Lecture 9: G-protein coupled receptors

GPCR Amplification

receptor activates many GP's; adenylyl cyclase makes many cAMP's; PK's phosphorylate many target proteins

No Sweet Taste

in cats, dolphins, etc; from no TAS1R2 (Gq GCPR)

G beta gamma Subunit

1. Muscarinic receptors=activates K channels; 2. NE a2-adrenergic receptors= inhibits Ca channel from opening to reduce transmitter release

Structure of G-protein coupled receptor

7 transmembrane spanning regions=serpentine receptors: 450 functional in humans 1. Receptor-G protein-Effector

G-q(gustatory) -alpha signaling (sweet receptor)

Activated GPCR-activated Gq protein-activates phospholipase C-B- changes PI 4, 5-bisphosphate to IP3 and DAG- IP3 binds to receptor on ER and causes release of Ca2+, DAG activates PKC

G-olfactory-alpha signaling

Activated GPCR-activates Adenylyl cyclase-activated subunit of Golf- changes ATP to cyclic AMP-CAMp can modify growth cones, bind to a CNG channel that allows the flow of Na+ and Ca+ inside the cell so that Ca can activate the Cl- out the cell to depolarization, or/and can activate Kinases to affect gene transcription.

G-inhibitory-alpha and G-stimulatory-alpha signaling

Activated GPCR-activates Adenylyl cyclase-activated subunit of Gs or Gi- changes ATP to cyclic AMP-which activates PKA by binding to the regulatory subunits and activating the catalytic subunits-PKA activated phosphorylates CREB-gene transcription. Convergence between Gi and Gs possible: acting on the same Adenylyl cyclase with opposite effects

GRK

GPCR kinase; phosphorylates GPCR

GTP analogs

GTPgS & Gpp(NH)p: cause permanent activation of GP b/c they're non-hydrolyzable

The Life cycle of GPCR

Off position: Alpha, beta and gamma with GDP Activated: Beta and gamma separates from alpha, GDP becomes GTP Aftermath: Hydrolyzes GTP to GDP, beta and gamma binds with alpha.

Rho family: Rho A

Rho A activation: activates Rho kinase, actin retraction, inhibits growth cones and axon outgrowth. inhibition of it causes dendritic elongation

Classes of GPCR-alpha

Six classes: Gi, Gs, Golf, Gq, Gt, and G12/13

GPCR Desensitization

activate GRK, which phosphorylates receptor; phosphorylation promotes GP uncoupling & attracts arrestin, which binds to the receptor; arrestin attracts AP2, dynamin, clathrin; endocytose receptor; in tolerance to opioids

Rho family: Cdc42

activated by adenylyl cyclase, CamKII, IGF-1 Cdc42: filopodia elongation

Rho family: Rac

activates Pak Rac: lamellipodia extension---RhoA antagonist

AP2

adaptor protein 2; binds to arrestin

GTPase Activating Proteins (GAPs)

bind to GP & activate its intrinsic GTPase activity, decreasing its functional activation

GDPbS

binds to GP & doesn't exchange, inhibiting GP activity

Dynamin

binds to GTP; in endocytosis of GPCR (desensitization), pinches off clathrin-coated vesicles

Miracle Fruit

binds to T1R2: agonist for acidic ph, antagonist for neutral ph=makes everything sweet

Clathrin

binds to arrestin; in endocytosis of GPCR (desensitization)

Cholera Toxin

catalyzes ADP-ribosylation of arginine residue in Gas/Gat, inhibiting GTPase activity of a & keeping em free & active; modifies GTP so it's bound to Gas so it can't be hydrolyzed to GDP; causes persistent activation of adenylyl cyclase

Nicotinamide Adenine Dinucleotide (NAD)

donates ADP-ribose group used by cholera/pertussis - add it to specific amino acid residues in a subunit

b-Arrestin

inhibits GPCR after it's been phosphorylated

Non-Canonical Use of Arrestin

internalization & trafficking for intracellular signaling cascade; internalize GPCR & use it somewhere inside cell

Rhodopsin

light-activated GPCR found in rods; optin + 11-cis-retinal; 7-TM Isomerization: rearrangement of rhodopsin's 11-cis-retinal to all-trans-retinal; due to absorbing photon; causes conformational change in opsin, activating Gt

Pertussis Toxin

modifies Gi alpha subunit and blacks activation of these subunits to prevent inhibition. Results in Whooping cough: persistant coughing, wheezing

G-t(vision)-alpha signaling

rhodopsin absorbs light --> activate GPCR --> Gt activates cGMP PDEs --> cGMP breakdown --> fewer cation (Na, Ca) channels open --> membrane hyperpolarization--> less mitter release from cell; light perceived as decrease in rod impulse

G12/13a

small GTPases, monomeric

Guanine Nucleotide Exchange Factors (GEFs)

stimulate release of GDP from inactive GP; exchanges GDP for GTP, activating GP

Guanosine Nucleotide Dissociation Inhibitors (GDIs)

stop GP from re-entering cycle again (from getting reactivated) - prevent dissociation from GDP

Chimeric GPCR Studies

take extracell domain of one type of GPCR & combine it w/ intracell domain of other GPCR; use to identify ligand of orphan receptor w/ unknown signaling pathway or to optimize screening of new classes of compounds for well-known therapeutic targets