L10 - GPCR classification, signalling and regulation Flashcards
describe the structural features of the GPCR superfamily
- extracellular N terminus
- cytoplasmic C terminus
- seven transmembrane α-helices
- six alternating extracellular and
intracellular loops
- similar basic typology
- Highly diverse in primary
structure
describe the different forms of GPCR ligands
detect and respond to a broad range of stimuli e.g. Photons, odorants, nucleotides, aa and ions, peptides, biogenic amines, and lipids.
Orphan GPCRs = non-known endogenous ligand
Rhodopsin (Class A)
Largest family, High diversity in primary structure and ligand preference
wide variety of small molecules, neurotransmitters, peptides and
hormones, as well as olfactory receptors, visual pigments, taste type 2 receptors and five pheromone receptors
Palmitoylation group in C-tail
Secretin (Class B)
15
Receptors include glucagon, secretin, vasoactive intestinal peptide, calcitonin, corticotropin-releasing hormone receptors
Large N-terminus with Cys-Cys bridges – hormone binding domain
Glutamate (Class C)
22
Receptors include metabotropic
glutamate receptors, two GABA
BRs, the calcium-sensing receptor (CASR), the sweet and umami taste receptors (TAS1R1–3)
exist as obligate homomers or heteromers.
long N-terminus (>600aa) that
forms a ‘Venus flytrap’ domain
3 very short, highly conserved
intracellular loops
Adhesion
33
Receptors are mostly orphans
Bind extracellular matrix molecules
Highly diverse and very large N termini
- contains “GPCR proteolysis site”
Frizzled (Class F)
11
10 Frizzled proteins (FZD(1-10)) and
Smoothened (SMO).
- FZDs are activated by secreted lipoglycoproteins of the WNT family.
- SMO is indirectly activated by the Hedgehog (HH) family of proteins acting on the transmembrane protein
Patched (PTCH).
describe components of GPCR signal transduction
inactive/active receptor, G-protein heterotrimer with a, b, y subunits, GDP/GTP and an inactive/active effector
describe the processes of G protein activation and deactivation
- binding of hormone induces a conformational change in intracellular receptor
- activated receptor recruits and binds G-protein at the Ga subunit
- G protein binding to the receptor causes a conformational change in the Ga that dissociates the GDP
- GTP binds to G-protein caused by GDP disassociation and causes Ga to disassociate from beta, gamma subunits
- hormone leaves receptor; Ga binds to downstream effector activating it
- hydrolysis of GTP to GDP causing Ga to return to heterotrimer with b and y at rest
G-protein accessory proteins
G protein activation can be accelerated by GEFs.
- Guanine nucleotide exchange factors GDP->GTP
GPCR inactivation can be accelerated by GAPs
- GTP->GDP
describe the different G protein subunits and the effectors they modulate
G-alpha
Gai, Gas: these activate and inhibit adenylyl cyclase ATP -> cAMP
Gaq: Activate phospholipase C to convert phosphatidylinositol 4,5 -> Pi and DAG
Ga12/13: activate RhoGEF -> Rho
G-Beta/Gama
no subunits but they mediate downstream signalling pathways
Gαi and Gαs modulation of adenylyl cyclase
Gαq initiation of calcium signalling
describe the processes of GPCR desensitisation and internalisation
Desensitisation
GPCR kinases (GRKs) phosphorylate the receptor
- At serine and threonine residues in the intracellular loops and/or the C tail.
This initiates the recruitment of the adaptor protein arrestin.
- 4 Arrestins 2 are visual, 2 are non visual in all cells
Internalisation:
GPCR bound Arrestin scaffolds clathrin and AP2 that begin to form clathrin-coated pits in the plasma membrane that pinch off and form endosomes
describe the various functions of arrestin
Desensitisation: Arrestin binding to the receptor uncouples the receptor from G protein signalling by sterically hindering access to the receptor’s binding domains.
Initiation of GPCR endocytosis: and Activation of signaling pathways, e.g. MAPK and Jun: arrestin is a scaffold protein that can recruit many other proteins onto it
