Which statement about Ras signaling termination?

• A. Ras has intrinsic GTPase activity that hydrolyzes GTP to GDP, turning off the signal.
• B. Ras is degraded after GTP binds.
• C. The receptor ubiquitinates Ras to stop signaling.
• D. The stimulus continues indefinitely unless the ligand dissociates.

Answer: (A)

Ras signaling is shut off when the active GTP-bound Ras is converted to the inactive GDP-bound form. Ras has intrinsic GTPase activity, which slowly hydrolyzes GTP to GDP, turning off the signal as the active effector interactions cease. In cells, GTPase-activating proteins can accelerate this hydrolysis to ensure timely termination. Once GDP-bound, Ras cannot promote downstream pathways, so the signal ends even if the upstream receptor and ligand remain engaged. The other ideas don’t describe the normal way Ras signaling terminates: degradation after GTP binding isn’t how Ras is typically regulated, receptor-mediated ubiquitination of Ras isn’t the standard mechanism of turning off Ras signaling, and a ligand staying bound doesn’t guarantee indefinite signaling because the GTPase cycle inherently resets the signal.