NBEO Ocular Physiology Practice Exam - Prep, Practice Test & Study Guide

In the phototransduction pathway, the key G protein that activates phosphodiesterase 6 (PDE6) is transducin. When light hits the photoreceptors in the retina, it causes a conformational change in rhodopsin, which is a light-sensitive receptor. This conformational change activates transducin, leading to the activation of PDE6.

PDE6 plays a critical role in the phototransduction cascade by hydrolyzing cGMP to GMP. This reduction in cGMP levels is essential for the closure of cGMP-gated ion channels in the photoreceptor cell membrane, resulting in hyperpolarization of the cell and ultimately leading to the transmission of visual signals to the brain.

Other options, such as rhodopsin, while crucial in the initial step of the phototransduction process, do not directly activate PDE6. Gustducin is more involved in taste transduction pathways, and cholecystokinin is a peptide hormone that plays roles in digestion and does not have any direct connection with the phototransduction pathway. Thus, transducin is accurately identified as the G protein responsible for activating PDE6 in this vital sensory process.