2.6 Adaptive Mechanisms of Extreme Alkaliphiles · conditions at once (see also > Chaps. 2.1 Introduction and History of Alkaliphiles, > 2.4 Anaerobic Alkaliphiles and Alkaliphilic

2.6 Adaptive Mechanisms ofExtreme Alkaliphiles

Koki Horikoshi# Springer 201

Terry Ann Krulwich1 . Jun Liu1 . Masato Morino2 . Makoto Fujisawa2 .

Masahiro Ito2 . David B. Hicks1

1Mount Sinai School of Medicine, New York, NY, USA2Toyo University, Ora-gun, Gunma, Japan

Prologue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120

Cytoplasmic pH Homeostasis: The Central Challenge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121

Growth of Extreme Alkaliphiles at Alkaline Cytoplasmic pH Values Not Tolerated by

Neutralophiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123

Na+/H+ Antiporters: Key Participants in Cytoplasmic pH Homeostasis of Alkaliphiles . 124

Redundancy in the Cation/Proton Antiporter Complements of Alkaliphiles, and the

Importance of Mrp-Type Antiporters in Alkaliphily . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125

Additional Structural, Enzymatic, and Metabolic Strategies for Cytoplasmic pH

Homeostasis, and Their Built-In Redundancies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130

A Low Bulk Protonmotive Force (pmf) Resulting from Successful pH Homeostasis,

and Its Bioenergetic Consequences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131

Strategies for Oxidative Phosphorylation at Low Protonmotive Force . . . . . . . . . . . . . . . . . . . 132

Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135

Cross-References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135

(ed.), Extremophiles Handbook, DOI 10.1007/978-4-431-53898-1_2.6,

1

120 2.6 Adaptive Mechanisms of Extreme Alkaliphiles

Prologue

" ‘‘Most engineers accept the ‘no free lunch’ principle, which states that any mechanism that

increases robustness in one setting (i.e., to one type of perturbation, or with respect to one

type of output) always compromises it in another’’ (Lander et al. 2009).

Introduction

Extreme alkaliphiles, like extremophiles in general, possess numerous structural, metabolic,

physiological, and bioenergetic adaptations that enable them to function well under their

particular ‘‘extreme’’ condition or, in the case of poly-extremophiles, under several extreme

conditions at once (see also >Chaps. 2.1 Introduction and History of Alkaliphiles,> 2.4 Anaerobic Alkaliphiles and Alkaliphilic Poly-Extremophiles). If they are facultative

extremophiles, many of the adaptations are present even under non-extreme growth condi-

tions. That is, the adaptations to the extreme condition are ‘‘hard-wired’’ although their expres-

sion may increase further when the bacteria confront the extreme condition(s). The constitutive

hard-wiring is presumed to be a mechanism that anticipates the need to survive and grow upon

a sudden shift to the extreme condition(s). Here, we will summarize a number of different

adaptations of alkaliphiles that support their ability to grow optimally at pH values well above

9.0. Some of these species or strains are obligate alkaliphiles that exhibit little or no growth at pH

values closer to neutral. Other, facultative alkaliphiles, grow in a range from pH 7.5 to �11

(Guffanti and Hicks 1991; Yumoto 2007). The facultative alkaliphiles exhibit trade-offs of the

type predicted by the ‘‘no free lunch’’ principle defined above. They have a remarkable capacity

for growth at pH values much higher than the outer limit of pH 8.5–9.0 for growth of typical

neutralophilic bacteria. Facultative alkaliphiles also transition almost seamlessly through

a sudden shift from near neutral pH to extremely alkaline pH (Krulwich 1995; Krulwich et al.

2007; Padan et al. 2005; Slonczewski et al. 2009). However, these alkaliphiles exhibit a cost of

these remarkable alkaliphilic properties. This deficit is reflected in lower growth rates at near

neutral pH than at high pH even though there are greater energy costs for growth at alkaline

pH, for example, pH homeostasis and ATP synthesis (Krulwich et al. 2007). We suggest that

extreme, obligate alkaliphiles represent extreme examples of the ‘‘no free lunch’’ principle

in having entirely lost the capacity to grow at neutral pH while excelling at highly alkaline

pH values.

Each alkaliphile strain that has been examined in some detail displays multiple adaptations

that address specific aspects of the challenge of growth at very high pH. For example, they have

multiple types of strategies and apparently redundant transporters or enzymes to achieve

alkaline pH homeostasis (Padan et al. 2005; Slonczewski et al. 2009). This is in accord with

a corollary of the ‘‘no free lunch’’ principle which posits that such multiple strategies and their

built-in redundancies confer robustness upon the system under the specific condition targeted

by the strategies, that is, survival and growth at alkaline pH in our case. It further predicts that

a cost will be exacted in the form of low robustness relative to other organisms under other

conditions, that is, non-alkaline conditions in our case. Thus the ‘‘no free lunch’’ principle is

also called the principle of ‘‘conservation of fragility’’ in the engineering literature (Lander et al.

2009). There follows a set of examples of how this conceptualization of alkaliphily explains data

emerging from physiological and bioenergetic studies of several alkaliphiles.We note that a deeper

Adaptive Mechanisms of Extreme Alkaliphiles 2.6 121

and broader understanding of the design principles underpinning alkaliphily awaits more wide-

spread studies of different extreme alkaliphiles in which mutations can be made in the native

setting. Genetically tractable strains are almost entirely unavailable for most extreme alkaliphile

types, including extreme Gram-negative alkaliphiles and poly-extremophiles that are alkaliphilic

as well as thermophilic and/or halophilic (Ma et al. 2004; Mesbah et al. 2007). Without many

more genetically tractable and extensively characterized alkaliphile strains, we cannot test the

hypotheses about models of robustness and adaptation that are developed from biochemical,

genome, transcriptome, and proteome data. The ‘‘omics’’ studies can be complemented by

studies of mutations of specific molecules in heterologous settings. However, the impact of

mutational alterations in specific molecules will often be affected by systems-level adaptations

in the native host in ways that cannot be anticipated by work outside the native setting.

Cytoplasmic pH Homeostasis: The Central Challenge

A small number of alkaliphilic bacteria were first described in the 1920s (>Chap. 2.1 Intro-

duction and History of Alkaliphiles). Only a few studies appeared after those first reports until

the 1970s when Koki Horikoshi initiated work that spurred a sustained interest in these

extremophiles, their physiology, and the potential products they might yield (Horikoshi

1991). It was immediately clear to bioenergeticists that alkaliphiles would require a capacity

for robust cytoplasmic pH homeostasis. This expectation has been validated in numerous

species and strains of alkaliphiles (Cook et al. 1996; Guffanti andHicks 1991; Olsson et al. 2003;

Sturr et al. 1994; Yumoto 2002). Among the most intensively studied strains is alkaliphilic

Bacillus pseudofirmus OF4, an extreme facultative alkaliphile that is genetically tractable and

grows well over a pH range from 7.5 to at least 11.4 (Krulwich et al. 2007; Sturr et al. 1994). B.

pseudofirmus OF4 is able to maintain a cytoplasmic pH of 8.2 when the external pH is 10.5,

a pH gradient of 2.3 pH units, inside acidic relative to the outside (> Fig. 2.6.1, upper left);

similar capacities for pH homeostasis are found in other extremely alkaliphilic Bacillus species,

such as the facultative alkaliphile B. halodurans C-125 (Ito and Aono 2002), the obligate

alkaliphile B. alcalophilus (Guffanti and Hicks 1991), as well as the alkaliphilic cyanobacterium

Spirulina platensis (Pogoryelov et al. 2003) (> Fig. 2.6.1, upper right). In accord with the notion

that robust adaptation to one perturbation tempers the robustness in the face of other

challenges, thermophilic alkaliphiles typically do not exhibit cytoplasmic pH homeostasis

that is as robust as that of non-thermophilic extreme alkaliphiles. Examples of this difference

are in> Fig. 2.6.1, which shows the cytoplasmic pH at external pH values near the upper end of

the optimal pH range for alkaliphilic B. pseudofirmusOF4, B. haloduransC-125, and S. platensis

in comparison with thermoalkaliphiles Clostridium paradoxum (Cook et al. 1996) and Bacillus

sp. TA2.A1 (recently proposed to be the TA2.A1 strain of Caldalkalibacillus thermarum

(McMillan et al. 2009). It has been suggested that even more ‘‘poly-extremophilic’’ bacteria,

such as alkaliphilic, thermophilic, and halophilic Natranaerobius thermophilus (Mesbah et al.

2007), may approach a physico-chemical limit of adaptations to the multiple stresses (Bowers

et al. 2009) (see >Chap. 2.4 Anaerobic Alkaliphiles and Alkaliphilic Poly-Extremophiles).

The detailed pH homeostasis profile for B. pseudofirmusOF4, and its relationship to growth

rate, was revealed by a set of assays on cells growing in continuous culture onmalate-containing

media at different rigorously controlled pH values from 7.5 to 11.4 (Sturr et al. 1994). These

analyses showed that: (1) the growth rate was lower at pH 7.5, at which the doubling time was

54min, than in the range of external pH from 8.5–10.5, at which the doubling time was 38min;

O.M. Trichome

Respiration ATP synthase

SCWP

Mrp andother CPAs

Mot PS

CheACheWMCP

NavBPNa+

Na+

Na+ Na+

Na+-coupledmotility

Na+/solutesymporters

Na+ Solutes

pH 6.4 pH 8.2

Thylakoid

ATPsynthase

ATP

ADP + Pi

Photosystemsand

cytochromes


Na+-coupled ATPaseNa+

SCWP

ADP + PiADP + Pi

55°C65°C

− pH 7.8

+ pH 9.1

Na+

Na+


Solutes Na+


SolutesNa+/protonantiporters

and otherCPAs

Na+ H+

− pH 8.4

+ pH 9.5

pH 8.2

pH 10.5

−

+Na+

Na+

?Na+

Mot?

ATPATP

Clostridium paradoxum

Solute

pH 10.0

Spirulina(Arthrospira)

platensis

CPAs

H+

H+ H+ H+

Respiration ATP synthase

SCWP

H+

H+

H+

CPAs

Bacillus sp. TA2.A1(Caldalkalibacillus thermarum TA2.A1)

H+

H+

H+

NDH

ADP + Pi

ATP

MrpBacillus pseudofirmus OF4/Bacillus halodurans C-125

H+H+

H+

F0

F0

F0F0

F1

F1

F1F1

Na+

MQ

. Fig. 2.6.1

Diagrammatic illustration of the pH homeostasis capacity of two alkaliphiles and two

thermoalkaliphiles and elements of their membrane-associated Na+ and H+ translocation

pathways. The cytoplasmic pH at a high external pH that supports optimum growth is shown in

the top two panels for extreme alkaliphiles, Bacillus pseudofirmus OF4, Bacillus halodurans C-125

(which have comparable patterns), and Spirulina platensis, and in the bottom two panels for two

thermoalkaliphiles, aerobic Bacillus sp. TA2.A1 and anaerobic Clostridium paradoxum. Elements of

their Na+ and H+ pathways are shown schematically and are described in the text. The two

extremely alkaliphilic B. pseudofirmus OF4 and B. halodurans C-125 have robust Mrp-dependent

pH homeostasis and have voltage-gated sodium channels that play a role in Na+ circulation in

support of pH homeostasis as well as chemotaxis (Fujinami et al. 2009; Ito et al. 2004; Ren et al.

2001). For C. paradoxum, motility has been shown (Li et al. 1993) while Na+-coupling of motility or

of solute uptake have not yet been directly shown but are proposed (Ferguson et al. 2006). The

arcs outside the membrane are shown to indicate secondary cell wall polymers (SCWP)(Schaffer

and Messner 2005) or outer membrane (OM) and trichome layers (Ciferri 1983)


(2) the cytoplasmic pH was maintained at about pH 7.5 at external pH values �9.5. At more

alkaline pH values the cytoplasmic pH rose, but still remained between 2.3 and 1.8 pH units

lower than the outside pH values up to the highest pH tested, pH>11 (see > Fig. 2.6.2); (3) at

pH�10.5, the doubling time increased roughly in parallel with the increasing cytoplasmic pH;

and (4) the cells were still capable of growth at pH 11.4, at which the cytoplasmic pH was 9.6.

6

7

8

9

10

5 6 7 8 9 10 11 12pHout

pHin

NeutralophilesAlkaliphiles

. Fig. 2.6.2

Alkaline pH homeostasis by representative alkaliphilic and neutralophilic bacteria. Reported

cytoplasmic pH data at the indicated external pH values are shown for two model neutralophilic

bacteria, B. subtilis ( ) (Shioi et al. 1980) and Escherichia coli ( ) (Padan et al. 1981) and for the

following alkaliphilic bacteria: B. pseudofirmus OF4 (■) (Sturr et al. 1994); B. halodurans C-125 ( )

(Ito and Aono 2002); B. cohnii YN-2000 ( ) (Suigyama et al. 1986); B. pseudofirmus RAB ( ) (Kitada

et al. 1982), B. alcalophilus ATCC 27647( ) (Hoffmann and Dimroth 1991)


The centrality of pH homeostasis for alkaliphiles is demonstrated by the direct relationship

between the decrease in growth rate (monitored as increasing doubling time) and the increase

in cytoplasmic pH beyond 8.2. It is also notable that, in keeping with the ‘‘no free lunch’’

principle, B. pseudofirmusOF4 grows better at pH 10.5, where the cytoplasmic pH is 8.2 than at

pH of 7.5 where the cytoplasmic pH is 7.5. The optimal cytoplasmic pH for typical

neutralophiles is 7.5–7.6 (see the Escherichia coli and B. subtilis patterns in > Fig. 2.6.2)

(Padan et al. 2005; Slonczewski et al. 2009). For B. pseudofirmus OF4, an optimal growth rate

is observed together with a cytoplasmic pH of 7.5 only when the external pH is higher than 7.5,

that is, in the pH 8.5–9.5 range. This suggests that at external pH values near neutral, factors on

the outer surface limit growth of this alkaliphile.

Growth of Extreme Alkaliphiles at Alkaline Cytoplasmic pH ValuesNot Tolerated by Neutralophiles

An unanticipated finding was that optimal growth of alkaliphiles would occur at cytoplasmic

pH values such as 8.2 (Sturr et al. 1994) (see> Fig. 2.6.1). When neutralophilic bacteria such as

E. coli and B. subtilis are exposed to alkaline conditions that lead to cytoplasmic pH values

above 8, growth arrest results (Padan et al. 2005; Slonczewski et al. 2009). Thus it was even

more surprising that slower but significant growth of B. pseudofirmus OF4 persists when the

cytoplasmic pH is as high as 9.6 (> Fig. 2.6.2) (Krulwich et al. 2007; Sturr et al. 1994).


We hypothesize that extreme alkaliphiles have adaptations of multiple cytoplasmic compo-

nents that underpin this remarkable ability to tolerate high cytoplasmic pH values. Evidence

for this hypothesis comes from the observation of apparent adaptations that foster alkaline

stability and functional integrity of the cytoplasmic enzyme phosphoserine aminotransferase

in both B. alcalophilus and B. circulans ssp. alcalophilus (Dubnovitsky et al. 2005; Kapetaniou

et al. 2006) (and see >Chap. 2.7 Bioenergetics: Cell motility and chemotoxis of extreme

alkaliphiles). Much more investigation into the adaptations of the cytoplasmic components

of the cells and their relative importance in extreme alkaliphily will be of great interest. We

hypothesize that some of these adaptations of cytoplasmic components lead to poorer growth

of extreme alkaliphiles at near neutral pH. For example, key processes such as translation,

transcription, or function of the divisome may be optimized for a cytoplasmic pH >7.5–8.5.

Na+/H+ Antiporters: Key Participants in Cytoplasmic pHHomeostasis of Alkaliphiles

In spite of their tolerance of unusually high cytoplasmic pH values, alkaliphiles must maintain

the cytoplasmic pH well below the external pH at the alkaline edge of their pH range for

growth. Mutations that inactivate the major mechanism of alkaline pH homeostasis are either

lethal to extremely alkaliphilic bacteria, as is apparently the case with B. pseudofirmus OF4

(Swartz et al. 2005), or render them non-alkaliphilic, as is the case for B. halodurans C-125

(Hamamoto et al. 1994). This major mechanism of pH homeostasis is the active uptake of H+

mediated by Na+/H+ antiporters (exchangers). Na+/H+ antiporters are membrane transport

systems that are energized by the substantial transmembrane electrical potential (DC) across

the cytoplasmic membrane (indicated by the +/� in > Fig. 2.6.1). The DC is generated by

active ion extrusion by primary ion pumps such as the respiratory chain components (see top

two panels of > Fig. 2.6.1), light-driven cation pumps, ATPases that extrude cations (as in

C. paradoxum, see > Fig. 2.6.1, bottom right), or membrane-embedded exergonic enzymes

whose activity is coupled to cation extrusion (von Ballmoos et al. 2009). Thus Na+/H+

antiporters are secondary active transporters. The bacterial Na+/H+ antiporters that play

a role in cytoplasmic H+ accumulation, relative to the outside milieu, specifically extrude

cytoplasmic Na+ in exchange for H+. A greater number of H+ are taken up than Na+ extruded,

that is, H+ taken up >Na+ extruded, so that the overall exchange is electrogenic, with net

positive charge moving inward during each turnover of the antiporter. This makes it possible

for the antiporter-mediated H+ uptake that is coupled to Na+ efflux to be energized by the

negative-inside transmembrane potential (see > Fig. 2.6.1). In neutralophilic bacteria such as

E. coli and B. subtilis, both Na+(Li+)/H+ and K+/H+ antiporters participate significantly in

alkaline pH homeostasis, whereas in the alkaliphilic bacteria studied to date, Na+(Li+)/H+

antiporters have an essential, dominant, and perhaps exclusive role in this central physiological

function (Hanhe et al. 2009; Padan et al. 2005; Slonczewski et al. 2009). Na+ is therefore

required for alkaliphile growth and pH homeostasis.

Although K+/H+ antiporter activity has been shown for members of the cation/proton

antiporter complements of alkaliphiles (Fujisawa et al. 2007; Mesbah et al. 2009; Wei et al.

2007), we know of no evidence from mutants or expression data for their involvement in the

specific process of alkaline pH homeostasis in alkaliphiles. The use of Na+ as the coupling ion is

advantageous because Na+ accumulation is cytotoxic in most bacteria; that cytotoxicity is

elevated at high pH (Padan et al. 2005;Wei et al. 2007). Therefore, it is advantageous to use Na+


to support alkaliphily via pH homeostasis since it concomitantly prevents cytotoxic accumu-

lation of Na+. It is further advantageous because of the inwardly directed electrochemical

gradient of Na+ that is generated by the high activity of Na+/H+ antiporters. This ‘‘sodium

motive force’’ (smf) is vitally important as a mode of energizing other bioenergetic work, such as

ion-coupled solute uptake, toxin extrusion, and motility under conditions in which the

protonmotive force (pmf, the transmembrane electrochemical gradient of protons) is low, as

discussed below. We have further hypothesized that use of K+/H+ antiporters for alkaline pH

homeostasis in extreme alkaliphiles would be problematic and there are probably mechanisms to

preclude its use at very alkaline pH. Because extraordinarily high levels of antiport activity are

required for alkaliphile growth at very high pH, use of K+/H+ antiporters would create the risk of

lowering the levels of cytoplasmic K+. Such reductions in cytoplasmic [K+] would be cytotoxic

because sufficient levels of this cation are required for optimal function of many cytoplasmic

proteins. In addition, the cytotoxicity of Na+ is exacerbated when K+ is depleted (Padan et al.

2005; Wei et al. 2007).

Redundancy in the Cation/Proton Antiporter Complements ofAlkaliphiles, and the Importance of Mrp-Type Antiporters inAlkaliphily

With rare exceptions, individual bacterial strains possess multiple cation/proton antiporters

(CPAs) that exchange Na+, Li+, K+ , or Ca2+ or some combination of these cytoplasmic cations

for external H+ (Krulwich et al. 2009; Padan et al. 2005; Slonczewski et al. 2009). For non-

marine bacteria, a typical range is 5–9 CPAs from several distinct antiporter families, as charac-

terized by sequence-based analyses in the Transporter Classification system (Ren et al. 2007; Saier

2002). For example, the B. subtilis complement is 6–7 antiporters and the complement from

E. coli contains a total of 8 CPAs (> Table 2.6.1). While the total CPA antiporter complement of

most of the examined alkaliphiles is within the range of neutralophiles, two alkaliphiles stand

out as exceptions. B. pseudofirmus OF4 has an unusually high number of antiporters, with a

total of 12, principally because of the large number of NhaC candidate proteins in its

genome (6). The organism with the largest complement, however, is the poly-extremophilic

N. thermophilus (> Table 2.6.1). We have hypothesized that bacteria that are challenged by

alkaline pHas their central and only extreme challenge rely heavily on a single, especially adapted

antiporter, a Na+/H+ antiporter with the requisite properties to meet that challenge (Krulwich

et al. 2009). Poly-extremophiles, on the other hand, have an overlapping set of challenges in

which antiporters may play a role, necessitating deployment of many more antiporters that

play roles under conditions in which the different aspects of the extremophily dominate.

CPA3 family members are widespread among both Gram-negative and Gram-positive

bacteria (as well as archaea) although they are absent in largely fermentative bacteria whose

ecological challenge is more commonly acid than alkali, for example, enteric bacteria, strep-

tococci, and lactobacilli (Swartz et al. 2005). The common themes that emerge from studies of

CPA3, Mrp-type antiporter roles and/or expression patterns in neutralophiles and alkaliphiles

are alkali and salt resistance (Hanhe et al. 2009; Kosono et al. 2005; Krulwich et al. 2007; Swartz

et al. 2005; Swartz et al. 2007). One Mrp-type antiporter system, Pha1 of Sinorhizobium

meliloti, catalyzes both K+/H+ and Na+/H+ antiport activities and the former activity has

a physiological role under nitrogen fixation conditions (Putnoky et al. 1998; Yamaguchi et al.

2009). In extremely alkaliphilic Bacillus species, B. halodurans C-125 and B. pseudofirmusOF4,

and in the alkaliphilic cyanobacterium, Anabaena sp. PCC7120, mutational evidence indicates

.Table

2.6.1

Cation/protonantiporter(CPA)candidatesforNa+(K

+)(Ca2+)/H+antiportcapacity

asrevealedbygenomicanalysesofselectedalkaliphilicandtw

omodel

neutralophilicbacteriaa

Organism

Alkaliphile/

neutralophile

Gram

+

or�

CPA1

CPA2

CPA3

NdhF-a

bNhaA

NhaB

NhaC

NhaD

CaCAa

Total

Bacilluspseudofirm

usOF4

Extremealkalip

hile

+3

21

00

06

00

12

BacillushaloduransC-125

Extremealkalip

hile

+1

11

00

02

00

5

BacillusclausiiK

SM-K16

Moderate

alkalip

hile

+1

02

00

04

00

7

Alkalilim

nicola

ehrlicheiMLH

E-1

Moderate

alkalip

hile

�2

02

00

00

00

4

NatranaerobiusthermophilusJW

/NM-W

N-LFc

Moderate

alkalip

hile;

poly-extremophile

+2

21

10

011

00

17

Synechocystissp.P

CC6803

Moderate

alkalip

hile

�3

31d

00

00

01

8

Bacillussubtilis

subsp.subtilis

str.168

Neutralophile

+1

21d

(1)

00

20

07(6)

Escherichia

coliK-12str.K-12substr.MG1655

Neutralophile

�2

30

01

10

01

8

Antiportersofthreemajoran

tiportercategoriesthat

areintheTransporterClassificationsystem

arefoundintherepresentative

bacteriashown:C

PA-typ

es,cation/protonan

tiporters

that

aresingle

ortw

o-geneproduct

antiporters

(CPA1orCPA2)orhetero-olig

omericMrp-typ

eCPA3an

tiporters

that

contain

6–7hyd

rophobicproteins;Nhaan

tiporters

that

are

hyd

rophobicmonomerordim

erproductsofa

singlegene;andasubsetoftheCaC

Afamily

antiportersthat

may

havemonovalentcation/protonan

tiportactivity.TheNdhF-aproteins

includeNt-Nhaan

tiporterfrom

N.thermophilum

andahomologuefrom

B.subtilis

that

isunlikelyto

havean

tiportactivity,asdiscussedinthetext.Therefore,thetotalnumberofCPAs

listedforB.subtilis

indicatesin

parenthesesthat

itshould

perhap

snotbecounted.Sim

ilarly,thetw

oNhaD

antiportercandidatesin

thedraftsequence

ofB.p

seudofirm

usOF4

may

instead

bearsenitetran

sportersan

dparenthesesareag

ainusedto

indicatethispossibility

that

theCPAtotalw

ould

be8instead

of10forthisalkalip

hile.

aAntiportdataaretakenfrom

www.m

embranetran

sport.org

(allorgan

ismsexceptN.thermophilus,Synecoccussp.P

CC7002,andB.p

seudofirm

usOF4,thelatterdatabeingderived

from

adraftgenomesequence)an

dsupplementedwithsearchesofan

notatedgenomesin

www.ncb

i.nlm

.nih.gov/genomes/lproks.cgiu

singan

tiportersoran

tiportersubunitsfrom

thedifferenttypes.CaC

Amembersshownto

beCa2

+-specificorthat

areap

parentNa+/Ca2

+an

tiportersareomitted.

bThistran

sportertypeisnotclassifiedin

www.m

embranetran

sport.org

butisdescribedin

(Krulwichetal.2009).

cN.thermophilusistheonlyan

aerobein

thistable.

dNotlistedin

www.m

embranetran

sport.org.


http://www.membranetransport.org

http://www.ncbi.nlm.nih.gov/genomes/lproks.cgi








that Na+/H+ antiport-specific members of the Mrp-type antiporters are crucial for alkaline pH

homeostasis in the upper ranges of pH (Hamamoto et al. 1994; Swartz et al. 2005). There is

a striking continuum with respect to the dominance of the Mrp role in the alkaline pH

homeostasis capacity of these alkaliphiles and neutralophilic B. subtilis. In B. pseudofirmus

OF4, the Mrp antiporter is apparently necessary for viability (Swartz et al. 2005). In

B. halodurans C-125, its mutation leads to an inability to grow at pH values above 9 and also

to increased sensitivity to inhibition by Na+ (Hamamoto et al. 1994). In Anabaena sp.

PCC7120 disruption of Mrp leads only to the inability to grow at pH values above 10 as well

as a pronounced increase in Na+ sensitivity (Blanco-Rivero et al. 2005). In neutralophilic

B. subtilis, loss of Mrp function leads to a modest increase in alkali sensitivity but a very

pronounced sensitivity to inhibition by Na+ (Ito et al. 1999; Ito et al. 2000; Kosono et al. 1999;

Swartz et al. 2005). Thus Mrp antiporters in extreme alkaliphiles are particularly adapted to

essential roles at very high pH, at which pH homeostasis is the central challenge to viability.

They also play roles in Na+-resistance that are not essential to viability; other antiporters of the

alkaliphile complement may contribute substantially to Na+-resistance, for example, NhaC

of B. pseudofirmus OF4 (Ito et al. 1997). It is notable that as the essentiality of a Mrp system in

alkaline pH homeostasis declines in this series of organisms, its relative role in Na+-resistance

increases. The Mrp continuum across these bacterial examples seems, in this way, to illustrate

the ‘‘no free lunch’’ principle at the molecular level. Interestingly, all eight antiporters from

N. thermophilus that have been assayed in vitro so far (2 CPA1s, 1 CPA2, 1 Nt-Nha, and 4

NhaCs) have a capacity for K+/H+ antiport (Mesbah et al. 2009). By contrast, as noted above,

the Mrp system of the more extremely alkaliphilic B. pseudofirmus OF4 as well as the homol-

ogous system called Mnh from the highly alkaline-tolerant Staphylococcus aureus are highly

specific for Na+(Li+)/H+ antiport and do not catalyze K+/H+ antiport (Swartz et al. 2007).

Again, the multiple stresses that must be managed by poly-extremophilic N. thermophilusmay

limit the adaptive constraints it can put on antiporter-based pH homeostasis without decreas-

ing its responses to salt and temperature challenges.

What is unique about Mrp systems that may make them adaptable to a special role in

alkaline pH homeostasis? Themost intensively studiedMrp antiporter systems to date are from

either the Group 1 or Group 2 Mrp systems (Swartz et al. 2005). Group 1 systems are encoded

by operons that contain seven genes, each of which has a hydrophobic membrane protein

product, MrpA-G proteins. Group 2 systems have a fusedMrpA and B so that the operons have

six genes encoding six hydrophobic products, MrpA’, C-G. Two properties of Group 1 and 2

systems led to their classification in their own category, the CPA3 family of the Transporter

Classification system. First, the two largest Mrp proteins, MrpA and MrpD, share significant

homology with a sub-complex of membrane-embedded subunits of the H+-pumping respira-

tory chain Complex I (NADH dehydrogenase) that is thought to be involved in H+ translo-

cation (Mathiesen and Hagerhall 2002). They share an oxidoreductase motif with these

Complex I subunits, for example, NuoL, M, and N in the E. coli nomenclature, that also

have homologues among ion-pumping bacterial hydrogenases (Friedrich and Weiss 1997;

Swartz et al. 2005). Similarity between MrpC and the NuoK of Complex I has also been

cited, albeit less striking than between MrpA and D and its homologues among subunits of

Complex I and hydrogenases (Mathiesen and Hagerhall 2003). > Figure 2.6.3 shows an

unrooted tree based on alignments of the family of MrpA/MrpD/NuoL/NuoM/NuoN proteins

(as well as Nt-Nha and homologues thereof that are discussed further below). The MrpA

proteins (which are larger than MrpD proteins, i.e., close to 90 KDa versus the approximate

55 KDa size of MrpD proteins) are more closely related to NuoL of Complex I than to most

Sa_MrpD Bc_MrpD

Bs_MrpD

Bp_MrpD

Bh_MrpDS6803_MrpD1

S6803_MrpD2

Ec_NuoL

Vc_MrpA

Pa_MrpA

Sa_MrpA

Bp_MrpA

Bs_MrpA Bh_MrpABc_MrpAVc_NdhF

Bs_NdhF

Sa_NdhF

Ec_NuoN

Ec_NuoM

Nt_MrpA

Nt_MrpD

Nt_Nha

Pa_MrpDVc_MrpD

0.1

. Fig. 2.6.3

The family of antiporter proteins MrpA, MrpD, and Nt-Nha; Nt-Nha homologues; and

respiratory chain complex I proteins NuoL, NuoM, and NuoN. Unrooted tree (TreeView) of

ClustalW (DSGene) analysis shows relationships between the two large subunits of Mrp

antiporters encoded in diverse mrp operons, the Nt-Nha antiporter protein, three

neutralophile homologues of MrpD/MrpA/Nt-Nha that, as discussed in the text, are unlikely to

have cation/proton antiport activity, and the homologues NuoL, M and N subunits of the

respiratory Complex I (H+-translocating NADH dehydrogenase) from E. coli. The bacterial species

and accession numbers for the protein sequences used are: Bc, B. clausii KSM-K16, MrpA

(YP_174287.1), MrpD (YP_174290.1); Bh, B. halodurans C-125, MrpA (NP_242185.1), MrpD

(NP_242182.1); Bp, B. pseudofirmus OF4, MrpA (AAF21812.2), MrpD (AAF21815.2); Bs, B. subtilis

subsp. subtilis str N16961, MrpA (NP_391038.2), MrpD (NP_440574.1), NdhF (NP_388064); Ec, E.

coli K-12 MG1655, NuoL, NuoM, NuoN (NP_416781.1, NP_416780.1 and NP_416779.2); Nt,

Natranaerobius thermophilus JW/NM-WN-LF, MrpA (YP_001916693.1), MrpD (YP_001916692.1),

Nt-Nha (YP_001916294.1); Pa, Pseudomonas aeruginosa PA)1, MrpA (NP_249745.1), MrpD

(NP_232555.1): S6803, Synechocystis 6803, MrpD1 and MrpD2 (NP_440572.1, NP_440574.1); Sa,

Staphylococcus aureus subsp. aureus COL (in which MrpA and MrpD are usually called MnhA

and MnhD (Hiramatsu et al. 1998)), MrpA (YP_185821.1), Mrp D (YP_185821.1), NdhF

(YP_185382.1); Vc, Vibrio cholera O1 biovar El Tor str. M16961, MrpA (NP_232557.1), MrpD

(NP_232555.1), NdhF (NP_231221.1)



MrpD proteins and the NuoM and NuoN pair of Complex I proteins. The exception is the

MrpA that is encoded in an operon in N. thermophilus but is significantly smaller and lacking

a ‘‘MrpB-like’’ domain commonly found in operon-encoded MrpA proteins. The

N. thermophilus MrpA as well as most MrpD proteins are more closely related to NuoM and

NuoN; the NuoM and NuoN subunit pair from Complex I are also more closely related to each

other than they are to NuoL. Only the two cyanobacterial Synechococcus strain 6803 MrpD

proteins cluster closer to the NuoL than to NuoM and NuoN, consistent with several points of

divergence of cyanobacterial Mrp operon patterns from the Group 1 and Group 2 patterns

(Krulwich et al. 2007; Swartz et al. 2005).

The second property that was recognized shortly after Mrp antiporter systems were discov-

ered is that antiport activity requires all of the 6–7 gene products of theGroup 1 or 2mrp operon.

This was first shown for the Mnh antiporter of the CPA3 family from S. aureus using in vitro

assays of truncated products of a full cloned mnh operon expressed in an antiporter-deficient

E. coli strain (Hiramatsu et al. 1998). Subsequently, nonpolar deletions were made in each gene

of the B. subtilis chromosomal copy of themrp operon (Ito et al. 2000) and in a cloned copy of

the B. pseudofirmusOF4mrp operon expressed in an antiporter-deficient E. coli strain (Morino

et al. 2008). Studies of both of these mutant panels confirmed the need for every Mrp protein

for significant antiport activity. The only modest exception was a small residual antiport

activity in the absence of MrpE (Morino et al. 2008; Yoshinaka et al. 2003). The requirement

for 6–7 hydrophobic proteins and the similarity of three of the Mrp proteins to subunits of

Complex I and hydrogenases that form sub-complexes led to the expectation that Mrp

functions as a hetero-oligomeric complex (Hiramatsu et al. 1998). This was confirmed by

demonstration of complexes containing each of the 7 Mrp proteins, first for B. subtilis Mrp

(Kajiyama et al. 2007) and then for B. pseudofirmus OF4 Mrp (Morino et al. 2008). Although

the analyses of the neutralophile Mrp complex and the alkaliphile Mrp complex were

conducted under different conditions and with different detection strategies, both prepara-

tions contained hetero-oligomeric Mrp complexes that included all 7Mrp proteins and were of

a size close to the expected size of a complex with one copy of each Mrp protein. However, the

alkaliphile samples also contained larger species, including a significant amount of a putative

full Mrp dimeric hetero-oligomer (Morino et al. 2008). We hypothesize that the unusual

hetero-oligomeric Mrp proteins are a consortium of transporters that may include two

Na+(K+)/H+ antiporters, or two Na+/H+ antiporters in extreme alkaliphiles, as well as other

transporter proteins that function synergistically with the antiporters. Additional activities would

account for several reports of transport capacities of Mrp systems that are not related to antiport

(Dzioba-Winogrodzki et al. 2009; Kashyap et al. 2006). The large hetero-oligomeric Mrp

complex is hypothesized to support antiport at high external values of pH by presenting

a large protein surface on the outside surface of the membrane. Such a surface could be

engineered to be an effective proton-gathering element, that could resemble, for example,

a larger and more extensive version of proton-gathering funnels observed in the high resolu-

tion crystal structure of E. coli antiporter NhaA (Padan 2008). Larger, dimeric hetero-

oligomeric complexes observed in alkaliphiles might be better adapted to serve this role than

monomeric Mrp complexes.

The requirement for all 6–7 gene products of operon-encoded Mrp systems in order to

observe antiport has made it difficult to test hypotheses that MrpA and MrpD are the actual

antiporter proteins (Mathiesen and Hagerhall 2002) or a more recent suggestion that MrpA,


MrpD, and MrpB constitute the critical catalytic core (Kajiyama et al. 2009). Therefore, the

recent study of the antiporter complement of N. thermophilus (Mesbah et al. 2009) that

included Nt-Nha constitutes a breakthrough. Nt-Nha is a MrpD/MrpA-like protein that has

antiport activity without any additional Mrp-like proteins. Nt-Nha is more closely related to

MrpD than MrpA (> Fig. 2.6.3). It lacks the MrpB domain that is found in the MrpA but not

MrpD proteins encoded in Group 1 and Group 2 operons. The MrpB domain is not present in

the MrpD-like homologues of cyanobacterial and the N. thermophilus operon-encoded Mrp

systems; these subunits are sometimes (probably erroneously) annotated as MrpA. The fact

that Nt-NhaA catalyzes Na+(K+)/H+ antiport without any other Mrp proteins supports the

idea that a MrpD/MrpA-like protein is the antiporter and argues against the absolute essen-

tiality of any other Mrp proteins to produce the antiport activity. There are other examples of

bacterial genes that are not in an apparent mrp operon but encode MrpA/MrpD/Nt-Nha-like

proteins. The genes are most commonly annotated as NdhF because they are closely related to

genes found in cyanobacterial and plant gene clusters that encode ion-pumping NADH dehydro-

genases (Battchikova and Aro 2007). The three examples shown in > Fig. 2.6.3, from B. subtilis,

S. aureus, and V. cholerae are not themselves likely to be active Na+(K+)/H+ antiporters since

they lack one or more residues that have been shown to be essential for such activity (Kajiyama

et al. 2009). These proteins may catalyze an antiport activity with different substrates and/or

their activity may depend upon modulation by a conserved protein that is not a hydrophobic

Mrp protein but is predicted to be co-transcribed with their genes. Currently, Mrp systems

constitute a structure-function puzzle of central interest with respect to alkaliphile pH homeo-

stasis mechanisms and the Nt-Nha antiporter may be one productive avenue to gain insights

that will suggest experiments with the larger more complex Mrp systems.

Additional Structural, Enzymatic, and Metabolic Strategies forCytoplasmic pH Homeostasis, and Their Built-In Redundancies

Extreme alkaliphiles employ additional strategies to supplement the contribution of antiporter-

dependent pH homeostasis. Secondary cell wall polymers (SCWP, see > Fig. 2.6.1), especially

teichuronopeptides, play a major role in supporting pH homeostasis in extremely alkaliphilic

B. halodurans C-125 (Aono et al. 1999). By contrast, the slightly more robustly alkaliphilic

B. pseudofirmusOF4 lacks comparable SCWPs. B. pseudofirmusOF4 expresses an acidic S-layer

both at pH 7.5 and 10.5. Interestingly, deletion of the slpA gene that encodes the S-layer has

only a modest adverse effect on the response of the alkaliphile to a sudden alkaline shift, that is,

the S-layer confers a modest advantage for alkaliphily. Growth of the slpA mutant at pH 7.5 is

greatly enhanced relative to the wild type, indicating that expression of slpA at near neutral pH

is quite detrimental to the alkaliphile (Gilmour et al. 2000; Krulwich et al. 2007). This is

a cogent example of the ‘‘no free lunch’’ idea that there is a palpable cost to this alkaliphile, with

respect to growth at near neutral pH, for its expression of a gene that modestly optimizes its

readiness for a transition to the extreme condition of alkaline pH.

Metabolic strategies are used by neutralophilic bacteria when challenged with alkali.

A major one is up-regulation of deaminases that produce cytoplasmic acids (Slonczewski

et al. 2009). This might be a problematic strategy for extreme alkaliphiles because the ammonia

released would be protonated and could thus tend to accumulate in the cytoplasm under the

prevailing condition of a higher cytoplasmic [H+] than the external medium during growth in

the highly alkaline pH range. Use of deaminases to produce acids that lower cytoplasmic pH


would depend upon robust ammonium ion extrusion systems. Such systems have been

suggested but their adequacy to support a robust deaminase strategy of pH homesotasis will

require further investigation (Fujisawa et al. 2007; Wei et al. 2003). Fermentations that produce

metabolic acids can also spare the use of energy-dependent antiport (Krulwich et al. 2007). The

general physiology of alkaliphiles is reviewed elsewhere in this handbook (>Chap. 2.4 Anaer-

obic Alkaliphiles and Alkaliphilic Poly-Extremophiles)

A Low Bulk Protonmotive Force (pmf) Resulting from Successful pHHomeostasis, and Its Bioenergetic Consequences

The pmf energizes H+-coupled ‘‘bioenergetic work’’ such as ion-coupled solute transport or

toxin efflux, flagellar motility, and ATP synthesis using an F1F0-ATP synthase. This transmem-

brane electrochemical proton gradient is composed of two components. In the orientation that

is productive for energization of H+-coupled bioenergetic work, these pmf components are

an electrical component (DC, inside negative relative to the outside) and a chemical

component (DpH, inside alkaline relative to the outside) (Mitchell 1961). However, because

of successful pH homeostasis, the DpH component is in the reverse orientation at the high

end of the alkaliphiles’ pH range for growth, that is, the cytoplasm is acidic relative to the

outside pH. Moreover, in alkaliphiles studied to date, the DC increases at high pH but not

enough to offset the adverse effect of the reversed DpH on the pmf. As a result the bulk pmf

is much lower in facultative alkaliphiles at the upper edge of their pH range than at near

neutral pH, for example, three times lower in B. pseudofirmus OF4 during growth at pH 10.5

than during growth at pH 7.5 (Krulwich et al. 2007; Sturr et al. 1994). The low pmf problem is

particularly acute because the cost of growing at pH 10.5 is higher because of the need for

high levels of DC-driven antiport and because there is a proton involved in the catalytic

reaction by which ATP is synthesized, which makes the reaction less favorable at high pH.

Still B. pseudofirmusOF4, grows as well at pH 10.5 as at pH 7.5, with comparable molar growth

yields and even faster generation times (Guffanti and Hicks 1991; Sturr et al. 1994). For solute-

coupled uptake and motility (see >Chap. 2.6 Adaptive mechanisms of extreme alkaliphiles),

a major strategy of alkaliphiles, including many cyanobacteria, is use of Na+-coupling instead

of H+-coupling as indicated in > Fig. 2.6.1 (Krulwich et al. 2007; Peddie et al. 2000). The

sodium motive force (smf) is larger than the pmf at highly alkaline pH since the antiport that

establishes the ‘‘reversed’’ DpH concomitantly generates an inwardly-directed chemical gradi-

ent of Na+ that adds to the significant, productively oriented DC to make a large smf. Na+-

coupled transporters of neutralophiles often work side-by-side with H+-coupled solute trans-

porters (see > Fig. 2.6.1) or can couple to both cations (Zani et al. 1993). By contrast, the Na+-

coupled solute transporters of extreme alkaliphiles cannot use H+ and are instead inhibited at

the elevated [H+] at near neutral pH. This accounts for the otherwise puzzling observation that

a higher [Na+] is required in the medium for growth of B. pseudofirmus OF4 on malate, which

is transported by Na+-dependent transport, at pH 7.5 than at pH 10.5 (Gilmour et al. 2000;

Krulwich et al. 2007). Some alkaliphiles may partially bypass this suboptimal property of

alkaline-adapted Na+-coupled solute transport at low pH, by employing transporters that are

not coupled to either the pmf or smf. It was noted in the B. halodurans C-125 genome that this

alkaliphile makes much greater use of ABC type, ATP-coupled transporters than found in

neutralophilic B. subtilis (Takami et al. 2000).


Strategies for Oxidative Phosphorylation at LowProtonmotive Force

The biggest conundrum with respect to bioenergetic work is that both extreme and moderate

alkaliphiles use H+-coupled rather than Na+-coupled ATP synthases (Krulwich et al. 2007;

von Ballmoos et al. 2008). Na+-coupled F1F0-ATPases are found in alkaliphilic anaerobes

(see C. paradoxum panel in > Fig. 2.6.1) that do not synthesize ATP with the enzyme under

physiological conditions. They use it in the hydrolytic direction that generates an smf

(Ferguson et al. 2006). Use of a Na+-pumping ATPase that generates an smf fits the overall

Na+-based energetics, reduces Na+ cytotoxicity, and also avoids cytoplasmic proton loss that

would occur using a H+-coupled ATPase at high pH. For cyanobacteria, use of a H+-coupled

ATP synthase for ATP synthesis may not be problematic because the low pmf problem applies

to ATP synthases that function in the cytoplasmic membrane across which this pmf is the

driving force for synthesis. Recent reports (Liberton et al. 2006; Nevo et al. 2007; Schneider

et al. 2007) indicate that the thylakoid membranes in which the synthase is localized is not

continuous with the cytoplasmic membrane. In that case, as shown in> Fig. 2.6.1, the problem

of low bulk pmf across the cytoplasmic membrane would not affect the energetics of ATP

synthesis. On the other hand, the low pmf conundrum is a major issue for extreme alkaliphiles

whose respiratory chains and ATP synthases, which constitute the machinery for oxidative

phosphorylation (OXPHOS), are in the cytoplasmic membranes. The problem may be especially

acute for thoseGram-positive alkaliphiles that lack the outermembrane ofGram-negative bacteria

or particularly protective SCWPs.

Solutions have been proposed to the conundrum of the robust H+-coupled ATP synthesis

observed in alkaliphiles at high pH and low bulk pmf and evidence for adaptations of the

OXPHOS machinery has also emerged. Proposals that explain the discrepancy between

the bulk pmf and ATP synthesis have focused on different ways in which the H+ pumped

by the respiratory chain may be sequestered from equilibration with the bulk medium. For

example, it has been hypothesized that rapid transfers of pumped H+ along the membrane

surface allow the H+ to reach the ATP synthase before they are equilibrated with the bulk phase

outside the cell. These surface-sequestered H+ would kinetically bypass the longer term

thermodynamic inadequacy of the low [H+] concentration in the bulk phase since the effective

near-membrane driving force would be higher than the low bulk pmf. Such proposals are similar

to models of energization of bioenergetic work proposed originally by RJP Williams

(Williams 1978). There are different recent models, including the suggestion based on computa-

tional modeling of forces near the membrane of interfacial barriers that promote retention of H+

near the surface in a delocalized manner and the suggestion based on in vitro experimental

models of H+ microcircuits near the membrane that depend upon proximity of the pumps to

the synthase and on properties of the membrane lipids (Branden et al. 2006; Mulkidjanian

et al. 2005). Some form of sequestered H+ translocation is likely to be a critical part of the

alkaliphile solution to the problem of OXPHOS at low bulk pmf but the suggestions are

currently hypotheses that still await more experimental data, especially in biological settings

(Krulwich 1995; Krulwich et al. 2007). The alkaliphileOXPHOS capacitymay, as predicted by the

microcircuit hypothesis, rely upon close proximity of critical respiratory chain pumps, especially

the terminal oxidases, to the ATP synthase; these partners in H+ transfers during alkaliphile

OXPHOSmight be found in clusters (Goto et al. 2005; Liu et al. 2007). The high concentration

of membrane cardiolipin has been suggested as a possible mediator of proximity and/or H+

transfer itself (Haines and Dencher 2002).


A more firmly established facet of OXPHOS in alkaliphilic Bacillus species such as

B. pseudofirmus OF4 is its dependence upon specific adaptations of the ATP synthase. There

also appear to be parallel adaptations in the terminal oxidase of the OXPHOS respiratory chain

component (Krulwich et al. 2007; Slonczewski et al. 2009). Such special adaptations of

OXPHOS machinery in response to specific environmental challenges are in fact widespread

(Ferguson and Ingledew 2008; von Ballmoos et al. 2008). Alkaliphile-specific sequence motifs

have been identified in both the F1F0-ATP synthase (examples in > Fig. 2.6.4, top) and the Cta

caa3-type cytochrome oxidase (examples in > Fig. 2.6.4, bottom) (Ivey and Krulwich 1992;

Quirk et al. 1993). In addition, unusual redox features of alkaliphile respiratory chain com-

ponents have been described (Goto et al. 2005; Hicks and Krulwich 1995; Muntyan and

Bloch 2008). Mutational work has not yet been conducted on the respiratory chain motifs

but ATP synthase motifs have been shown to play important roles in OXPHOS at high pH and

low pmf. For example, the ‘‘alkaliphile-specific’’ a-subunit Lys180 plays a critical role in

synthesis at high pH and is proposed to be required for H+ capture from the outside surface

of the synthase and passage to the interface between the a-subunit and the c-ring rotor (see> Fig. 2.6.4, top) (McMillan et al. 2007; Wang et al. 2004). There are also critical motifs on both

helices of the hairpin-like c-subunits that constitute the oligomeric rotor; their mutational

change to the consensus sequence for non-alkaliphiles severely compromises ATP synthesis at

high pH (Liu et al. 2009; Wang et al. 2004). Part of the adaptive value of the ATP synthase

motifs of alkaliphiles appears to be in preventing H+ loss to the outside, since several mutations

to the consensus sequence result in significant H+ leakiness (Liu et al. 2009; Wang et al. 2004).

Another proposed adaptation that could address or partially address the alkaliphile

OXPHOS conundrum relates to the number of c-subunits that comprise the c-ring rotor

(> Fig. 2.6.4, top). The stoichiometry of the c-ring varies among different organisms in

which it has been determined from 10–15 (von Ballmoos et al. 2008; von Ballmoos et al.

2009). An especially high c-subunit stoichiometry would help address the problem of

OXPHOS at low pmf (Meier et al. 2007; von Ballmoos et al. 2008). The thermoalkaliphile

Bacillus sp.TA21.A1 has a stoichiometry of 13 subunits/ring, which is indeed toward the high

end of the known range of stoichiometries, but even though the organism is a moderate

alkaliphile, the stoichiometry does not account for the energetics observed (Matthies et al.

2009; Meier et al. 2007). There also does not appear to be a strong correlation between a high

c-ring stoichiometry andOXPHOS at low pmf. The cyanobacterium S. platensis has the highest

c-ring stochiometry reported to date, at 15 subunits/ring (Pogoryelov et al. 2005; Pogoryelov

et al. 2009), even though the localization of its ATP synthase in intracellular thylakoids

probably avoids the low pmf problem of alkaliphile OXPHOS that takes place in the cytoplas-

mic membrane (> Fig. 2.6.1). The high stoichiometry in S. platensis may relate to other

properties of the setting such as the relative contributions of the two pmf components (von

Ballmoos et al. 2008; von Ballmoos et al. 2009).

Even if a somewhat high c-subunit stoichiometry plays, at most, a minor role in the

alkaliphile OXPHOS condundrum, we note another point of interest connected with this

stoichiometry. There is now a strong consensus that the stoichiometry of the c-ring apparently

does not vary within a single organism under different conditions such as growth pH or growth

substrate (Meier et al. 2005; von Ballmoos et al. 2008). If alkaliphiles follow the Bacillus sp. TA2.

A1 trend of higher than average c-subunit stoichiometry, we would cite it as one more example

of the ‘‘no free lunch’’ principle. A c-ring with a high stoichiometry supports a higher H+/ATP

stoichiometry for the ATP synthase and that, in turn, facilitates synthesis at low pmf. However,

an ATP synthase with high H+/ATP stoichiometry is a less economical energy converter than

Predictedpl

# basicresidues

H+ c ring

F0

F1

ααδ

β

εγ

ATP

H+

H+

a

H+

b2

ADP+ Pi

# acidicresidues

Outer C-terminal helixof each c -subunit hasPXXEXXP motif.Inner N-terminal helix hasAXAXAXA motifLys180

227

207

3.3

3.310.810.17.5

7.57.4

3.01

1654

46

19

8244

46

11

. Fig. 2.6.4

Examples of putative alkali-adaptive motifs in the ATP synthase and cytochrome oxidase

involved in oxidative phosphorylation by extremely alkaliphilic Bacillus species. Top: The

schematic diagram of a bacterial ATP synthase depicts an H+-translocating synthase that is found

in alkaliphiles. The membrane-embedded F0 and cytoplasmic F1 are indicated; they are,

respectively the proton-conducting/rotor elements and the catalytic elements involved in

ATP synthesis. The oligomeric ring of c-subunits, that is, the rotor for the rotary coupling

mechanism for ATP synthesis is shown in green. The stator elements of the synthase are shaded in

blue. As downward movement through the F0 energizes synthesis, rotation of the c-ring in the

direction shown leads to concomitant rotation of the interactive F1 subunits g and « that result in

a conformational change in the three catalytic sites (at the a/b interfaces). Arrows point to

regions of three major adaptations of the F0: a-subunit (aLys180 in the putative H+ uptake path)

and c-subunits (the PXXEXXP and AXAXAXAmotifs in the C- and N-terminal helices, respectively).

Bottom: An alignment of a region of subunit II of Bacillus cytochrome caa3 complexes that

shows that this region in alkaliphiles (species in blue) is significantly more acidic than the same

stretches found in neutralophiles. Acidic residues are shown in red and basic residues in blue.

The acidic and basic residue composition of each species is shown on the right, along with

the predicted isoelectric point (pI) of that region. The numbering refers to the mature form of

the B. pseudofirmus OF4 subunit II



a synthase with a lower c-subunit stoichiometry (Pogoryelov et al. 2005). Therefore, the c-ring

with 13 subunits/ring would help the alkaliphile ATP synthase function better at high pHand low

pmf than if it had a stoichiometry of 10, but at near neutral pH and high pmf, the synthase would

function less well than those of neutralophiles that have stiochiometries of 10 subunits/ring.

Conclusions

Adaptations of extreme alkaliphiles optimize growth and survival at pH values above pH 10 but

these adaptations convey a cost at near-neutral pH. A fuller understanding of the range and

utility of different adaptations will depend upon concerted ‘‘omics’’ efforts in a greater range of

genetically tractable alkaliphiles so that mutations can be used to test hypotheses about

structure-function and systems level relationships.

Cytoplasmic pH homeostasis is centrally important to alkaliphily and depends upon active

H+ accumulation by electrogenic, secondary cation/proton antiporters that in extremely

alkaliphilic Bacillus use Na+ as the efflux substrate coupled to H+ uptake.

The tolerance of alkaliphiles for unusually high cytoplasmic pH values and a very limited

amount of experimental data suggest that there are likely to be important adaptations of

cytoplasmic components in extreme alkaliphiles. Bioinformatic approaches to the increasing

number of fully sequenced genomes may lead to testable hypotheses about such adaptations and

also to suggestions for components on the outside of alkaliphile cells whose poor function at near

neutral pH contributes to limitations on growth in this range of pH for many alkaliphiles.

The hetero-oligomeric Mrp antiporter is the dominant cation/proton antiporter of

extremely alkaliphilic Bacillus species examined to date. The basis of the complexity of these

antiporters remains unknown. Experiments also should be designed to test the hypotheses that

the large outer surface of theMrp antiporter plays a role in H+ capture at elevated pH. Roles for

the other members of the antiporter complements still await definition.

Further experimental tests and models based on established parameters of alkaliphile

energetics should also be developed to test various proposals of sequestered H+ pathways

from respiratory and other H+ pathways to the ATP synthase both in alkaliphiles and beyond.

Progress has been made on alkaliphile-specific motifs of the ATP synthase that await

correlation with structural data and parallel development of insights into apparent adaptations

of the respiratory H+ pumps. The hypothesized importance of specific membrane lipids,

especially cardiolipin, also awaits experimental examination.

Acknowledgments

Work conducted in the authors’ laboratories was supported by research grant GM28454 and

the Systems Biology Center-NY grant P50-GM071558 from the National Institute of General

Medical Sciences (T.A.K) and a grant from the 21st Century Center of Excellence program of

the Ministry of Education, Culture, Sports, Science and Technology of Japan (M.I.).

Cross-References

> 2.1 Introduction and History of Alkaliphiles> 2.4 Anaerobic Alkaliphiles and Alkaliphilic Poly-Extremophiles> 2.5 General Physiology of Alkaliphiles> 2.7 Bioenergetics: Cell Motility and Chemotaxis of Extreme Alkaliphiles


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Documents

2.6 Adaptive Mechanisms of Extreme Alkaliphiles · conditions at once (see also > Chaps. 2.1 Introduction and History of Alkaliphiles, > 2.4 Anaerobic Alkaliphiles and Alkaliphilic