Final answer:
The G protein alpha subunit deactivates itself by hydrolyzing the bound GTP to GDP, terminating the signal and reassociating with the beta and gamma subunits to reset the G protein for a new signaling event.
Step-by-step explanation:
The G protein alpha subunit switches itself off by hydrolyzing its bound GTP to GDP in a process that is integral to cell signaling. This occurs in a cyclic series of events involving G-protein-coupled receptors. Initially, a signaling molecule binds to the receptor, causing a GDP molecule associated with the alpha subunit to be exchanged for GTP. As a result of the GTP binding, the beta and gamma subunits dissociate from the alpha subunit, initiating a cellular response.
Following activation, the G protein alpha subunit possesses intrinsic GTPase activity, which allows it to hydrolyze the bound GTP to GDP. This action of hydrolysis is what turns off the signal, as the hydrolysis of GTP to GDP leads to a conformational change in the alpha subunit that reduces its affinity for the effector proteins, ultimately resulting in the termination of the signal. Subsequently, the inactive alpha subunit reassociates with the beta and gamma subunits, resetting the G protein for another cycle once a new signaling event occurs.