transducin

Transducin is a retina-specific heterotrimeric G protein that transduces light signals from activated opsins to phosphodiesterase. It consists of a G alpha t (Gαt) subunit and a G beta-gamma (Gβγ) dimer. In mammals there are rod- and cone-specific Gαt isoforms: GNAT1 in rods and GNAT2 in cones. The beta and gamma subunits also have retina-specific isoforms, with rod transducin typically formed by Gβ1 and Gγ1 and cone transducin by cone-specific subunits Gβ3 and Gγt.

In the phototransduction cascade, light activation of rhodopsin (R) promotes GDP–GTP exchange on Gαt. Gαt–GTP then dissociates from Gβγ and activates the catalytic subunits of phosphodiesterase 6 (PDE6) by relieving the gamma-subunit–mediated inhibition. Activated PDE6 hydrolyzes cGMP to GMP, lowering intracellular cGMP concentration. The resulting closure of cGMP-gated channels causes the photoreceptor to hyperpolarize and reduces neurotransmitter release, signaling light detection to downstream bipolar and ganglion cells.

Termination of the signal occurs as Gαt hydrolyzes GTP to GDP; this GTPase activity is accelerated in the retina by regulators such as the RGS9–Gβ5–R9AP complex. The Gαt subunit then re-associates with Gβγ, returning the system to the dark state.

Transducin localizes to photoreceptor outer segments and is essential for both scotopic and photopic vision, with rod transducin supporting low-light signaling and cone transducin supporting daylight and color vision. Mutations in GNAT1, GNAT2, or associated regulatory proteins can cause inherited retinal diseases, including congenital stationary night blindness and cone-rod dystrophy.