Rhodopsin
Rhodopsin is the photoreceptor protein in rod cells of vertebrate retina [1]. It is also known as opsin 2.
Rhodopsin is a member of the GPCR family, which is coupled to the G-protein, transducin. Rhodopsin, unlike many other GPCRs is activated by a photon of light not by the binding of a ligand. Infact, the ligand (11-cis retinal) is already bound in its inactive state. The photon causes a conformational change in the GPCR which causes activation of the transducin, whose α subunit, then activates cyclic GMP phosphodiesterase [2].