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 c ell membrane to close. It has long been a puzzle that the cGMP phospho diesterase (PDE) whose activation causes this decrease contains not on ly catalytic sites for cGMP hydrolysis but also noncatalytic cGMP bind ing sites. Recent work has shown that occupancy of these noncatalytic sites slows the rate of PDE inactivation. We report here that PDE acti vation induced by activated transducin lowers the cGMP binding affinit y for noncatalytic sites on PDE and accelerates the dissociation of cG MP from these sites. These sites can exist in three states: high affin ity (K-d = 60 nM) for the nonactivated PDE, intermediate affinity (K-d approximate to 180 nM) when the enzyme is activated in a complex with transducin, and low affinity (K-d > 1 mu M) when transducin physicall y removes the inhibitory subunits of PDE from the PDE catalytic subuni ts. Activation of PDE by transducin causes a 10-fold increase in the r ate of cGMP dissociation from one of the two noncatalytic sites; physi cal removal of the inhibitory subunits from the PDE catalytic subunits further accelerates the cGMP dissociation rate from both sites >50-fo ld. Because PDE molecules lacking bound cGMP inactivate more rapidly, this suggests that a prolonged cGMP decrease may act as a negative fee dback regulator to generate the faster, smaller photoresponses charact eristic of light-adapted photoreceptors.