As in all G protein–coupled signalling pathways, timely termination of the rhodopsin signalling pathway requires that all the activated intermediates be inactivated rapidly, restoring the system to its basal state, ready for signalling again. Which of the following mechanism does not terminate rhodopsin signalling?

The complex composed of the inhibitory γ subunit of PDE and Gαt∙ GTP recruits two additional proteins, RGS9 and Gβ5, that together act as a GAP to enhance the rate of hydrolysis of bound GTP to GDP. Hydrolysis of GTP, in turn, causes release of the PDE γ subunit, which re-joins the PDE α and β subunits, terminating PDE activation.

Light-triggered closing of the cGMP-gated cation channels causes a drop in the cytosolic Ca2+ concentration inside the rod cell. The drop in the intracellular Ca2+ concentration is sensed by a class of Ca2+-binding proteins known as guanylate cyclase–activating proteins, which bind to guanylate cyclase and stimulate its activity, thereby elevating the level of cGMP and causing the cGMP-gated ion channels to reopen.

A major process that suppresses and helps to terminate the visual response involves phosphorylation of rhodopsin when it is in its activated (R*). Rhodopsin kinase, is the enzyme that catalyzes this phosphorylation reaction. Arrestin bound to the phosphorylated R* completely prevents interaction with Gαt, blocking formation of the active Gαt∙ GTP complex and stopping further activation of PDE.

Binding of acetylcholine triggers activation of the Gαi subunit and its dissociation from the Gβγ subunit in the usual way. In this case, however, the released Gβγ subunit (rather than Gαi∙ GTP) binds to and opens the associated effector protein, a K+ channel. The increase in K+ permeability hyperpolarizes the membrane, which reduces the frequency of PDE activation.