Which statement best describes G_i protein signaling?

Which statement best describes G_i protein signaling?

• A. It increases cAMP by activating adenylate cyclase.
• B. It decreases cAMP by inhibiting adenylate cyclase.
• C. It has no effect on second messenger systems.
• D. It directly opens potassium channels.

Answer: (B)

Gi proteins convey inhibitory signals by stopping the production of cAMP. When a Gi-coupled receptor is activated, the alpha subunit inhibits adenylyl cyclase, so less ATP is converted to cAMP. With lower cAMP, protein kinase A activity drops, leading to changes in phosphorylation and downstream cellular responses. This decrease in cAMP is the defining effect of Gi signaling. There can be additional actions through the Gβγ subunits, such as modulation of ion channels that can influence excitability, but the key, most characteristic description is the inhibition of adenylyl cyclase and the resulting drop in cAMP.

Gi proteins convey inhibitory signals by stopping the production of cAMP. When a Gi-coupled receptor is activated, the alpha subunit inhibits adenylyl cyclase, so less ATP is converted to cAMP. With lower cAMP, protein kinase A activity drops, leading to changes in phosphorylation and downstream cellular responses. This decrease in cAMP is the defining effect of Gi signaling.

There can be additional actions through the Gβγ subunits, such as modulation of ion channels that can influence excitability, but the key, most characteristic description is the inhibition of adenylyl cyclase and the resulting drop in cAMP.