in the classic paradigm of G-protein signaling, GDP-bound Galpha remains associated with the non-dissociable betagamma subunits and, in combination with a G-protein coupled receptor (GPCR), represents the inactive state of the signaling pathway. Signal perception by GPCR facilitates the exchange of GTP for bound GDP on Galpha. The GTP-bound Galpha dissociates from the Gbetagamma dimer and both entities can interact with specific downstream effectors to transduce the signal. Galpha is restored to its GDP-bound conformation by its own intrinsic GTPase activity which leads to its reassociation with the Gbetagamma dimer and GPCR. The switch-like signaling mechanism has two distinct regulatory steps: the rate of GDP-GTP exchange facilitated by a cognate GPCR, which involves GDP release and GTP binding; and the rate of GTP hydrolysis by the Galpha protein
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GTP phosphohydrolase (signalling)
This group comprises GTP-hydrolysing systems, where GTP and GDP alternate in binding. This group includes stimulatory and inhibitory G-proteins such as Gs, Gi, Go and Golf, targetting adenylate cyclase and/or K+ and Ca2+ channels; Gq stimulating phospholipase C; transducin activating cGMP phosphodiesterase; gustducin activating cAMP phosphodiesterase. Golf is instrumental in odour perception, transducin in vision and gustducin in taste recognition. At least 16 different alpha subunits (39-52 kDa), 5 beta subunits (36 kDa) and 12 gamma subunits (6-9 kDa) are known.