In response to a ligand binding a GPCR, a particular G protein (G _s) activates adenylate cyclase. If a cell has a mutation where the G protein can no longer hydrolyze GTP, which of the following would be the result?

• A. Cellular levels of cAMP would go down in response to ligand binding.
• B. Cellular levels of cAMP would go up independent of ligand binding.
• C. The ligand would not be able to bind to the receptor.
• D. Cellular levels of cAMP would not change.
• E. Adenylate Cyclase would always remain inactive.

Answer: (B)

Verified step by step guidance

1. Understand the role of G proteins in signal transduction: G proteins are molecular switches that activate or deactivate signaling pathways in response to ligand binding to GPCRs (G protein-coupled receptors).
2. Recognize the function of G_s protein: G_s is a type of G protein that, when activated, stimulates adenylate cyclase, leading to the production of cyclic AMP (cAMP) from ATP.
3. Consider the impact of GTP hydrolysis: Normally, G proteins hydrolyze GTP to GDP, which turns off the signal. If a mutation prevents GTP hydrolysis, the G protein remains active longer than usual.
4. Analyze the consequences of a non-hydrolyzing G protein: Without GTP hydrolysis, G_s remains active, continuously stimulating adenylate cyclase, leading to increased levels of cAMP regardless of ligand binding.
5. Conclude the effect on cellular cAMP levels: Since adenylate cyclase is persistently activated, cellular levels of cAMP would rise independently of ligand binding, as the G protein cannot turn off the signal.