Signal Transduction Pathways Practice Test

In cultured liver cells, hormone H binds membrane receptor R and activates G protein Gα, which stimulates adenylyl cyclase to convert ATP to cAMP. cAMP binds protein kinase A (PKA), releasing active catalytic subunits that phosphorylate enzyme E, increasing its activity 6-fold within 30 s. A phosphodiesterase (PDE) hydrolyzes cAMP to AMP. When cells are treated with a PDE inhibitor, the same dose of H produces a larger and longer-lasting increase in E activity; however, without H the inhibitor causes little change. Which change would most likely eliminate the effect of the PDE inhibitor on E activity?

In an experiment, cells are exposed to ligand L that activates receptor R and increases cytosolic cAMP. A FRET sensor reports PKA activity. With L alone, PKA activity peaks at 30 s and returns to baseline by 3 min. With L plus a drug that blocks receptor internalization, PKA activity remains elevated for 10 min. Ligand binding affinity is unchanged. Which explanation is best supported by these data?