In the dark, the α subunit of transducin is bound to GDP and is inactive. When a photon of light activates the photoreceptor cell, the visual pigment rhodopsin undergoes a conformational change. Activated rhodopsin then interacts with transducin, causing the exchange of GDP for GTP on the α subunit. This activation leads to the dissociation of the α subunit from the βγ complex. The activated α subunit then goes on to activate phosphodiesterase (PDE), which hydrolyzes cGMP to GMP, leading to the closure of cGMP-gated ion channels and hyperpolarization of the photoreceptor cell.
