Final answer:
Sar-1 transitions from its GDP-bound state to GTP-bound state upon activation by a G-protein-coupled receptor, which triggers a change in G-protein shape and causes GDP to GDP exchange on the α subunit. This change activates the protein, leading to downstream cellular responses until intrinsic GTPase activity resets the protein to its inactive state.
Step-by-step explanation:
Sar-1 is a monomeric small GTPase protein, which is involved in vesicle trafficking, particularly during the coat protein II (COPII) mediated transport from the endoplasmic reticulum to the Golgi apparatus. The change from its guanosine diphosphate (GDP) bound state to the guanosine triphosphate (GTP) bound state in Sar-1 is a critical step in vesicle formation. When a signaling molecule binds to a G-protein-coupled receptor, which is the activation event, the G protein α subunit exchanges GDP for GTP. The receptor undergoes a conformational change upon ligand binding that facilitates this exchange on the α subunit. After the exchange, the α subunit with the GTP bound separates from the βγ subunit complex. This change activates the G protein, with the active GTP-bound α subunit then being able to interact with other target proteins in the cell, such as adenylyl cyclase in the case of signal transduction or other effector proteins involved in vesicular transport. Eventually, intrinsic GTPase activity of the α subunit hydrolyzes the GTP back to GDP, which leads to the reassociation of the α subunit with the βγ complex, thereby resetting the cycle.