Final answer:
G proteins cannot get near the intracellular domain of the receptor. The G-protein α subunit exchanges GDP for GTP upon receptor binding, leading to signal transduction. GTP hydrolysis deactivates the G protein, resetting the signaling cycle.
Step-by-step explanation:
G proteins cannot get near the intracellular domain of the receptor. In the signaling process, the receptor undergoes a shape change upon binding its effector signal molecule, allowing it to recognize and bind to the G-protein trimer on the cytoplasmic surface of the plasma membrane. Following this binding, GTP displaces GDP on the α subunit of the G-protein. This exchange is crucial for G-protein activation and subsequent signal transduction inside the cell.
The G protein, composed of α, β, and γ subunits, becomes activated when the α subunit exchanges GDP for GTP. The β and γ subunits dissociate from the α subunit, which can then trigger a cellular response by activating other proteins or enzymes like adenylate cyclase. Hydrolysis of GTP to GDP terminates the signaling event, allowing the G protein to revert to its inactive form.