Which one of the following is the first step in the sequence of actions in the G-protein receptor activity cycle?
A. ? chain activates a target protein.
B. Neurotransmitter binds with receptor.
C. Membrane channels open, or intracellular target proteins are activated.
D. Receptor protein changes shape.
ANS: B
Rationale: The G-protein receptor activation cycle consists of the following steps: (1) neurotransmitter binds to the G-protein; (2) receptor protein changes shape; (3) G-protein subunits break free as cytoplasmic shuttling units; (4) subunits bind to the membrane ion channel; (5) ion channel changes shape; and 6) subunits deactivate and reassociate with the G-protein receptor.
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