Cell Signaling (BIO-203)
Lecture 3
Types of G proteins
Humans have 21 different Gα subunits (3652 kDa)
 6 Gβ subunits (35-35 kDa)
 12 Gγ subunits (8-10kDa)
 Different Gβγ function similarly
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http://highered.mcgraw-hill.com/sites/0072943696/student_view0/chapter10/animation__membranebound_receptors__g_proteins__and_ca2__channels.html
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The alpha subunit of the G-protein is activated by..
A)separating from the gamma and beta subunits.
B)the G-protein changing conformation.
C)binding to the calcium ions
D)replacing the GDP with GTP.
E)replacing the GTP with GDP.
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The activated alpha subunit then binds to...
A)the calcium ion channel
B)the calcium ions.
C)the gamma and beta subunits.
D)GDP
E)GTP
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As a result...
A)the calcium channel opens and calcium ions leave the cell
B)the calcium channel opens and calcium ions enter the cell.
C)the calcium channel closes.
D)the calcium ions bind to calmodulin.
E)the calcium ions bind to the ligand receptor site.
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The G-protein changes chemical composition when the GTP replaces the GDP on the alpha
subunit.
A)True
B)False
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The combination of the calcium and the calmodulin produces the response of the cell to the
ligand.
A)True
B)False
Answers
1.
2.
3.
4.
5.
D
A
B
B
A
Acetylcholine receptors in the heart muscle
activate a G Protein that opens K+ Chanel
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Activation of acetylcholine receptors in cardiac
muscle slows the rate of heart muscle contraction.
It is coupled to a complex of Gαi-Gβγ protein.
Binding of acetylcholine triggers activation of
Gαi subunit followed by its dissociation from
the Gβγ subunit.
The released Gβγ subunit binds to and opens
a K+ channel.
The increase in K+ permeability hyperpolarizes
the membrane which reduces the frequency of
heart muscle contraction.
Acetylcholine receptors in the heart muscle
activate a G Protein that opens K+ Chanel
Hyperpolarization
It is a change in a cell's membrane potential that
makes it more negative. It is the opposite of
a depolarization.
 It is often caused by efflux
of K+through K+ channels, or influx of Cl–
through Cl– channels. On the other hand, influx
of cations, e.g. Na+ through Na+ channels or
Ca2+ through Ca2+ channels inhibits
hyperpolarization.
 The efflux of K+ ions from the cytosol increases
inside-negative ion potential across the plasma
membrane that lasts for several seconds. it
reduces the frequency of muscle contraction.
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Light activates Gαt- Coupled
rhodopsins
Human retina contains 2 types of
photoreceptor cells:
 Rods stimulated by moonlight over a range
of wavelengths.
 Cones involved in color vision.
 They are the primary recipients of visual
stimulation.
 Rhodopsin consists of the protein opsin
which has a usual GPCR structure covalently
bonded to light-absorbing pigment 11-cisretinal.
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The trimeric G Protein
couple to rhodopsin is
called transducin (Gt).
 It contains Gαt subunit.
 Rhodopsin and Gαt
subunit are found only in
rod cells.
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Rhodopsin is sensitive enough
to respond to a single photon
of light, this response takes
place in the form of
isomeriztion,
Therefore, when a photon of
light enters the eye, it is
absorbed by the retinal and
causes a change in its
configuration
from 11-cis retinal to all-trans
retinal.
This isomeriztion induces
conformational changes in
Rhodopsin that activates the
G-protein.
Light activated rhodopsin pathway
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In dark adopted rod cells:
Light absorption generated activated opsin
Opsin binds inactive GDP-bound Gαt protein and
mediates replacement of GDP with GTP
The free Gαt-GTP activates cGMP
phosphodiesterase (PDE) by binding to its inhibitory
γ subunits and dissociating them from the catalytic α
and β subunits.
The free α and β subunits convert cGMP to GMP.
The resulting decrease in cGMP leads to dissociation
of cGMP from the nucleotide-gated channels in the
plasma membrane and closing of channels.
The membrane then becomes hyperpolarized.
Light activated rhodopsin pathway