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Sensory
rhodopsin II (rainbow colored) embedded in a
lipid bilayer (heads red and tails blue) with Transducin below it. G
tα is colored red, G
tβ blue, and G
tγ yellow. There is a bound
GDP molecule in the G
tα-subunit and a bound retinal (black) in the rhodopsin. The
N-terminus terminus of rhodopsin is red and the
C-terminus blue. Anchoring of transducin to the membrane has been drawn in black.
Transducin (also called Gt) is a heterotrimeric G protein that is naturally expressed in vertebrate retina rods and cones (a different Transducin gene is expressed in each cell type). 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 ion channels and a hyperpolarized membrane potential. This chain of signaling events is also called "the vertebrate phototransduction cascade".
The termination of Transducin activity occurs when (active) GTP bound Transducin is hydrolyzed to Transducin-GDP, a process which is accelerated by a complex containing an RGS (Regulator of G-protein signaling)-protein and the gamma-subunit of the effector, cyclic GMP Phosphodiesterase.
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