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