Download 5.3 G Protein-Coupled Receptors

Gerhard Krauss
Biochemistry of Signal Transduction and Regulation(3rd
Edition) ISBN: 3-527-30591-2
LOGO
G Protein-Coupled
Signal Transmission
Pathways
授課老師: 褚俊傑副教授 (生物科技系暨研究所)
聯絡電話: 0986-581835
電子信箱: [email protected]
The G protein cycle.
The receptor–G-protein complex remains the only major G protein conformation for which atomic-scale structural information is unavailable. In the
resting state, G proteins are heterotrimers of GDP-bound α- (blue), β- (green) and γ- (yellow) subunits (Gαt/iβ1γ1). On binding of an extracellular
stimulus (light purple), receptors (pink) (such as bovine rhodopsin) undergo a conformational change that permits G protein binding and catalyses GDP
release from Gα. Once GDP is released, a stable, high-affinity complex is formed between the activated receptor (R*) and G protein. Binding of GTP
(green) to Gα destabilizes this complex, allowing both subunits, Gα(GTP) (Gαt(GTPγS) ND2) and Gβγ, to interact with downstream effector proteins
(purple) (Gαi/q(GDP·AlF4?)·GRK2·Gβ1γ2). The signal is terminated on hydrolysis of GTP to GDP by Gα, which may be catalysed by regulator of G
protein signalling (RGS) proteins (dark red) (Gαt/i(GDP·AlF4).
Outline
 5.1 Transmembrane Receptors: General Structure and
Classification
 5.2 Structural Principles of Transmembrane Receptors
 5.3 G Protein-Coupled Receptors
 5.4 Regulatory GTPases
 5.5 The Heterotrimeric G Proteins
 5.6 Effector Molecules of G Proteins
5.1 Transmembrane Receptors: General Structure and Classification
 Signal transmission into the cell interior takes place by reaction
chains, in which several individual reactions generally run in
sequence and involve many signal proteins. The nature of the
extracellular signal can be very diverse and may include
extracellular signal molecules, such as low-molecular-weight
messenger substances or proteins, or sensory signals such as light
signals.
 The cell uses two principal ways to transduce signals into the
interior of the cell. One way is exemplified by nuclear receptor
signaling, where the signaling molecule crosses the cell membrane
and activates the receptor in the interior of the cell. In the other
major way the signal is registered at the cell membrane and
transduced into the cell by transmembrane proteins.
 Three different types of transmembrane proteins participate in this
mode of signaling:
Transmembrane LOGO
Receptors
www.themegallery.com
5.1 Transmembrane Receptors: General Structure and Classification
 Signaling via Transmembrane Receptors
Transmission of the signal implies specific
communication with the effector protein, the next
component of the signal transmission pathway on the
inner side of the cell membrane. In this process
enzymatic activities can be triggered and/or the activated
receptor engages in specific interactions with
downstream signal proteins.
An intracellular signal chain is set in motion, which
finally triggers a defined biochemical response of the
target cell (Fig. 5.1a). Sensory signals (light, pressure,
odor, taste) can be received as well by transmembrane
receptors and can be transmitted into intracellular
signals.
Fig. 5.1 Mechanism of signal transduction at membranes. a) Signal transmission
via ligand-controlled transmembrane receptors. The ligand L binds to the
extracellular domain of a transmembrane receptor, whereby the receptor is
activated for signal transmission to the cytosolic side. The cytosolic domain of the
activated receptor R* transmits the signal to signal proteins next in sequence.
5.1 Transmembrane Receptors: General Structure and Classification
 Signaling via Ligand-gated Ion Channels
One simply designed path of signal transmission is
found in neuronal communication. Transmembrane
receptors are also used for signal transmission here.
These have the character of a ligand-gated ion channel
(Fig. 5.1b).
Binding of a ligand (neurotransmitter or neurohormone)
to the transmembrane receptor leads to a conformational
change of the receptor that enables the flow of ions
through the membrane.
In this case, the receptor presents itself as an ion
channel with an open state controlled by ligand binding
to the outer side (or also to the inner side).
Fig. 5.1 Mechanism of signal transduction at membranes. b) Signal transduction via
ligand-controlled ion channels. The ligand binds to the extracellular side of a
receptor that also functions as an ion channel. Ligand binding induces the opening
of the ion channel, there is an ion efflux and a change in the membrane potential.
5.1 Transmembrane Receptors: General Structure and Classification
 Signaling via Ligand-gated Ion Channels
Another mechanism of signaling across the cell
membrane uses changes in membrane potential. A
change in membrane potential induces the opening of an
ion channel, and ions cross the membrane.
In this case, the change of the ion’s milieu is the
intracellular signal. Ion channels with an open state
regulated by changes in membrane potential are known
as voltage-gated ion channels (Fig. 5.1c).
The potential-driven passage of ions through ion
channels is the basis for stimulation in nerves.
Fig. 5.1 c) Signal transduction via voltage-gated ion channels. A change in the
membrane potiential V is registered by an ion channel which transitions from the
closed to the open state.
5.1 Transmembrane Receptors: General Structure and Classification
 Intracellular Activation of Receptors
We also know of transmembrane receptors for which the
reception of the signal and activation take place on the inner
side of the membrane. The cGMP-dependent ion channels
involved in signal conduction in the vision process are ligandregulated ion channels with an open state controlled by
intracellularly created cGMP.
Another example is the receptors for inositol triphosphate
which are localized in the membrane of Ca2+ storage
organelles and also have the character of ligand-controlled
ion channels. Inositol triphosphate is an intracellular
messenger substance that binds to the cytosolic side of the
corresponding receptor located in the membrane of cell
organelles.
5.2 Structural Principles of Transmembrane Receptors
 5.2.1 The Extracellular Domain of Transmembrane
Receptors
 5.2.2 The Transmembrane Domain
 5.2.3 The
Receptors
Intracellular
Domain
 5.2.4 Regulation of Receptor Activity
of
Membrane
5.2 Structural Principles of Transmembrane Receptors
 Transmembrane receptors are integral membrane
proteins, i.e., they possess a structural portion that
spans the membrane. An extracellular domain, a
transmembrane domain and an intracellular or cytosolic
domain can be differentiated within the structure (Fig.
5.2a).
Fig. 5.2 Structural principles of transmembrane receptors. a) Representation of the
most important functional domains of transmembrane receptors.
5.2.1 The Extracellular Domain of Transmembrane Receptors
 In many receptors, the extracellular domain contains the
ligand-binding site. Glycosylation sites, i.e. attachment
sites for carbohydrate residues, are also located nearby
in the extracellular domain.
 The extracellular localized protein portion may be formed
from a continuous protein chain and may include several
hundred amino acids. If the receptor crosses the
membrane with several transmembrane segments, the
extracellular domain is formed from several loops of the
protein chain that may be linked by disulfide bridges.
 Transmembrane receptors may show homotrophic
composition (identical subunits) or heterotrophic
composition (different subunits, Fig. 5.2b), so that the
extracellular domain may be made up of several identical
or different structural elements.
Fig. 5.2 Structural principles of transmembrane receptors. b) Examples of subunit
structures. Transmembrane receptors can exist in a monomeric form (1), dimeric
form (2) and as higher oligomers (3,4). Further subunits may associate at the
extracellular and cytosolic domains, via disulfide bridges (3) or via non-covalent
interactions (4).
5.2.2 The Transmembrane Domain
 The transmembrane domains have different functions,
according to the type of receptor. For ligand-controlled
receptors, the function of the transmembrane domain is
to pass the signal on to the cytosolic domain of the
receptor. For ligand-controlled ion channels, the
transmembrane portion forms an ion pore that allows
selective and regulated passage of ions.
 The transmembrane receptors span the 5 nm thick
phospholipid bilayer of the cell membrane with structural
portions known as transmembrane elements. The inner
of a phospholipid layer is hydrophobic and,
correspondingly, the surface of the structural elements
that come into contact with the inner of the phospholipid
double layer also has hydrophobic character.
5.2.2 The Transmembrane Domain
 Structure of Transmembrane Elements
High-resolution structural information about the
transmembrane elements of transmembrane receptors
could recently be obtained on the example of rhodopsin,
the light-activated G protein coupled receptor of the vision
process (Fig. 5.3).
These data, together with earlier data on the structures
of
other
transmembrane
proteins
(e.g.,
bacteriorhodopsin), have confirmed that a-helices are the
principal structural building blocks of the transmembrane
elements of membrane receptors.
Fig. 5.3 Three-dimensional structure of rhodopsin. Two views of rhodopsin. A) The
seven a-helices of the G protein-coupled receptor rhodopsin weave back and forth
through the membrane lipid bilayer (yellow lines) from the extracellular environment
(bottom) to the cytoplasm (top). The chromophore 11-cis retinal (yellow) is nested
among the transmembrane helices .B) View into the membrane plane from the
cytoplasmic side of the membrane. Roman numerals indicate numbered helices.
5.2.2 The Transmembrane Domain
 Structure of Transmembrane Elements
In addition to a-helices, proteins also use b-structures to
cross the membrane. The transmembrane domain of the
bacterial OmpF porin is made up of b-elements (see Fig.
5.4). The b-elements, in this case, are not mostly made
up of hydrophobic amino acids and form a barrel-like
structure.
Fig. 5.4 The OmpF porin from Eschericia coli is an integral membrane channelforming protein which spans the outer membrane in Gram-negative bacteria. The
structure of a monomer of the OmpF porin is shown. In total, 16 b-bands are
configured in the form of a cylinder and form the walls of a pore through which
selective passage of ions takes place.
5.2.3 The Intracellular Domain of Membrane Receptors
 Two basic mechanisms are used for conduction of the
signal to the inner side of the membrane (Fig. 5.5):
 Starting from the activated receptor, a large number of
reactions can be set in motion (Fig. 5.5). One main route
of signal transmission takes place by activation of G
proteins, another via activation of tyrosine-specific
protein kinases, and a further route is via activation of
ion channels. In the further course of G protein-mediated
signal transmission, secondary diffusible signals are
often formed: the “second messenger” molecules (see
Chapters 3 and 6).
Fig. 5.5 General functions of transmembrane receptors. Extracellular signals convert the
transmembrane receptor from the inactive form R to the active form R*. The activated
receptor transmits the signal to effector proteins next in the reaction sequence. Important
effector reactions are the activation of heterotrimeric G-proteins, of protein tyrosine kinases
and of protein tyrosine phosphatases. The tyrosine kinases and tyrosine phosphatases may
be an intrinsic part of the receptor or they may be associated with the receptor. The activated
receptor may also include adaptor proteins in the signaling pathway or it may induce opening
of ion channels.
5.2.4 Regulation of Receptor Activity
 The cell has various mechanisms available, with the help
of which the number and activity of transmembrane
receptors can be regulated. The aim of regulation is, for
example, to weaken signal transmission via the receptor
during conditions of long-lasting hormonal stimulation.
 Furthermore, signal transduction by transmembrane
receptors may be modulated via crosstalk with other
signaling pathways. The structural elements involved in
regulation of receptor activity are generally located in the
cytosolic domain.
5.2.4 Regulation of Receptor Activity
 These are, above all, protein sequences that permit
phosphorylation of the receptor by protein kinases.
Phosphorylation at Ser/Thr or Tyr residues of the
cytosolic domain may lead to inactivation or activation of
the receptor and thus weaken or strengthen signal
transmission.
 In this way, Ser/Thr-phosphorylation is used in the
process of internalization of receptors in order to remove
the receptor from circulation after it has been activated
(see Section 5.3.4). The protein kinases involved are
often part of other signaling pathways and can link the
activity of the transmembrane receptors to other
signaling networks.
5.3 G Protein-Coupled Receptors
 5.3.1 Structure of G Protein-Coupled Receptors
 5.3.2 Ligand Binding
 5.3.3 Mechanism of Signal Transmission
 5.3.4 Switching Off and Desensitization of 7-Helix
Transmembrane Receptors
 5.3.5 Dimerization of GPCRs
5.3 G Protein-Coupled Receptors
 Of the transmembrane receptors that receive signals and
conduct them into the cell interior, the G protein-coupled
receptors form the largest single family. About 5% of the
genome of the worm Caenorhabditis elegans is occupied
by genes encoding G protein-coupled receptors.
 In vertebrates, more than 1000 different G proteincoupled receptors are found that may be activated by
extracellular ligands or sensory signals. The ligands
include biogenic amines, such as adrenaline and
noradrenaline, histamine, serotonin, lipid derivatives,
nucleotides, retinal derivatives, peptides such as
bradykinin and large glycoproteins such as luteinizing
hormone, and parathormone (see also Table 3.1).
Tab. 5.1 Classification of the heterotrimeric G proteins according to the α-subunits.
5.3 G Protein-Coupled Receptors
5.3.1 Structure of G Protein-Coupled Receptors
 Examplary for the G-protein-coupled receptors, Fig. 5.6
shows the two dimensional model of bovine rhodopsin. The
three-dimensional structure of rhodospsin has been directly
visualized high resolution X-ray analysis that shows a bundle
of 7 transmembrane helices, as predicted from a multitude of
biochemical and biophysical studies.
 This structure provides a frame upon which the 3D-structure
of the huge family of 7-helix transmembrane receptors can be
modeled.
Sequence
comparisons,
biochemical
and
biophysical data indicate that the transmembrane bundle
structure is conserved among GPCRs and possibly within the
entire GPCR family.
 By contrast, the loops and termini are more divergent in
amino acid sequence and possibly in three-dimensional
structure.
Fig. 5.6 Two dimensional model of rhodopsin. The extracellular (intradiscal) and
intracellular regions of rhodopsin each consist of three interhelical loops (given the
prefixes E(extracellular)-I to E-III or C(cytoplasmic)-I to C-III. A conserved disulfide
bridge is found on the extracellular side linking EII with E-III. On the intracellular
side, a short helix runs parallel to the membrane surface. In the native protein, the
C-terminus carries two palmitoylated Cys-residues which function as membrane
anchors
causing formation of a putative fourth intracellular loop.
5.3.1 Structure of G Protein-Coupled Receptors
 The only structural feature common to all G protein-coupled
receptors is the presence of the seven transmembrane
helices connected by alternating extracellular and intracellular
loops, with the amino terminus located on the extracellular
side and the carboxy terminus on the intracellular side. Apart
from that feature, the overall sequence homology among the
G protein-coupled receptors is low.
 Significant sequence homology is found, however, within
three subfamilies, designated family A, B and C receptors.
The classification is based on the size of the extracellular
loops, the presence of key residues and the formation of
disulfide bonds (Fig. 5.7). Family A includes the rhodopsin/badrenergic receptor, family B includes calcitonin receptors
and family C includes receptors for c-amino butyric acid,
Ca2+ and glutamate.
Fig. 5.7 Classification of GPCRs. The G protein-coupled receptors can be divided
into
three major subfamilies (see Gether, 2001)
Family A receptors are characterized by a series of highly conserved key residues
(black letter in white circles). In most family A receptors, a disulfide bridge is
connecting the E-II and E-III loops. In addition, a majority of the receptors have a
palmitoylated cysteine in the cytoplasmic C-terminus. Ligands include the biogenic
amines (adrenaline, serotonine, doapmine, histamine), neuropetide Y, adenosine,
chemokines and melatonine, among others.
Fig. 5.7 Classification of GPCRs. The G protein-coupled receptors can be divided
into
three major subfamilies (see Gether, 2001)
Family B receptors are characterized by a long extracellular N-terminus containing
a series of cysteine residues presumably forming a network of disulfide bridges.
Representative members of the family B receptors include calcitonine receptor,
glucagon receptor and parat hormone receptors.
Fig. 5.7 Classification of GPCRs. The G protein-coupled receptors can be divided
into
three major subfamilies (see Gether, 2001)
Family C receptors are characterized by a very long N-terminus forming the
extracellular ligand binding site. There is only one putative disulfide bridge and the
third cytoplasmic loop is very small. The taste receptors, the metabotropic
glutamate receptors, the c-aminobutyric acid (GABA) receptors and Ca2+-receptors
belong to this class, among others.
5.3.2 Ligand Binding
 The area of ligand binding has been particularly well defined
for the receptors of classical “small ligands” (adrenaline,
noradrenaline, dopamine, serotonine, histamine). Targeted
mutagenesis, biochemical, biophysical and pharmacological
investigations have shown that these ligands are bound in a
binding crevice formed by the transmembrane helices.
 In agreement with this model, it has been shown that the
extracellular and intracellular sequence portions of the
receptors are not needed for ligand binding in these cases.
Rhodopsin is a unique case, since the retinal ligand is
covalently attached by Schiff-base linkage to a Lys residue of
transmembrane helix VII. In that case too, the binding site is
deeply buried in the interior of the transmembrane segment
(see Fig 5.3).
5.3.3 Mechanism of Signal Transmission
 The heterotrimeric G protein, which exists as the inactive
GDP form, now binds via its a- and possibly γ-subunit to the
activated receptor and is activated itself. An exchange of GDP
for GTP takes place, and the βγ-subunit of the G protein
dissociates (see Section 5.5.3). Once the G protein is
activated, it frees itself from the complex with the receptor,
which either returns to its inactive ground state or activates
further G proteins.
5.3.4 Switching Off and Desensitization of 7-Helix Transmembrane Receptors
 A phenomenon often seen in transmembrane receptors
in general, and in G protein coupled receptors in
particular, is desensitization (Fig.5.8). Desensitization
means a weakening of the signal transmission under
conditions of long-lasting stimulation by hormones,
neurotransmitters or sensory signals.
 Despite the persistent effect of extracellular stimuli, the
signal is no longer passed into the cell interior, or only in
a weakened form, during desensitizing conditions. This
is a mechanism with which both short-term and longterm regulation of receptor activity is possible.
Fig. 5.8 General principle of desensitization of G-protein-coupled receptors.
Desensitization of a hormone-bound receptors can take place by two principle
routes, schematically represented in the figure. A suppressing influence may be
exerted on the receptor system via proteins (X) of a signal chain, triggering inhibition
of the signal chain. Receptor systems may also mutually influence one another in
that a signal protein X formed in one signal chain mediates the desensitization of
another receptor system R*, and vice versa.
5.3.4 Switching Off and Desensitization of 7-Helix Transmembrane Receptors
 Phosphorylation by cAMP-dependent protein kinases (Fig.
5.8)
Phosphorylation of the cytoplasmic domain of 7-helix
transmembrane receptors can take place via cAMPdependent protein kinases (protein kinase A) or via protein
kinase C (Chapter 7) (Fig. 5.9). This is a feedback
mechanism. The hormonal activation of the receptor leads,
via G proteins and adenylyl cyclase/cAMP, to activation of
protein kinases of type A (see Sections 5.6.1 and 6.1, and
Chapter 7).
The activated protein kinases phosphorylate the receptor in the
region of the cytoplasmic domain on Ser/Thr residues.
Regulation via adenylyl cyclase/cAMP/proteinkinase A is an
example of a heterologous desensitization, since adenylyl
cyclase can be activated by a variety of signals originating
from different signaling pathways (see Section 5.6.1).
Fig. 5.9 Desensitization of G-protein-coupled receptors via cAMP-dependent protein kinases.
Starting from an activated receptor R*, the signal is transmitted via the Ga-subunit of the Gprotein to adenylyl cyclase. The latter is activated and forms cAMP. This activates a protein
kinase of type A that passes the signal in the form of a Ser/Thr-specific protein phosphorylation
to substrate proteins. One of the substrates is also the receptor that is phosphorylated in the
region of the cytoplasmic domain by the activated protein kinase A. The ligand-bound receptor
is preferentially phosphorylated. As a consequence of phosphorylation, activation of further Gproteins by the receptor is suppressed.
5.3.4 Switching Off and Desensitization of 7-Helix Transmembrane Receptors
 Phosphorylation via G protein coupled receptor protein
kinases (GRK)
The
major
mechanism
for
the
homologous
desensitization of agonist-bound 7-helix transmembrane
receptors consists of a two-step process in which the
agonist-bound receptor is phosphorylated by a GRK and
then binds an arrestin protein which interrupts signaling
to the G protein. Receptor phosphorylation by GRKs
triggers several reactions (Fig. 5.10)
Fig.5.10
Receptor
desensitization:
phosphorylation,
arrestin
binding and internalization.
The activated, agonist-bound
receptor is phosphorylated on
the cytoplasmic region by a G
protein-coupled receptor
protein kinase (GRK). The
phosphate residues serve as
attachment sites for b-arrestin
which has protein kinases of
the MAPK cascade associated.
This serves as a trigger for
internalization of the receptor
to endosomes. The receptor
may now be dephosphorylated
and transported back to the
cell
membrane (not shown in the
figure).
5.3.4 Switching Off and Desensitization of 7-Helix Transmembrane Receptors
 Binding of Arrestin
For some receptors, arrestin binding serves to activate a
protein kinase cascade, the MAPK cascade. In that
signaling mode, arrestins function as scaffolding proteins
that help to organize the three protein kinases of the
MAPK pathway into a cascade of sequentially acting
protein kinases delivering a signal to the level of, e.g.,
transcription factors (see Chapter 10).
In view of this observation, arrestins appear to function
not only as “signal terminators” but rather also as
activators of another signaling pathway, that of the
MAPKs, providing another example of a crosstalk
between different signaling pathways (Fig. 5.11).
Fig. 5.11 Activation of the MAPK cascade via b-arrestin and G protein-coupled
receptors. A complex is formed between b-arrestin and the various components of
a MAPK module (see Chapter 10) in the cytosol. This multiprotein complex
translocates to the plasma membrane bound receptor following ligand binding. barrestin functions as a scaffold for the MAPK module and promotes internalization
of the whole complex. This leads to generation of active MAPK and stimulation of
transcription. MAPK: mitogen activated protein kinase, MEK: MAPK/ERK kinase,
MEKK: MEK kinase.
5.3.5 Dimerization of GPCRs
 Dimerization has been shown to alter the ligand-binding,
signaling and trafficking properties of G protein-coupled
receptors.
 In addition to homodimers, the formation of heterodimers
with related members of the same subfamily has been
also reported. The structural, functional and mechanistic
consequences of the formation of the oligomeric receptor
complexes remain to be elucidated.
5.3.5 Dimerization of GPCRs
Cross-talk between G protein—coupled receptors (GPCRs), ligand-gated ion channels
(LGChs) and receptor protein tyrosine kinases (RTKs).
5.4 Regulatory GTPases
 5.4.1 The GTPase Superfamily: General Functions and
Mechanism
 5.4.2 Inhibition of GTPases by GTP Analogs
 5.4.3 The G-domain as Common Structural Element of
the GTPases
 5.4.4 The Different GTPase Families
5.4 Regulatory GTPases
 The heterotrimeric G proteins, the major effector proteins
of the 7-helix transmembrane receptors, belong to the
large family of regulatory GTPases ; these bind GTP and
hydrolyze it, thereby functioning as a switch in central
cellular processes.
 The family of regulatory GTPases is also called the
GTPase superfamily.
5.4.1 The GTPase Superfamily: General Functions and Mechanism
 The Switch Function of the GTPases
The regulatory GTPases are involved in signaling chains
by functioning as a switch. Incoming signals are received
by the GTPases and are passed on to downstream
components of the signaling chain. The switch function is
based on a cyclical, unidirectional transition between an
active, GTP-bound form and an inactive, GDP-bound
form (Fig. 5.12).
The binding of GTP brings about the transition into the
active form. Hydrolysis of the bound GTP by an intrinsic
GTPase activity converts the protein into the inactive,
GDP-bound form.
Fig. 5.12 The switch function of the
regulatory GTPases. The GTP
form of the regulatory GTPases
represents the “switched on” form
of the GTPase, the GDP form, in
contrast, the “switched off” form.
The switch function of the
regulatory GTPases may be
controlled by guanine nucleotide
exchange factors, by GTPase
activating proteins (GAPs) and by
G-nucleotide dissociation inhibitors
(GDIs). The regulatory GTPases
run through a GTPase cycle which
signals flow into via GEFs and are
conducted further in the form of the
GTPase-GTP complex to effector
molecules
further
down
the
sequence. Hydrolysis of the bound
GTP ends the activated state. The
rate of GTP hydrolysis is either
intrinsically determined or may be
accelerated via GAPs.
5.4 Regulatory GTPases
Regulatory GTPases have a common mechanism that enables them to switch a
signal transduction chain on and off.
5.4.2 Inhibition of GTPases by GTP Analogs
 Nonhydrolyzable GTP analogs are an indispensable tool in
the identification and structural and functional characterization
of GTPases. The GTP analogs shown in Fig. 5.13, GTPγS,
β,γ-methylene GTP and β,γ-imino GTP, are either not
hydrolyzed by GTPases or only very slowly. Addition of these
analogs fixes the G protein in the active form; it is
permanently “switched on”. For cellular signal transduction,
this means permanent activation of the signal transmission
pathway.
 In many cases, a role of G proteins in a signal chain was
inferred from the observation that nonhydrolyzable GTP
analogs bring about a lasting activation of signal transmission.
The GTP analogs were equally important for structural
determination of the activated form of GTPases.
Fig. 5.13 Examples of non-hydrolysable GTP analogs.
5.4.4 The Different GTPase Families
 The superfamily of GTPases, with over 100 members, is
subdivided according to sequence homologies,
molecular weight and subunit structure into further
(super)families.
 These are the families of the heterotrimeric G proteins,
the Ras superfamily of small GTPases and the family of
initiation and elongation factors (Fig. 5.15).
Fig. 5.15 The GTPase superfamily.
5.5 The Heterotrimeric G Proteins
 5.5.1 Classification of the Heterotrimeric G Proteins
 5.5.2 Toxins as Tools in the Characterization of Heterotrimeric G
Proteins
 5.5.3 The Functional Cycle of Heterotrimeric G Proteins
 5.5.4 Structural and Mechanistic Aspects of the Switch Function of
G Proteins
 5.5.5 Structure and Function of the βγ-Complex
 5.5.6 Membrane Association of the G Proteins
 5.5.7 Regulators of G Proteins: Phosducin and RGS Proteins
5.5 The Heterotrimeric G Proteins
Regulatory cycle of heterotrimeric G proteins, for abbreviations
5.5 The Heterotrimeric G Proteins
 A common structural feature of the G proteins is their
construction from three subunits (Fig. 5.16), a large αsubunit of 39–46 kDa, a β-subunit of 36 kDa and a γsubunit of 8 kDa. The a-subunit has a binding site for
GTP or GDP and carries the GTPase activity.
 The β- and γ-subunits exist as a tightly associated
complex and are active in this form. All three subunits
show great diversity, so that at least 20 different genes
for α-subunits, 5 for β-subunits and 12 for γ-subunits are
known in mammals. Some G protein are ubiquitous,
whereas others only occur in specialized tissue.
Fig. 5.16 Structure and activation of the heterotrimeric G-proteins. Reception of a signal by
the receptor activates the G-protein, which leads to exchange of bound GDP for GTP at the asubunit and to dissociation of the βγ-complex. Further transmission of the signal may take
place via Gα-GTP or via the βγ-complex, which interact with corresponding effector molecules.
The α- and γ-subunits are associated with the cell membrane via lipid anchors. Signal
reception and signal transmission of the heterotrimeric G-proteins take place in close
association with the cell membrane. This point is only partially shown in the figure.
5.5.1 Classification of the Heterotrimeric G Proteins
 Most functions of signal transmission by G proteins are
realized by the a-subunit. Since different G proteins
interact with very different partners, there are significant
differences in the structure of the a-subunits.
 Based on comparison of the amino acid sequences, the
Ga proteins are divided into four families, the Gs, Gi, Gq
and G12 families.
 These families are summarized in Table 5.1, together
with representative members and their characteristic
properties.
Tab. 5.1 Classification of the heterotrimeric G proteins according to the α-subunits.
5.5.2 Toxins as Tools in the Characterization of Heterotrimeric G Proteins
 Two bacterial toxins, namely pertussis toxin and cholera
toxin, were of great importance in determining the
function of G proteins.
 Both toxins catalyze ADP ribosylation of proteins. During
ADP ribosylation, an ADP-ribose residue is transferred
from NAD+ to an amino acid residue of a substrate
protein (Fig. 5.17).
Fig. 5.17 ADP-ribosylation of the Ga-subunit of transducin by cholera toxin. Cholera
toxin catalyzes the ADP-ribosylation of the a-subunit of the G-protein transducin.
During the reaction, the ADP ribose residue of NAD+ is transferred to Arg174 of
Ga,t, which inactivates the GTPase activity of Ga,t.
5.5.3 The Functional Cycle of Heterotrimeric G Proteins
 Signal transmission via G proteins takes place in close
association with the inner side of the cell membrane.
Both the a-subunit and the βγ-complex are associated
with the membrane via membrane anchors (see Section
5.5.6).
 Like all regulatory GTPases, the heterotrimeric G
proteins run through a cyclical transition between an
inactive, GDP-bound form and an active, GTP-bound
form. Thereby, the activated G protein-coupled receptor
functions as a nucleotide exchange factor, GEF. Figure
5.18 sketches the different functional states and the role
of the individual subunits.
Fig. 5.18 Functional cycle of the
heterotrimeric Gproteins. a) The Gproteins exist in the ground state as a
heterotrimeric complex (GαGDP) · (βγ). b)
The activated receptor binds to the
inactive heterotrimeric complex of the Gprotein and leads to dissociation of the
bound GDP and the βγ-complex. c)
Binding of GTP to the “empty” Gα-subunit
transforms the latter into the active
GαGTP state. GαGTP interacts with an
effector molecule in the sequence E1 and
activates the latter for further signal
transmission. The released βγ-complex
may also take part in signal conduction by
binding to a corresponding effector
molecule E2 and activating the latter for
further signal conduction. d) Hydrolysis of
the bound GTP terminates the signal
transduction via the α-subunit.
5.5.4 Structural and Mechanistic Aspects of the Switch Function of G Proteins
 A structural model of the trimeric G protein and the
receptor is presented in Fig. 5.19. In this model, the
known structures of the ground state of rhodopsin and
the structures of the transducin Gtα· GDP · (βγ) complex
have been modeled, taking into account the location of
the lipid anchors and the known interaction sites
between the receptor and the G protein.
Fig. 5.19 Model of the assembly of rhodopsin and transducin at the cell membrane
(from Hamm, 2001). Models are based on the crystal structure and are to scale.
The C-terminal residues after S316 are not shown. The orientation of transducin
(Gt,α, Gt,βγ) with respect to rhodospin and the membrane is based on the charge
and hydrophobicity of the surface, the known rhodosin binding sites on transducin
and the sites of lipidation of Gt,α, and Gt,βγ.
5.5.5 Structure and Function of the βγ-Complex
 Structures of the βγ-complex could be obtained both in
the free state and in the Gα-GDP-bound state. The βγcomplex has an interesting configuration of seven bsheet structures for the β-subunit.
5.5.6 Membrane Association of the G Proteins
 Signal transmission via G proteins is inseparably linked
with their membrane association.
 The preceding reaction partners are transmembrane
proteins, and the subsquent effector molecules, such as
adenylyl cyclase, are either also transmembrane
proteins or are associated with the membrane (Fig.
5.23).
Fig. 5.23 Membrane anchor of the heterotrimeric G-proteins. The lipid anchoring in the system
of G-protein- coupled receptors and the corresponding G-proteins is shown. In the figure, it is
assumed that the lipid anchors are located in the membrane. A possible involvement of the
lipid anchor in protein interactions is not shown. The G-protein coupled receptor carries a
palmitoic acid anchor at the C-terminus. The α-subunit of the heterotrimeric G-protein is
associated with the membrane via a myristoic acid anchor at the N-terminus, whilst the γsubunit of the βγ-complex uses a prenyl residue as a membrane anchor.
5.5.7 Regulators of G Proteins: Phosducin and RGS Proteins
 The regulation is mostly of a negative, suppressing
character and serves two purposes in particular. Firstly,
the cell must try to weaken the cytoplasmic answer
under conditions of persistent activation of the receptor.
Secondly, the cell needs mechanisms to rapidly
terminate the signal. Typically, the rate of GTP
hydrolysis of the a-subunit is very slow, about 4 min–1.
 The cell must be able to shorten the associated long
lifetime of the activated state in a regulatable way. The
most important regulatory attack points at the level of the
G proteins and their receptors are (Fig. 5.24)
Fig. 5.24 Regulation of G protein-coupled receptors and of G proteins. Signal
transduction by activated G protein-coupled receptors (GPCRs) is mainly regulated
by phosphorylation via GPCR kinases (GRKs) leading to downregulation and
desensitization. Signaling by GαGTP can be negatively controlled by regulators of
G protein signaling (RGS) and by the effector proteins themselves. A negative
control of βγ-complex signaling is mediated by phosducin.
LOGO
www.themegallery.com