Which is NOT typically a step in the cAMP second-messenger system?

Question

Quizplus · Accepted Answer

The correct sequence of steps in the cAMP second-messenger system is: 
1. A first-messenger (e.g. hormone) binds to a transmembrane receptor
2. The receptor activates a G-protein
3. The G-protein subunits dissociate and one of them activates adenyl cyclase
4. Adenyl cyclase converts ATP into cyclic AMP (cAMP)
5. cAMP activates protein kinase A (PKA)
6. Active PKA phosphorylates target proteins in the cell
Thus, step C is not typically a step in this process. Instead, an activated G-protein subunit activates adenyl cyclase, which ultimately leads to phosphorylation of PKA by cAMP.

Which Is NOT Typically a Step in the CAMP Second-Messenger