The ability of rod cells in the eye to respond to light and transmit that response to the optic nerve requires that the 11-cisform of vitamin A be attached to which of the following proteins?
(A) cGMP phosphodiesterase
(B) Na+ channel
(D) Both rod and cone cells contain a photoreceptor pigment in their membranes. The photosensitive compound (photoreceptor) of most mammalian eyes is a complex of protein and an aldehyde form of vitamin A. The protein component is a member of the opsin family called scotopsin. The photoreceptor of rod cells is specifically called rhodopsin (choice C) or visual purple, which is a complex between the protein scotopsin and the 11-cis-retinal form of vitamin A. Intracellularly, rhodopsin is coupled to a specific G-protein called transducin (choice E). When the rhodopsin is exposed to light it is bleached releasing the 11-cis-retinal from opsin. Absorption of photons by 11-cis-retinal triggers a series of conformational changes on the way to conversion all-trans-retinal. The release of opsin results in a conformational change in the photoreceptor. This conformational change activates transducin, leading to an increased GTP-binding by the alpha-subunit of transducin. Binding of GTP releases the alpha-subunit from the inhibitory beta- and gamma-subunits. The GTPactivated alpha-subunit in turn activates an associated phosphodiesterase (choice A), an enzyme that hydrolyzes cyclic-GMP(cGMP) to
GMP. Cyclic GMP is required to maintain the Na+ channels (choice B) of the rod cell in the open conformation. The drop in cGMPconcentration results in complete closure of the Na+ channels