Transducin

Transducin (G_t) is a protein naturally expressed in vertebrate retina rods and cones and it is very important in vertebrate phototransduction. It is a type of heterotrimeric G-protein with different α subunits in rod and cone photoreceptors.

Light leads to conformational changes in rhodopsin, which in turn leads to the activation of transducin. Transducin activates phosphodiesterase, which results in the breakdown of cGMP. The intensity of the flash response is directly proportional to the number of transducin activated.

Transducin is activated by metarhodopsin II, a conformational change in rhodopsin caused by the absorption of a photon by the rhodopsin moiety retinal. The light causes isomerization of retinal from 11-cis to all-trans. Isomerization causes a change in the opsin to become metarhodopsin II. When metarhodopsin activates transducin, the guanosine diphosphate (GDP) bound to the α subunit (T_α) is exchanged for guanosine triphosphate (GTP) from the cytoplasm. The α subunit dissociates from the βγ subunits (T_βγ.) Activated transducin α-subunit activates cGMP phosphodiesterase cGMP phosphodiesterase breaks down cGMP, an intracellular second messenger which opens cGMP-gated cation channels. Phosphodiesterase hydrolyzes cGMP to 5’-GMP. Decrease in cGMP concentration leads to decreased opening of cation channels and subsequently, hyperpolarization of the membrane potential.

Transducin is deactivated when the α-subunit-bound GTP is hydrolyzed to GDP. This process 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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