Lecture Notes Series I. 2008
Biology 4407B, Biochemistry 4806B, Neuroscience 4376B
G protein Coupled Receptors
and G proteins
Dr. Melanie Kelly, Department of Pharmacology,
G proteins Couple Receptors to
Receptors are Seven Transmembrane Serpentine
Receptors are specialized for interaction with G
Effectors are enzymes (Phospholipases, Kinases,
Synthases) and other target molecules such as ion
1983 rhodopsin was the first G-protein-coupled
receptor to be cloned and was found to have seven
hydrophobic segments, constituting 7
7TM receptors now known to be the largest
superfamily of proteins in eucaryotes.
GPCRs signal via coupling to guanine nucleotide
7 TM receptors may exist as oligomeric protein
structures and linked to multiple G-proteins to initiate
G-Protein-Coupled Receptors Have
Evolved Through Divergent and
There are Several Families of
Rhodopsin- The Founding
member of Family A!
Members of Family A
constitute aprox. 90% of all
Rhodopsins are activated
by light and turn on signaling
pathways that leads to
Structural Characteristics of
Conserved residues largely located in transmembrane segments are a
“finger-print signature” of the Rhdopsin-like GPCR family 8
7TM Receptor Family B
Family B is a 7 TM family of peptide and neurohormone receptors that
do not have the identifying conserved residues of the rhodopsin-like
Family B have a large N-terminal extracellular domain containing 6
conserved cysteine residues.
All receptors in this family stimulate AC and couple to Gs proteins.
Some members of family B interact with chaperone proteins called
RAMPs (receptor activity modifying proteins).
7TM Receptor Family C
Contains receptors for metabotropic glutamate receptors, Ca2+-sensing
receptor (CaSR), the -aminobutyric acid (GABA)B receptor, some
pheromone receptors and taste receptors.
Most family 3 GPCRs have a large extracellular domain responsible for
ligand recognition and all Family C receptors function as dimers. 10
Guanine Nucleotide Binding
GTP binding proteins act as transducers between receptors and
Two major classes of G proteins have been described in the
literature: Heterotrimeric G proteins consisting of , subunits and
low molecular weight monomeric G proteins.
* subunits of trimeric G proteins (39-52 kDa) characterizes the
individual G protein and have a high affinity Mg2+ -modulated
nucleotide binding site.
* subunits are 35-36 kDa and the -subunit is 7-16 kDa.
* Low-molecular weight monomeric G proteins are 18-32 kDa and
include ras-related proteins and many oncogene products.
Heterotrimeric G Protein
G proteins derived from large gene family
at least 16 genes encode -subunit, 4 genes
encode -subunits and multiple genes encode
A given subtype may have more than one
Gs includes 4 isotypes one of which is Golf
Gi consists of Gi1,G i2, G i3
Gq consists of G15, G16, G14, G11,
G12 consists of G12 and G13
In mammals 5 distinct -subunits and
several different -subunits have been
Interactions between G proteins
(a) Agonist binds to receptor
(b) G protein interacts with
activated receptor and GDP
exchanged for GTP on subunit
and subunits dissociate (?)
(c) Activated subunit interacts
with other signaling molecules.
subunits also signal.
(d) -subunit possess slowly
hydrolysing intrinsic GTPase
activity. RGS proteins enhance
hydrolysis of GTP to GDP.
(e) -subunit inactivated and
recycled back to associate with
The Life-cycle of GPCR
Agonists and Antagonists Act by
Stabilizing Active and Inactive
7TM Receptors are in equilibrium between active and inactive
conformations. -Dynamic allosteric proteins that interchange between
different conformations that are stabilized by ligands
An agonist -stimulates the receptor by stabilizing an active
by various mechanisms including binding between transmembrane
An antagonist is a compound that stabilizes one or more of the
many different inactive conformations of the receptor and thus
prevents the receptor from going into the active signaling
Desensitization Mechanisms Turn
Signaling Off During Prolonged
G protein-coupled receptor signalling is attenuated by
phosphorylation by protein kinases and interaction with an intracellular
protein, arrestin, which binds to the phosphorylated receptor and
promotes dissociation of G protein
Ligand-occupied receptors are then sequestered into endocytic
intracellular vesicles where the ligand is degraded and the receptor
dephosphorylated and shunted back to the membrane.
After repeated or prolonged activation downregulation, receptors
can be delivered to lysosomes and degraded after internalization. This
results in a decreased response which is only recoverable with
synthesis of new receptor molecules.
Kinases and Arrestins involved in
internalization of GPCRs
GRKs – G protein receptor kinases –
Protein Kinase C and Protein Kinase A –
Diversity of GPCRs, G proteins and
their signaling pathways
Some G protein-coupled Receptors
Activate cAMP-dependent Signaling
Some G protein-coupled Receptors
Activate Phospholipid Signaling Pathways
G Protein Receptor Cross-
Talk for Signal Amplification
Selbie and Hill, 1998 22
TIPS 19: 87-93
Bacterial Toxins Can Regulate The
Activity of Specific G protein Subunits
Several bacterial toxins modify G protein activity by a process
called ADP-ribosylation. This process involves the addition of an
ADP-ribose group from nicotinamide adenine dinucleotide (NAD+)
to an amino acid residue in the subunit.
Cholera Toxin – ADP-ribosylates and irreversibly activates Gs
by inhibiting its GTPase activity. This leads to prolonged pathway
Pertussis Toxin - ADP-ribosylates and inactivates Gi and Go
by stabilizing their association with subunits. This leads to
inhibition of GPCR pathways that couple to Gi/o G proteins.