General structure of metabotropic receptor General structure of metabotropic receptor. 7 TM domains highly conserved, loops, tails sites of variability.

Small peptides bind inside the membrane.

Key feature is association with G proteins Key feature is association with G proteins. GDP bound inactive state, ligand binding induces exchange for GTP. Many types of heterotrimeric g proteins. Gs, Gi, Gq, G12—mainly distinguished on basis of the alpha subunit. Many different bg subunits too.

Regulation of the G protein alpha subunit activation state is complex Regulation of the G protein alpha subunit activation state is complex. Multiple factors act in both directions at both transitions.

GRK (G protein receptor kinase) is cytoplasmic—can’t phosphorylate membrane receptor. Upon activation, bg subunits bind GRK and recruit it to the membrane where it can phosphorylate the ligand occupied receptor. Bg is tethered, so it acts like a scaffold. Phosphorylation leads to desensitization of the receptor.

Gbg activation of MAPK pathway and inactivation by b-arrestin

Small G proteins, profound role in cancers, regulated much like heterotrimeric G proteins without the receptor. (GAPs, GEFs, GIPs etc). GEFs activate ras, GAPs inhibit ras by activating GTPase activity. Ras has very little endogenous GTPase activity, so GAPs are critical in turning off ras signaling. Ras can be directly activated by bg subunits, activate MAPK pathway Rab3 surrounds vesicles upon activation (GTP bound form) making them undockable. Depolarization leads to GAP activation, and GDP-Rab dissociates from the vesicle allowing docking and fusion.

Gai GPCR directly activates an ion channel. Gi typically. Bg subunits more important, but some ai can activate GIRK directly. a2-adrenergic, D2 dopamine, muscarinic, 5-HT1A, GABAB use this mechanism. These receptors link to bg subunits that can also bind and reduce opening of Ca2+ channels, especially L type.

Gas or i Adenylyl Cyclase. 2nd messenger system #1 generated cAMP, activated PKA, phosphorylates CREB (and others) induces gene transcription. AC stimulated by Gas, inhibited by Gai (may be indirect through Gi bg subunits inhibiting a Gas). Receptors that activate AC: D1, D5 dopamine, 5HT4, 7, b-adrenergic, and vasopressin. Receptors that inhibit AC: D2, D3, muscarinic M2, M4, opioid, adenosine A1, 5HT1A, 5HT1B, 5HT1D.

Many kinds of AC. Panel A: types 1,3,7 activated by both Ca/CaM and Gas synergistically, can be inhibited by bg. Found highest in hpc and ctx. Panel B: types 2,4,activated by Gas and bg, phophorylation by PKC but not PKA. Pretty wide expression Panel C: types 5 and 6 inhibited by Gai and free Ca2+, stimulated by Gas, inhibited by both PKA and PKC. Thus cells with same NT receptor but different AC isoform can have different signaling response to NT binding.

G proteins are targets of many toxins G proteins are targets of many toxins. Useful in determining what kind of G protein is involved in a process. Gs: cholera toxin: ADP ribosylates an arginine—inhibits GTPase activity (prolongs activation) and allows Gs to stimulate downstream targets (AC) Gi: pertussis toxin ADP ribosylates Gi, Go—inhibits GDP-GTP exchange (prevents activation) and doesn’t allow Gi to inhibit downstream signaling (AC, K+ channels etc)

Amplification of 2nd messenger cascade—slow but long lasting and powerful response.

Gaq or o Phospholipase C: #2 second messenger system. Often linked to Go proteins. Both a and bg subunits activate PLC to cleave membrane phospholipids. TRK receptors can also activate PLC.

Gaq or o Cleavage of PIP2 in the membrane releases IP3 (diffusible) and DAG (diacylglycerol). IP3 binds receptor on ER, releases Ca from stores, Ca may activate CaMK to phosphorylate targets. DAG remains in membrane and activates isoforms of PKC to phosphorylate targets. In both cases, the second messenger enables a conformational change (via Ca) in the kinase exposing or activating the catalytic domains. Receptors that activate PLC: muscarinic M1, M3, M5, 5HT1C, some peptides and mGluR.

Phospholipase A2 can be activated downstream of glutamate receptors Phospholipase A2 can be activated downstream of glutamate receptors..produce AA, PAF, etc. roles in inflammation, stroke, etc.

Interplay between fast and slow transmitter responses Interplay between fast and slow transmitter responses. Much opportunity to overlap. Trk receptors.

Trk receptors mainly bind growth factors, cytokines Trk receptors mainly bind growth factors, cytokines. Monomers form dimers upon binding, autophosphorylate each other, then other targets. Often promote survival or inhibit cell death. One main ras GEF, called sos, is also linked to trk receptors via adaptor proteins, highlighting another opportunity for interaction between pathways.

Convergence on Calcium—whole lecture coming up next week.