G alpha (12/13) auto-inactivates by hydrolysing GTP to GDP

When a ligand activates a G protein-coupled receptor, it induces a conformational change in the receptor (a change in shape) that allows the receptor to function as a guanine nucleotide exchange factor (GEF), stimulating the exchange of GDP for GTP on the G alpha subunit. In the traditional view of heterotrimeric protein activation, this exchange triggers the dissociation of the now active G alpha subunit from the beta:gamma dimer, initiating downstream signalling events. The G alpha subunit has intrinsic GTPase activity and will eventually hydrolyze the attached GTP to GDP, allowing reassociation with G beta:gamma. Additional GTPase-activating proteins (GAPs) stimulate the GTPase activity of G alpha, leading to more rapid termination of the transduced signal. In some cases the downstream effector may have GAP activity, helping to deactivate the pathway. This is the case for phospholipase C beta, which possesses GAP activity within its C-terminal region (Kleuss et al. 1994).

当配体激活G蛋白偶联受体时，会引起受体构象的变化（形态的改变），使受体能够充当鸟苷酸交换因子（GEF），从而刺激Gα亚基上的GDP与GTP的交换。在异源三聚体蛋白激活的传统观点中，这种交换触发了活性化的Gα亚基与β:γ二聚体的解离，进而启动下游的信号转导事件。Gα亚基具有内在的GTP酶活性，最终会水解结合的GTP为GDP，允许其与Gβ:γ重新结合。额外的GTP酶激活蛋白（GAPs）刺激Gα的GTP酶活性，导致信号转导的更快终止。在某些情况下，下游效应器可能具有GAP活性，有助于途径的失活。例如，磷脂酶Cβ在其C端区域具有GAP活性（Kleuss等，1994年）。