MBG301 Class IV Classification of GPCRs according to their effector function (according to Lodish) 1. Adenylcyclase activation by GPCRs 2. Ion channel regulation by GPCRs 3. Phospholipase C (PLC) activation by GPCRs 4. Activation of gene transcription by GPCRs 1. Adenylcyclase activation by GPCRs 1
Similar but distinct G proteins can have opposing effects on the effector molecule. These GPs will in turn, bind different receptors which bind different ligands. α: 20 ;β 5; γ 6 Fig. 15-21 All GPCRs that activate Adenylate synthase share a common structure -orientation -7 trans-membrane α-helices -4 extra-cellular segments -4 cytosolic segments Fig. 15-10 no crystallography data! but: a) Hydropathy analysis determined the transmembrane regions b) Chou-Fasman analysis determined that these were alpha helices 2
Kobilka et al. Science 240: 1310 (1988) Fig. 15-12 Kobilka et al. Science 240: 1310 (1988) Inject cells with mrna coding for Adrenergic Receptors Fig. 15.7 Xenopus oocytes Xenopus oocytes 1mm 3
Kobilka et al. Science 240: 1310 (1988) : epinephrine, binds both receptors β2 AR α2 AR Kobilka et al. Science 240: 1310 (1988) 1. Which domains are critical for ligand binding? 2. Which domains are critical for G protein activation? STUDENT PRESENTATION 4
camp phosphodiesterase AMP adenylatekinase ATP GTP associated Gαs is able to associate with both of the catalytic domains of AC. (AC activator) Fig. 15.22 camp binding activates PKA by dissociating its catalytic domains, which are S/T kinases X-R-R/K-X-S/T-Φ Fig. 15.23 5
Hunger Epinephrine Hunger Glycogen Glucose AC GPCR Epinephrine Fig 15-25 Fig. 15-26 Fig. 15.6 6
Regulation of G-protein mediated signaling 1. GPCR affinity for its ligand decreases upon GDP replacement by GTP (on the G-prot). 2. GTP bound to Gs is quickly hydrolyzed (following dissociation of the G-prot. subunits 3. camp phosphodiesterase converts camp to AMP 4. Activated PKA will phophorylate the GPCR that led to its activation, decreasing its affinity for its ligand (feedback suppression) 5. Activated PKA will phophorylate GPCRs other than those which activated it (heterologous desensitization) 6. A kinase specific to the β adrenergic receptor (BARK) will phosphorylate and thus inactivate this receptor only when the receptor binds its ligand (homologous desensitization) 6 & 7. β-arrestin binding to the adrenergic receptors that have been extensively phosphorylated by BARK Arrestin: 1. Completely inhibits Gs binding to receptor; 2. Induces endocytosis of receptor/ligand complex 3. Functions as an adaptor protein Fig. 15-27 Fig. 14-32 7
8. A-kinase associated proteins (AKAPs) anchor PKA to a particular subcellular compartment (e.g. nuclear membrane, outer cytosolic membrane etc.) Fig. 15.23 8. A-kinase associated proteins (AKAPs) anchor PKA to a particular subcellular compartment (e.g. nuclear membrane, outer cytosolic membrane etc.) Fig. 15.28 AKAP co-localized PKA with camp phosphodiseterase (PDE) providing very tight control of PKA activity. The catalytic units (C) of PKA phosphorylate and stimulate PDE activity. Some catalytic units move into the nucleus directly activating certain transcription factors 8. A-kinase associated proteins (AKAPs) anchor PKA to a particular subcellular compartment (e.g. nuclear membrane, outer cytosolic membrane etc.) - experimental evidence: Dodge et al. EMBO J. 20:1921 (2001) fig. 4 = mouse anti-akap antibody coupled to Cyan fluorescent protein (immunofluorescence) 8
Kinase inhibitor development is a major field of interest Examples to ligands of G protein-coupled receptors: Small molecules: -Acetylcholine -Cannabioids -Dopamine -Serotonin -Adrenaline/epinephrine -Leukotriens -Prostaglandins Peptides: -Angiotensin -Bombesin -Bradykinin -C5a -Calcitonin -Cholecystokinin -Endothelin -f-metleuphe -Glucagon -Neuropeptide Y -Neurotensin -Opioids -Oxytocin -Vasopressin Glycoproteins: -Lutropin -Thyrotropin -Follicule stimulating hormone 1. How can we determine trans-membrane domains of proteins in the absence of crytal structure? 2. How can we determine the specific function of a domain of a protein (e.g. ligand binding, effector activating) 3. Why is the physiological response to a ligand not necessarily linearly related to ligand concentration? 4. What are some mechanisms by which signaling through the β-adrenergic receptor is regulated? 9
Required reading: Lodish: Chapter 13.3 (15.4) Kobika et al. Science 240: 1310 (1988) Suggested reading: Dodge et al. EMBO J. 20:1921 (2001) 10