What is the role of cAMP in cellular signaling?

• A. It activates protein kinase C (PKC).
• B. It activates protein kinase A (PKA), which phosphorylates target proteins.
• C. It directly opens ion channels.
• D. It inhibits neurotransmitter release.

Answer: (B)

cAMP acts as a second messenger that transduces extracellular signals into cellular responses by activating a specific kinase that phosphorylates many target proteins. When a receptor coupled to Gs is stimulated, adenylyl cyclase converts ATP into cAMP. The rise in cAMP binds to the regulatory subunits of protein kinase A (PKA), releasing and activating the catalytic subunits. These catalytic subunits then phosphorylate serine/threonine residues on a variety of substrates, changing their activity, interactions, or location. This phosphorylation can alter metabolism, ion channel function, and even gene expression (for example, via CREB).

This explains why the role of cAMP is to activate PKA and drive phosphorylation, rather than directly opening ion channels (that’s not a direct action of cAMP) or activating PKC (PKC is triggered by diacylglycerol and calcium). The effect on neurotransmitter release is context-dependent and not a universal rule.