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Journal of Luminescence 122–123 (2007) 294–296

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Probing the connection of PBSs to the photosystems in Spirulinaplatensis by artificially induced fluorescence fluctuations

Heng Li, Shuzhen Yang, Jie Xie, Jingquan Zhao�

Key Laboratory of Photochemistry, Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, PR China

Available online 14 March 2006

Abstract

The molecular architecture and the structural connections of phycobilisomes (PBSs) to the photosystems in the intact cells of Spirulina

platensis were studied by taking advantage of glycerol- and betaine-induced fluorescence fluctuations. Generally, with a selective

excitation of C-phycocyanin (C-PC), glycerol could induce not only decoupling of PBSs from the photosystems but also of C-PC rods

from allophycocyanin (APC) cores, while betaine could strengthen the connection of PBSs to the thylakoid membrane but induce a

partial dissociation of PBS. On the other hand, glycerol did not exert an influence on the fluorescence spectra of the photosystems in

isolated thylakoid membrane. Therefore, it was deduced that glycerol could provide a molecular environment to weaken the hydrophobic

interactions of not only the LCM with the membrane but also the linker polypeptides to the water-soluble phycobiliproteins (C-PC and

APC), while the betaine could strengthen the hydrophobic interaction of LCM with the membrane but weaken the electrostatic

interaction of linker polypeptides to C-PC and/or APC.

r 2006 Elsevier B.V. All rights reserved.

Keywords: Fluorescence fluctuation; Energy transfer; Glycerol; Betaine; Phycobilisome

1. Introduction

It is well known that phycobilisomes (PBSs), photo-system II (PSII) and photosystem I (PSI) are the threeprincipal optical entities for photosynthesis in cyanobac-teria. A PBS plays a role of light harvesting and is linked tothe thylakoid membrane via a colored polypeptide (LCM)which is also an energy transfer intermediate from PBS toPSII [1], however, a direct energy transfer from PBSs toPSI was also suggested [2,3]. Up to now, the structuralconnection and functional association of PBSs with thephotosystems has long been and still is an interestingresearch field [4–6]. It is well known that PBSs, PSII andPSI are all highly fluorescent pigment-protein complexes;therefore, the structural and functional messages can bederived by probing the fluorescence responses to a specialphysical or chemical ‘stimulus’, such as irradiation,chemicals, temperature.

Glycerol is usually used as a cryoprotectant for low-temperature fluorescence measurement. However, it was

e front matter r 2006 Elsevier B.V. All rights reserved.

min.2006.01.145

ing author. Tel.: +8610 82617053; fax: +86 10 82617315.

ess: zhaojq@iccas.ac.cn (J. Zhao).

found that the connection of PBSs to the photosystemsmight be alternated in the presence of glycerol [7]. On theother hand, it was found that betaine could fix PBS firmlyon the thylakoid membrane [8–10]. How the small chemicalmolecules interact with the functional entities and themembrane is still not clear. In the current work, takingadvantage of glycerol- and betaine-induced fluorescencefluctuations, the structural matches and functional associa-tions of the three functional entities in the intact Spirulina

platensis cells were investigated. Besides suggesting aconcentration of glycerol used safely as a cryoprotectantfor measurement of the 77K fluorescence, some informa-tion about the structural connections was derived.

2. Materials and methods

2.1. Culture and growth conditions

Spirulina platensis, a cyanobacterium, was cultured in a5L bottle at 28 1C, bubbled with air and irradiated with40W fluorescence lamps continuously. Ten-day-old cul-tures were harvested by centrifugation, washed and

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Fig. 2. 77K fluorescence emission spectra of intact cells normalized at

707 nm in the presence of betaine (A) and those normalized at 670 nm in

the presence of both glycerol and betaine (B) with excitation at 580 nm.

The concentrations of glycerol (V/V%) are denoted by the percentages in

the figure.

H. Li et al. / Journal of Luminescence 122–123 (2007) 294–296 295

resuspended in fresh growth medium; the same capture wasused in all the experiments.

2.2. Spectral measurements

The thylakoid membrane of Spirulina platensis wasprepared based on the reported method [9]. Fluorescenceemission spectra were obtained on an F4500 spectro-fluorimeter (Hitachi, Japan) with a chlorophyll concentra-tion 1 mgChl-aml�1 at RT and 5 mgChl-aml�1 at 77K.The concentrations were estimated based on the absor-bance at 665 nm in methanol extracts [11]. For the 77Kspectral measurements, the samples were treated byglycerol for 10min at room temperature and then rapidlyfrozen into liquid nitrogen. To examine the effect ofbetaine, the sample was treated by betaine (1.0M) for30min at room temperature before treated by glycerol.

3. Results and discussion

3.1. The effect of glycerol and betaine on intact cells

Generally, it can be seen in Fig. 1 that PBSs fluorescenceincreases while the PSII and the PSI decrease (Fig. 1A)with glycerol concentration rising from 0% to 50%. On theother hand, the fluorescence spectra of the isolatedthylakoid membrane (PSII and PSI included) are notaffected by glycerol at all (Fig. 1B). That is to say, thefluorescence fluctuations of Fig. 1A must originate from analternation of the connection of PBSs with the membraneand/or a structural change of PBSs themselves, instead ofthe photosystems.

In comparison, betaine-induced fluorescence fluctuation,as shown in Fig. 2, is different from that shown in Fig. 1.Firstly, a much higher PSII fluorescence but a much lowerallophycocyanin (APC) fluorescence (Fig. 2A) as well as

Fig. 1. 77K fluorescence emission spectra for intact cells excited at 580 nm

but normalized at 670 nm (A) and those for thylakoid membrane excited

at 436 nm but normalized at 707 nm (B) in the presence of glycerol. The

concentrations of glycerol (V/V%) are denoted by the percentages in the

figure.

the less-affected 77K fluorescence spectra by glycerol (Fig.2B) suggest an improved connection of PBSs with PSII sopromote the energy transfer efficiency. Secondly, a muchhigher C-phycocyanin (C-PC) fluorescence than the APCmust originate from a less efficient energy transfer from C-PC to APC, which means, the C-PC rods might becomesomewhat detached from the APC cores.Further, to observe the fluorescence fluctuations quanti-

tatively, the fluorescence spectra at 50% glycerol concen-tration in the absence and presence of betaine aredeconvoluted into components of C-PC, APC, APC, PSIIand PSI [12,13], and the ratio of sub-band area to the totalarea is considered to be the relative fluorescence yield of acomponent, as described in [8,9]. The deconvolutedparameters are listed in Table 1.Based on the deconvolution, dependence of the relative

fluorescence yields of the four components on concentra-tion of glycerol are shown in Fig. 3. It clearly shows thatthe fluorescence spectra are less affected at a concentrationof glycerol less than 30%, while more and more PBSsbecome detached with increase in the concentration, untilmost of PBSs become detached at the 50% concentration.Furthermore, no matter the cells are treated by betaine ornot, there is a mirror-imaged linear correlation of the PSIIwith the APC and also the PSI with the C-PC, furtherconfirming the excitation energy transfer from C-PC rodsto PSI. In addition, for cells treated by betaine, C-PCfluorescence yield is always higher than APC no matter theglycerol is present or not, suggesting a betaine-induceddetachment of C-PC rods from the APC cores.

3.2. How did the glycerol- and betaine-induced structural

modification occur?

As mentioned above, betaine could strengthen thelinking of LCM to the membrane while weaken the coupling

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Table 1

The fluorescence percentages for the components in the presence of 50% glycerol

Component C-PC (648 nm) APC (660 nm) APC and PSII (685 nm) PSII (695 nm) PSI (720 nm)

Without betaine 30.2 29.3 17.7 6.5 16.3

With betaine 31.7 18.5 12.2 13.6 23.9

Fig. 3. The dependence of relative fluorescence yields for C-PC (squares),

APC (circles), PSII (open circles) and PSI (open squares) of the intact cells

on glycerol concentrations in the absence (A) and presence of betaine (B).

H. Li et al. / Journal of Luminescence 122–123 (2007) 294–296296

of C-PC with APC. How did the two opposite phenomenaoccur? It is known that betaine is a small zwitterionmolecule and possesses a polarity even stronger than awater molecule. At quite high concentration, betainemolecules could replace the water molecules at the interfacebetween the hydrophobic thylakoid membrane and thewater-soluble PBSs and then make the hydrophobicinteraction of the loop domain of LCM to the lipidmembrane much stronger, so PBSs fixed firmly. Otherwise,the presence of the significant polar groups in the linkerpolypeptides may interact with APC trimers via not onlyhydrophobic but also electrostatic interaction [14]. There-fore, the small betaine molecules may get into the centralhollow of an APC trimer or a C-PC hexamer and result in amuch loosened connection of C-PC in a rod as well as ofthe rods to APC cores. On the other hand, the effect ofglycerol is understandable for the lipid-soluble moleculescan not only get into the central hollow of a phycobilipro-tein but also penetrate into the thylakoid membrane toweaken the hydrophobic interaction. Based on the analysis,it can be proposed that the fixing of PBSs by betaine is atwo-dimensional interaction (the interface) while a three-dimensional interaction is responsible for the detachmentof PBSs from the membrane and also the dissociation ofPBSs by glycerol or by betaine.

In addition, it must be indicated that it should be verycautious to derive the structural and functional informa-tion from low-temperature fluorescence spectra withglycerol as a cryoprotectant for it may induce some

artificial changes on the native structures of the photo-synthetic systems from cyanobacteria.

4. Conclusion

Based on the results, it can be concluded that the glycerolcan induce a decoupling of PBSs from the photosystems viaweakening hydrophobic interaction of LCM to lipophilicthylakoid membrane. Further, at high concentration ofglycerol, PBS itself begins to be dissociated sequentially. Asa zwitterion molecule, betaine can improve the connectionof PBSs with the membrane via strengthening the hydro-phobic interaction between LCM and lipophilic thylakoidmembrane. On the other hand, the strong polarity ofbetaine can weaken electrostatic interactions between thelink polypeptides and C-PC hexamers or the APC trimers,so result the rods less coupled to cores. As a cryoprotectantfor low-temperature fluorescence measurement, glycerol athigh concentration would make the photosynthetic appa-ratus lose its integrity so lead to somewhat ‘artificial’ ratherthan ‘natural’ results.

Acknowledgement

This research was supported by the National NaturalScience Foundation of China (NSFC). (Nos. 90306013 and50221201).

References

[1] D.J. Lundell, R.C. Williams, A.N. Glazer, J. Biol. Chem. 256 (1981)

3580.

[2] A.N. Glazer, Biochim. Biophys. Acta 768 (1984) 29.

[3] X. Su, P.G. Fraenkel, L. Bogorad, J. Biol. Chem. 267 (1992) 22944.

[4] N. Murata, Biochim. Biophys. Acta 189 (1969) 171.

[5] J.F. Allen, N.G. Holems, FEBS Lett. 202 (1986) 175.

[6] J.F. Allen, Science 299 (2003) 1530.

[7] C.W. Mullineaux, Biochim. Biophys. Acta 1184 (1994) 71.

[8] Y. Li, J. Zhang, J. Xie, J. Zhao, Biochim. Biophys. Acta 1504 (2001)

229.

[9] D. Li, J. Xie, Y. Zhao, J. Zhao, Biochim. Biophys. Acta 1557 (2003)

35.

[10] D. Li, J. Xie, J. Zhao, Biochim. Biophys. Acta 1608 (2004) 114.

[11] R.J. Porra, W.A. Thompson, P.E. Kriedeman, Biochim. Biophys.

Acta 975 (1989) 384.

[12] D. Bruce, J. Biggins, T. Steiner, M. Thewalt, Biochim. Biophys. Acta

806 (1985) 237.

[13] D. Bruce, J. Biggins, Biochim. Biophys. Acta 810 (1985) 295.

[14] W. Reuter, G. Wiegand, R. Huber, M.E. Than, Proc. Natl. Acad. Sci.

96 (1999) 1363.