Cell signalling and intracellular messengers G-protein coupled receptors (GPCR) · Largest class of human cell surface receptors · Involved in smell taste, vision, hormone detection . Bind to huge array of molecules · Characterised by 7 transmembrane domains EXTRACELLULAR SPACE GDP CYTOSOL plasma membrane > GTP-bound Ga separates from the GPCR and Gbg. > Ga goes on to activate signalling cascades. An active GPCR can activate many G proteins G protein · G protein acts as molecular switches . In the active state they are bound to GDP · Activation of the G protein results in release of GDP . This GDP is then replaced with GTP producing an active G protein B G- proteins as molecular switches Downstream: G- proteins sit in the membrane and consists of 3 subunits o Alpha, beta and gamma Alpha subunit binds to GDP o Alpha and gamma have covalently attached lipid tails so they can stay in the membrane securely The G protein Activation Cycle · GPCR sits on the membrane o When not bound to a ligand it's in its inactive form · When ligand binds to receptor, it changes its property and the receptor becomes activated . It can now recruit G-protein · GDP on the alpha subunit is exchanged for GTP, activating the G-protein · Alpha dissociated from beta and gamma subunits · These separate subunits can now go on and activate the next member of the signalling cascade Another way of explaining it: > Ligand binds to the extracellular side. > Changes the conformation of the receptor. > This recruits the G protein. > At this point the G protein is inactive (GDP-bound). > Binding to the receptor induces GDP release; GDP is replaced by GTP. Inactive GTP360 GOP DE-ACTIVATOR GAP GEF ACTIVATOR GTPase Regulator of G protein signaling (RGS) = GAP Activated GPCRs = GEFs GTE Active Cellular function G-proteins can activate Adenylyl-cyclase · Adenylyl-cyclase is a transmembrane protein · Adenylyl-cyclase can form cyclic AMP (CAMP) from ATP · CAMP is potent SECONDARY MESSENGER second messengers Definition: A diverse family of low molecular weight compounds such as cyclic AMP and calcium ions, which transmit the biological signals initiated by receptor-ligand binding at the cell surface to intracellular targets such as gene expression. (from: Unified Medical Language System) · second messengers- alter metabolism or affect effector enzymes that modulate target proteins and eventually cell behaviour. Include: o CAMP o Ca 2+
o IP3 o DAG G protein families · 4 families based in sequence homology o Gs- stimulates adenylyl cyclase o Gi/o-inhibits adenylyl cyclase O Ga/11-stimulates PLCß o G 12/13-activares RhoGEFs Stimulatory and inhibitory G-proteins ?????? Cellular effects of CAMP · Comes as a tetramer with 2 regulatory subunits and 2 catalytic subunits · Activation of CAMP-dependent on protein kinase A (PKA) · Protein kinase A can phosphorylatxe multiple targets · Protein kinase A can modulate transcription (gene expression) by phosphorylating transcription factors. Gq can activate phospholipase C (PLCß) DDDD 4 × CAMP R R R R C c