GAP (GTPase-activating proteins)
GTPase-activating proteins (GAP) otherwise known as GTPase-accelerating proteins are a family of regulatory proteins which bind to activated G-proteins in order to stimulate GTPase activity . GAPs also known as RGS proteins are essential in regulating G protein activity, which is important as they control many cellular processes such as transduction of signalling from the G protein-coupled receptor. GAPs function in these processes is to turn off the activity of G proteins, making it produce an opposite effect to guanine nucleotide exchange factors (GEFs)
GAP proteins are closely linked to the activity of G-protein coupled receptors. The activity of these G-proteins is a result of them binding guanosine triphosphate (GTP). The G-proteins can hydrolyse bound GTP to GDP and release it, by breaking a phosphate bond, however not very effectively so it is a slow process. This results in a reduction of activity for G-protein coupled receptors and their corresponding processes as it takes longer for them to return to their initial state. GAP proteins therefore make the bound GTP a better target for nucleophilic attack by water, by altering the charge distribution and lowering the transition state , making it a more reactive substrate and thus speeding up hydrolysis. By doing this it allows G-protein coupled receptors to be activated more frequently within the cell.
GAPs that act on small GTP-binding proteins of the Ras family, have conserved structures and use similar mechanisms.