Hormone Transport, Receptors, Signal Transduction (3A) Practice Test

A researcher compares signaling from two receptors in the same immune cell. Receptor M is activated by a membrane-impermeant cytokine and leads to phosphorylation of STAT proteins that dimerize and enter the nucleus. Receptor N is activated by a steroid hormone that diffuses across the membrane and binds an intracellular receptor. In an experiment, a selective blocker prevents ligand binding to receptor M only.

Which outcome would be expected if receptor M is blocked?

In a cell line expressing a GPCR for hormone M, investigators measure second messengers after acute M exposure. M increases cAMP but does not change IP$_3$. A point mutation is introduced into the receptor’s cytosolic region that prevents coupling to heterotrimeric G proteins while leaving ligand binding intact. After mutation, M still binds the receptor at the cell surface, but cAMP no longer increases.

Which outcome would be expected if the receptor is blocked (or uncoupled) as described?