Transducin is a protein found in:

Retina REF: Ganong 22nd edition, chapter 8, http://en.wikipedia.org/wiki/Transducin Transducin (also called Gt) is a heterotrimeric G protein that is naturally expressed in veebrate retina rods and cones. Mechanism of action: Heterotrimeric Transducin (alpha-beta-gamma subunits) is activated by a conformational change in rhodopsin due to the absorption of a photon by rhodopsin's active group retinal. Activation causes the GDP bound to the alpha subunit to be exchanged with GTP from solution and results in activated alpha dissociating from beta-gamma. Active Transducin-alpha then causes cyclic GMP Phosphodiesterase to increase its activity, thereby lowering the concentration of cGMP, an intracellular second-messenger molecule. Decrease in cGMP concentration leads to the closure of cGMP-regulated Na+ and Ca2+ ion channels and a hyperpolarized membrane potential. This chain of signaling events is also called "the veebrate photo transduction cascade"