Transducina

Transducin, also called the transducin alpha subunit or G protein alpha t (Gαt), is a heterotrimeric G protein central to the vertebrate phototransduction cascade in retinal photoreceptors. In rods, the active visual pigment rhodopsin (Rhodopsin*) catalyzes the exchange of GDP for GTP on Gαt, causing the G protein to dissociate from the Gβ1γ1 dimer. The GTP-bound Gαt then stimulates the retina-specific phosphodiesterase PDE6, lowering intracellular cGMP. The fall in cGMP leads to closure of cGMP-gated ion channels, reduced inward current, and hyperpolarization of the photoreceptor, ultimately signaling light detection. In cones, a similar mechanism uses cone transducin.

Transducin is a heterotrimer composed of a Gαt subunit (rod version encoded by GNAT1; cone version encoded by GNAT2), and the Gβ1 and Gγ1 subunits (GNB1 and GNGT1). The Gαt subunits belong to the Gα family of small GTPases and cycle between GDP- and GTP-bound states, with intrinsic GTPase activity regulated by regulators of G protein signaling (RGS proteins).

Tissue distribution and evolution: Transducin is highly enriched in photoreceptor cells of the retina and is evolutionarily conserved among vertebrates. It interacts with rhodopsin or cone opsins to initiate the phototransduction cascade and is a key determinant of signal amplification and response kinetics in vision.

Clinical relevance: Variants in transducin alpha subunits GNAT1 or GNAT2 have been linked to inherited retinal disorders, including cone or cone-rod dystrophies and night vision impairment in some cases.