Transducin is a protein found in?

ANSWER: (B) RetinaREF: Ganong 22nd edition, chapter 8, http://en.mkipedia.org/uiki/TransducinRepeat from December 2010Repeat (***)"Transducin (also called Gt) is a heterotrimeric G protein that is naturally expressed in vertebrate 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 rhodopsins 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 vertebrate photo transduction cascade"