Transducin (also called Gt)'s a heterotrimeric G protein that's naturally expressed in vertebrate retina rods and cones (a different Transducin gene's expressed in each cell type). Heterotrimeric Transducin (alpha-beta-gamma subunits)'s 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 b… (More on Transducin)