Chapter 11:Cell Communication
Sig
nal-T
rans
duct
ion
Em
phas
is This chapter’s emphasis is on signals that are
released from one cell and allowed to freely diffuse to a second (or more) recipient cell(s)
These communications are deliberately initiated, received, and interpreted in order to increase the physiological coordination of the cells in multicellular organisms
We will consider in particular those events that follow the reception of a chemical signal
We will not dwell on the purpose of the signal We also will not dwell on why and how a
given cell releases a given signal
Sig
nalin
g w
ith D
irect
Con
tact
Local Signaling w/o Direct Contacte.g., interferon release by viral-infected cells
Long
-Dis
tanc
e S
igna
ling
Long
-Dis
tanc
e D
iffus
ion Note how specificity is determined by
presence/absence of receptor protein
Sig
nalli
ng, F
ree-
Livi
ng C
ells
Cel
l-Cel
l Che
mic
al S
igna
ling Three general categories of chemical
signaling:• Cytoplasmic connections between cells• Cell-to-cell contact-mediated signaling• Free diffusion between cells
• Distant cells (hormones)• Adjacent cells (within interstitial space)
All of latter involves the physical movement of ligands
• That is, ligand reception by a protein• Note that reception means molecule-to-
molecule contact
Liga
nds
e.g., steroid hormonese.g., nitric oxide
e.g., epinephrinee.g., insulin
Nothing to memorize on
this page
Signal Transduction
In this case the receptor protein is a membrane proteinLigan
d
Often turning on or off enzyme activity
Thr
ee S
tage
s 2a. Transduction
2b. Transduction
2c. Transduction
2d. Transduction
1. Reception
3. Response
Responses usually involve increasing or decreasing some protein’s function
Thr
ee S
tage
s
2a. Transduction
2b. Transduction
1. Reception
3. Response
Intr
acel
lula
r R
ecep
tor
Rap
idity
of R
espo
nse Slower
response if requiring protein
synthesis
G-P
rote
in-L
inke
d R
ecep
tor
G proteins bind GTP
G-Protein-Linked ReceptorThe more ligand binding, the greater the cellular response
Note lability of all signals
Pro
tein
Kin
ase
& P
hosp
hata
seP r o te in O H + A T P P r o te in O P
O
O
O
+ A D P
P i H 2 O
P r o te in K in a s e
P r o te in P h o s p h a t a s e
Like signal lability,
reversibility of phosphorylatio
n makes signaling reversible
Therefore, responses
tend to continue (or expand) only
so long as signaling continues
This reversibility contributes
to the dynamic nature of
cells
Tyrosine Kinase Receptor
Receptor Tyrosine KinaseNote steps involved:1. Ligand Reception2. Receptor Dimerization3. Catalysis (Phosphorylization)4. Subsequent Protein Activation5. Further Transduction6. Response
Ion-
Cha
nnel
Rec
epto
r
Reversibility is assured by pumping ions
back out again (using
separate protein)
Pho
spho
ryla
tion
Cas
cade
Cyclic AMP (cAMP)
“Second” Messenger
Second messengers
are not proteins
Note reversibility
2nd
Mes
seng
er, S
.T.P
.
Ca2+
-med
iate
d S
igna
l Am
p.
Releasing Ca2+ is a means of greatly amplifying signal
Sig
nal A
mpl
ifica
tion
(Cas
cade
)
Sig
nal A
mpl
ifica
tion
(Cas
cade
)
Note how, via catalysis, one ligand molecule binding gives rise to many new intracellullar molecules
Sig
nal A
mpl
ifica
tion
(Cas
cade
)
Sig
nal-T
rans
duct
ion
Cas
cade
Seek to understand the concept, rather
than memorize the specific protein
Nuc
lear
Res
pons
e
Var
ious
Res
pons
es Note that more than one
response can result from the reception of a single ligand
Var
ious
Res
pons
es Same ligand
gives rise to different responses
(here same receptor, different relay)
Cells differ in terms of their proteins
Different proteins respond differently to the same environmental signals
The End