G-proteins are

• A. Mobile membrane proteins with three subunits that bind GDP to GTP
• B. Cytosolic enzymes that hydrolyze ATP
• C. Receptor tyrosine kinases
• D. Transcription factors in the nucleus

Answer: (A)

G-proteins act as molecular switches in GPCR signaling. They are membrane-associated proteins that come as a trio of subunits—alpha, beta, and gamma—anchored to the inner surface of the plasma membrane. In the inactive state, the alpha subunit binds GDP. When a receptor is activated, GDP is exchanged for GTP on the alpha subunit, prompting the alpha subunit to dissociate from the beta-gamma dimer and regulate downstream effectors such as adenylate cyclase or phospholipase C. The intrinsic GTPase activity of the alpha subunit hydrolyzes GTP back to GDP, allowing the subunits to reassociate and return to the inactive state. This cycling enables the external signal to be translated into cellular responses. Other options describe different kinds of proteins (cytosolic ATPases, receptor kinases, or transcription factors) and do not match the membrane-anchored heterotrimeric G-protein signaling mechanism.