Figure 3.

Phototransduction. (Top) In the absence of light, inactive, 11-cis retinal bound, rhodopsin (pink) is associated with the GDP bound heterotrimeric Gα_T (orange, mauve). Phosphodiesterase 6 (PDE6; blue) is basally autoinhibited. Guanylate cyclase (not shown) generates cyclic guanosine monophosphate (cGMP) that keeps CNG channels (green) open. (Middle) Activation of the cascade begins when photon absorption isomerizes 11-cis-retinal to the all-trans conformation. The activated rhodopsin activates Gα_T; the GTP bound alpha subunit dissociates to bind PDEγ thus removing the autoinhibition of PDE6. The decrease in cGMP concentration closes CNG channels. (Bottom) The cascade is inactivated when the GTPase Activating Complex consisting of R9AP (brown), Gβ5 (light brown), and RGS9 (reddish-brown) activates GTP hydrolysis on Gα_T. Rhodopsin is phosphorylated by GRK (light gray) allowing arrestin (gray) binding which inactivates the receptor.