cGMP binding sites on photoreceptor phosphodiesterase: role in feedback regulation of visual transduction.

A central step in vertebrate visual transduction is the rapid drop in cGMP levels that causes cGMP-gated ion channels in the photoreceptor cell membrane to close. It has long been a puzzle that the cGMP phosphodiesterase (PDE) whose activation causes this decrease contains not only catalytic sites for cGMP hydrolysis but also noncatalytic cGMP binding sites. Recent work has shown that occupancy of these noncatalytic sites slows the rate of PDE inactivation. We report here that PDE activation induced by activated transducin lowers the cGMP binding afnit for noncatalytic sites on PDE and accelerates the disation of cGMP from these sites. These sites can exist in three states: gh affinity (Kd =60 nM) for the nonactivated PDE, intermediate affinity (Kd 180 nM) when the enzyme is activated in a complex with trasducin, and low aMnity (Kd \u3e 1 FM) when transducin physically removes the inhibitory subunits of PDE from the PDE catalytic subunits. Activation of PDE by htrnducin causes a 10-fold increase in the rate of cGMP discation from one of the two noncatalyc sites; physical removal of the inhibitory subunits from the PDE catalytic subunits further accelerates the cGMP disiation rate from both sites \u3e50-fold. Because PDE molecules lacking bound cGMP inactivate more rapidly, this suggests that a prolonged cGMP decrease may act as a negative feedback regulator to generate the faster, smaller photoresponses characteristic of liht-adapted photoreceptors