Final answer:
GTP-binding proteins are activated when GSM replaces GDP upon receptor binding, and inactivated when GTP is hydrolyzed back to GDP, reassociating the subunits to restart the signaling cycle.
Step-by-step explanation:
Turning a GTP-binding protein, or G-protein, on and off is a crucial process in cell signaling using G-protein-linked receptors. When a signaling molecule, often a hormone or neurotransmitter, binds to a receptor on the cell surface, it causes a conformational change that allows the G-protein to bind to the inside surface of the plasma membrane. At this point, the GDP molecule associated with the α subunit of the G-protein is replaced by GTP, activating the G-protein. The β and γ subunits then dissociate from the α subunit, which can either itself trigger a cellular response or, in some cases, the dissociated βγ pair can also elicit a response. The signal is turned off once the GTP attached to the α subunit is hydrolyzed back to GDP, reassociating all subunits and returning the G-protein to its inactive state, ready for a new cycle of activation.