Action of the alpha subunit of G-protein is:

The α-subunit has GTPase activity: the bound GTP is slowly hydrolysed to GDP: the α-subunit then dissociates from the effector to rejoin its other subunits, but not before the effector has been activated / inhibited for several seconds (much longer than the life-time of the activated receptor, which is in milliseconds) and the signal has been greatly amplified. The rate of GTP hydrolysis by the α subunit and thus the period for which it remains activated is regulated by another protein called ‘regulator of G protein signaling’ (RGS). The onset time of response through GPCRs is in seconds. KEY CONCEPT:- Alpha subunit of G protein contains GTPase which breaks GTP into GDP. Reference: Essentials of Medical Pharmacology Eighth Edition KD TRIPATHI page no 55