Receptors II

Receptors II: G-protein Coupled Receptors and G-proteins

G-Protein Signal Transduction—Three Players – each discussed in more detail below

(1) G-Protein Coupled Receptors (GPCR)

ligand – can respond to hormones, neurotransmitters, local effectors

ligands take the form of proteins, peptides, amino acid derivatives, catecholamines, lipids, nucleotides, or nucleosides, chemicals of taste and smell, and light (photoisomerization of rhodopsin)
intermediate affinity—less than enzyme-linked receptors, more than ligand-gated channel receptors

structure – similar to bacteriorhodopsin

7-transmembrane (serpentine) with extracellular N-terminus (site of glycosylation) and cytosolic C-terminus
3 extracellular loops have cysteine residues which form disulfide bonds
phosphorylation of serine and threonine near C-terminus desensitizes the receptor

sequence of events

ligand binds either TM regions (small neurotransmitters) or extracellular loops (large hormones)
this causes rearrangement, exposing G-protein recognition sites (antagonists bind without rearranging)
the C-terminus and 3^rd intracell loop interact with the G-protein
affinity for ligand is reduced, and the receptor dissociates

termination of signal by phosphorylation of receptor to inactivate it

either by PKA (bound and unbound) or beta-andrenergic receptor kinase (BARK, only if bound to ligand)
homologous desensitization—receptor causes its own phosphorylation by turning on PKA
heterologous desensitization—receptor causes phosphorylation of other receptors by PKA (Fig. 6)

(2) G-Proteins

structure – heterotrimeric (alpha-beta-gamma); other GTPases are generally monomeric

alpha (hydrophilic)—largest, contains GTP binding site and GTPase activity and interacts with effectors
beta-gamma (hydrophobic)—highly conserved among G-proteins, essential for interaction with receptors

different conformation when bound to GTP and to GDP

GDP form interacts with "nucleotide exchange factor" (here, the receptor) which causes it to bind GTP
GTP form interacts with effectors (transducing the signal), then hydrolyze GTP to GDP

cascade amplification—each step multiplies the effect

(3) Effectors

some effectors recognize the G-protein alpha subunit, others recognize the beta-gamma dimer

examples of effectors:

(1) adenylyl cyclase – G_s activates, alpha G_i inhibits
(2) phospholipases – G_q or G_i activates phosphlipase C (producing IP3 and DAG)
(3) ion channels – G_S, G_i, or G_o activate
(4) cyclic nucleotide phosphodiesterases – G_t (transducin) activates cGMP PDE (visual transduction)

Cycle of Activity

alpha-GDP exchanged for alpha-GTP, then alpha released from beta-gamma

either beta-gamma or alpha-GTP can interact with effectors, until GTP is hydrolyzed

GDP release is very slow (0.1sec), but an order of magnitude higher when interacting with bound receptor

Bacterial Poisoning of G-Proteins

alpha subunit of G-proteins can be ADP-ribosylated by cholera or pertussis bacterial toxins

cholera causes irreversible activation of G_s (no GTPase so always active, cAMP activates PKA which turns on Cl^- channels)

pertussis affects G_i, preventing interaction with the receptor