G-protein and G proteincoupled

Transcription

1 G-protein and G proteincoupled receptors (GPCR)

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3 1994 Nobel Prize in Physiology and Medicine for "their discovery of G-proteins and the role of these proteins in signal transduction in cells

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5 The birth of G-proteins 1950s/60s (Rall and Sutherland) Sutherland thought epinephrine receptor and AC might be the same molecule. 1960s/70s (Rodbell and colleagues) 1. Multiple receptors couple to AC in fat cells ( thus independent receptors linked to common effector AC via transducers (ca 1969) 2. Serendipitous addition of MgATP (required for AC assays) caused decrease in steady state levels of I 125 glucagon binding, apparently due to increased rate of dissociation of bound ligand. 4. Contaminant GTP was culprit, lowered affinity of the receptor for hormones. 5. There were many confusing reports because of contaminating aluminum fluoride. 1970s (Gilman and coworkers) 1. cyc- S49 lymphoma cells contained β-adrenoreceptors but no epinephrine stimulated AC activity. Ross and Gilman wanted to reconstitute coupling by adding membrane extracts containing AC to cyc- membranes. Surprisingly, even heat inactivated extracts (AC-dead) worked in the reconstitution! S49 had β-ar and AC but lacked intermediary factor (Gs heat stable!) 2. This reconstitution preparation allowed purification of Gs.

6 Role of aluminum fluoride in the presence of GDP

7 Classification of G Ptoteins Heterotrimeric G-protein subunits: α, β, γ (α binds GTP; βγ functional heterodimer) Non-heterotrimeric G proteins: small G proteins/ras-like GTPases, Elongation factor EFTu, Etc Effectors Adenylyl cyclases (AC) Phospholipases (PL) cgmp phosphodiesterase Kinases Ion channels (K channels, and voltage gated Ca channels)

8 Classification of heterotrimeric G Proteins based on activity G s stimulates adenylyl cyclase G i inhibits A.C., activates K+ channel (GIRK) G o ( o for other, abundant in brain), neither stimulates or inhibits AC, inhibits Ca channels G q stimulates PLCβ G t (transducin), stimulates cgmp phosphodiesterase G 12 RhoGEF

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10 1. α subunits G o, G t 2. β subunits :5 β subunits (highly similar sequence). 3. γ subunits: 12 γ subunits (more diverse) Gαβ heterotrimers do not form randomly. Some specificity in βγ dimerization (differing affinities)

11 GPCR 720 human genes GPCRs are also termed - Heptahelical receptors - Serpentine-like receptors - 7TM receptors - Magnificent Seven

12 Crystal structure of bacteriorhodopsin Gobind Khorana shared the 1968 Nobel Prize for Physiology and Medicine with Marshall Nirenberg and Robert Holley for identification of genetic code. Since mid 1970s Khorana studied the biochemistry of the membrane protein bacteriorhodopsin responsible for converting photon energy into proton gradient. 11-cis-and all-trans-retinal G-protein not involved

13 The importance of GPCRs 1. Number (C. elegans 1100; H. sapiens, ~800; D. melanogaster, 160; reflects number of olfactory receptor genes in worm [~1000] and mammal [several hundreds]), a few % of genome; non-olfactory GPCRs), 2. Diversity (mostly small molecule ligands) 3. Evolutionarily conserved yeast to man 4. Pharmaceutical importance: ~500 known molecular targets of drugs, 60% of these are cell surface receptors, 75% of these are GPCRs (GPCRs = ~45% of all known drug targets) ~100 orphan GPCRs are known in human. Reverse pharmacology to identify possible ligands (of great pharmaceutical interest) An orphan receptor is an apparent receptor that has a similar structure to other identified receptors but whose endogenous ligand has not yet been identified. If a ligand for an orphan receptor is later discovered, the receptor is referred to as an "adopted orphan

14 The G-protein cycle, the G-protein switch More on-off timer than switch inefficient hydrolysis of GTP is key timing from Alfred G. Gilman Nobel Lecture G proteins and regulation of adenylyl cyclase Nobel Lecture December 8, 1994

15 Classical examples of GPCR signalling. Activated receptor acts as binding partner And guanine nucleotide exchange factor (GEF) for heterotrimeric G protein. After agonist binding, a transient high-affinity complex of agonist, activated receptor and G protein is formed. GDP is released from the G protein and is replaced by GTP. This leads to dissociation of the G-protein complexes into α subunits and βγ dimers, which both activate several effectors. Gα s, for instance, activates adenylyl cyclase, which leads to an increase in cyclic AMP (camp). This increase in camp in turn activates protein kinase A (PKA), which is a serine/threonine kinase that phosphorylates many different substrates, including 7TM receptors, other kinases and transcription factors.

16 The switch function of the regulatory GTPases. The GTP form of the regulatory GTPases represents the switched on form of the GTPase, the GDP form, in contrast, the switched off form. The switch function of the regulatory GTPases may be controlled by GEFs, by GAPs and GDIs. Hydrolysis of the bound GTP ends the activated state. The rate of GTP hydrolysis is either intrinsically determined or may be accelerated via GAPs. GAP (GTPase-activating proteins) negatively regulate G-protein signaling by enhancing GTPase up to 10 5 fold. GDI (guanine nucleotide dissociation inhibitor) inhibit GDP or GTP dissociation from complex with G α

17 Regulation of the G-protein cycle Three types of regulatory protein control the G-protein cycle: 1. GEFs accelerate the release of bound GDP from Gα 2. GDIs inhibit release of bound GDP (ligand-bound GPCR is most important GEF for heteromeric G-proteins; b, g function like GDIs) 3. GAPs accelerate the rate of intrinsic GTPase reaction (downstream effectors often have GAP activity, e.g. PLC-β, retinal cgmp phosphodiesterase)

18 The G Protein Cycle

19 Heterotrimeric G Proteins consist of α, β, γ subunits Reception of a signal by the receptor activates the G-protein, which leads to exchange of bound GDP for GTP at the α subunit and to dissociation of the βγ complex. Further transmission of the signal take place via G α GTP or via the βγ complex, which interact with corresponding effector molecules. The α and γ subunits are associated with the cell membrane via lipid anchors. Signal reception and signal transmission of the heterotrimeric G-proteins take place in close association with the cell membrane.

20 Desensitization of GPCRs: Termination of receptor signaling in the face of persistent ligand ( desensitization is an umbrella term encompassing several distinct mechanisms that overlap in time course) 1.Heterologous desensitization Fig.5.10 One of PKA substrates is the receptor. The ligand-bound receptor is preferentially phosphorylated. As a consequence of phosphorylation, activation of further G-proteins by the receptor is suppressed.

21 Desensitization of GPCR: 2. Homologous desensitization Receptor desensitization: phosphorylation, arrestin binding and internalization. The activated, agonist-bound receptor is phosphorylated on the cytoplasmic region by a GRK (G-Protein coupledreceptor kinase). The phosphate residues serve as attachment sites of β-arrestin, which serves as a trigger for internalization of the receptor to endosomes. The receptor may now be dephosphorylated and transported back to the cell membrane.

22 Proteins that associate with the C-terminal of GPCRs: -Receptor-activity modifying proteins (RAMPs). These are TM proteins that modulate diverse functions of GPCRs belonging to class B. -Scaffolding proteins. Examples are members of the adaptor protein family A-kinase anchor proteins (AKAP), the scaffolding proteins Shank and spinophilin. - PDZ-containing proteins. Many GPCRs contain a PDZ-binding motif in the C-terminus. The PDZ serves as scaffolding or interaction module for the formation of higher protein complexes.

23 Toxins that affect G-proteins: 1. Constitutive activation of G s -proteins by cholera toxin is the cause of the devastating effect of the cholera bacterium, Vibrio cholerae, on the water content of the intestine. Because of the lack of deactivation of the G s -proteins, adentylyl cyclase next in the reaction sequence is constantly activated, so that the level of camp in the cells of the intestinal epithelium is greatly increased. This, in turn, leads to increased active transport of ions, and an excessive efflux of water and Na + takes place in the intestine. Cholera toxin is an enzyme that ADP ribosylates Gsα using cellular NAD+, locks Gs in active state (unable to hydrolyse GTP) and causing elevated camp in intestinal epithelium. 2. Pertussis toxin of the bacterium Bordetella pertussis causes increased camp by ADP-ribosylating Giα, inactivating Giα and preventing its inhibition of AC ADP-ribosylation of the Gα subunit

24 Membrane anchor of the heterotrimeric G-proteins. The lipid anchoring in the system of G-protein-coupled receptors and the corresponding G-proteins is shown. It is assumed that the lipid anchors are located in the membrane. The GPCR carries a palmitoic acid anchor at the C terminus. The α subunit of the heterotrimeric G-protein is associated with the membrane via a myristoic acid anchor at the N terminus, whilst the γ subunit of the βγ complex uses a prenyl residue as a membrane anchor.

25 (Nonhydrolyzable analogues)

26 2012 Nobel Prize in Chemistry "for studies of G-protein coupled receptors" Robert J. Lefkowitz Brian K. Kobilka

27 Adrenergic Receptor Ligands Epinephrine (adrenalin), Preferentially binds to Alpha2 adrenergic receptor Isoprenaline Preferentially binds to Beta 1 and 2 adrenergic agonist Norepinephrine Preferentially binds to Alpha 1 and Beta3 adrenergic receptor

28 Discovery of Gβγ subunit function A Late Bloomer 64 MD Columbia University 76 Appointed by Thomas Smith to the sole basic research faculty at the cardiovascular Division of Brigham and Women s Hospital, Harvard Medical School

29 G proteins (GTPases)

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31 The Ras superfamily of monomeric GTPases - Ras proteins - Rho/ Rac proteins - Rab proteins - Arf proteins - Ran proteins

32 Ras (189 aa) 1.Monomeric GTP-binding proteins discovered as oncogene products. 2. H-ras: Harvey murine virus, farnesylation K-ras: Kirsten sarcoma virus, farnesylation N-ras: discovered from neuroblastoma, palmitoylation 3. Lovastatin or simvastain (inhibitor of HMG CoA reductase) as drug for heart disese as well as cancer. 3. Yeast RAS activates adenyly cyclase, mammalian ras can replace yeast RAS function. 4. Cellular ras-gtp hydrolysis is very slw (t 1/2 several hours) ---led to the finding of GAP 5. Constitutively active ras(g12v); dominant negative ras (S17N)

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36 Loaded Spring mechanism of regulatory GTPases Schematic diagram of the universal switch mechanism involving switch Ⅰ and switch Ⅱ domains of regulatory GTPases. Invariant Thr35 and Gly60 residues (numbering for Ras) are fixed in a strained conformation by hydrogenbond interactions with the γ- phosphate after GTP. Release of the γ-phosphate after GTP hydrolysis allows the switch Ⅰ and switch Ⅱ regions to relax into the GDP-specific conformation. The extent of the conformational change is different for different proteins and may involve extra elements for some proteins.

37 Domain structure of p120 GAp. The functional domains of p120 GAp are shown together with their interaction targets. PIP 3 = phosphatidylinositol-3,4,5- trisphosphate.

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41 Fig.9.7

42 Site of modification: Cys in a Caax box Site of modification: N-terminus or Lys

43 Cholesterol synthesis and Statins (Lovastatin or Atorvastatin) Merck Atorvastatin (lipitor) Pfizer

44 Cholesterol synthesis

45 Dr. Vagelos is the former Chairman and CEO of Merck & Co, Inc. and then the Chairman of two biotech companies; Regeneron Pharmaceuticals and Theravance. Dr. Vagelos was the lead scientist in Merck s development of the statin drugs Lovastatin and Zocor, cardiovascular protective agents that serve to decrease blood cholesterol levels by inhibiting HMG-CoA reductase. He was also the key advocate in Merck s decision to make Ivermectin freely available to the people of Africa and Central America for the treatment of river blindness what was a widespread, chronic and debilitating disease disseminated by black flies. P. Roy Vagelos, MD

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