The addition of the nonhydrolyzable GTP-analog, GTPγS (shown below), is a common cell culture technique. If only affecting the inhibitory pathway, what effect would GTPγS have on cellular cAMP levels?

• A. cAMP levels increase because adenylate cyclase can no longer hydrolyze GTP.
• B. cAMP levels decrease because G_s remains in its inactive state and can no longer bind GDP.
• C. cAMP levels decrease because G_i remains in its active state since it can no longer hydrolyze GTP.
• D. cAMP levels increase because G_i remains in its inactive state since it can no longer hydrolyze GTP.

Answer: (C)

Verified step by step guidance

1. Understand the role of GTP in cellular signaling: GTP is a molecule that binds to G-proteins, which are involved in transmitting signals from outside the cell to the inside. When GTP is bound, the G-protein is active; when GDP is bound, it is inactive.
2. Identify the effect of GTPγS: GTPγS is a nonhydrolyzable analog of GTP, meaning it can bind to G-proteins but cannot be hydrolyzed to GDP. This means that G-proteins remain in their active state when bound to GTPγS.
3. Focus on the inhibitory pathway: In the context of the problem, we are interested in the inhibitory G-protein (Gi). Normally, Gi inhibits adenylate cyclase, reducing cAMP levels when active.
4. Analyze the effect of GTPγS on Gi: Since GTPγS cannot be hydrolyzed, Gi remains in its active state, continuously inhibiting adenylate cyclase. This leads to a decrease in cAMP levels because adenylate cyclase activity is reduced.
5. Conclude the effect on cAMP levels: With Gi remaining active due to the presence of GTPγS, cAMP levels decrease as adenylate cyclase is inhibited and cannot produce cAMP efficiently.