Transducin

It is a type of heterotrimeric G-protein with different α subunits in rod and cone photoreceptors.

Transducin activates phosphodiesterase, which results in the breakdown of cyclic guanosine monophosphate (cGMP).

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.

The Tα subunit of transducin contains three functional domains: one for rhodopsin/Tβγ interaction, one for GTP binding, and the last for activation of cGMP phosphodiesterase.

However, the isoforms exhibit functional interchangeability in the phototransduction cascade and shouldn't solely account for differences in light sensitivity.

[8] Interaction with photolyzed rhodopsin opens up the GTP-binding site to allow for rapid exchange of GDP for GTP.

A conformational change of the Tα by photolyzed rhodopsin causes the tilting of the helix, opening the GTP-binding site.

The GTP-induced conformational changes could also disrupt the rhodopsin/Tβγ binding site and lead to dissociation from the GTP-Tα complex.

[9] The excess Tβ and Tγ have been concluded to be floating freely around in the ROS, though it cannot be associated with the Tα at any given time.

Rhodopsin was found to specifically cause a conformational switch in the carboxyl terminal of the Tγ subunit.

Tα can accelerate the rate of activation of light-off induced Protein Kinase A due to binding to rhodopsin.

[7] Transducin activation ultimately results in stimulation of the biological effector molecule cGMP phosphodiesterase, an oligomer with α, β and two inhibitory γ subunits.

[16] The addition of Tβγ facilitates inhibition of the PDE catalytic moiety because it binds with the Tα-GTP complex.