Binding of cGMP to GAF domains in amphibian rod photoreceptor cGMP phosphodiesterase (PDE). Identification of GAF domains in PDE alphabeta subunits and distinct domains in the PDE gamma subunit involved in stimulation of cGMP binding to GAF domains.

Retinal cGMP phosphodiesterase (PDE6) is a key enzyme in vertebrate phototransduction. Rod PDE contains two homologous catalytic subunits (Palphabeta) and two identical regulatory subunits (Pgamma). Biochemical studies have shown that amphibian Palphabeta has high affinity, cGMP-specific, non-catalytic binding sites and that Pgamma stimulates cGMP binding to these sites. Here we show by molecular cloning that each catalytic subunit in amphibian PDE, as in its mammalian counterpart, contains two homologous tandem GAF domains in its N-terminal region. In Pgamma-depleted membrane-bound PDE (20-40% Pgamma still present), a single type of cGMP-binding site with a relatively low affinity (K(d) approximately 100 nm) was observed, and addition of Pgamma increased both the affinity for cGMP and the level of cGMP binding. We also show that mutations of amino acid residues in four different sites in Pgamma reduced its ability to stimulate cGMP binding. Among these, the site involved in Pgamma phosphorylation by Cdk5 (positions 20-23) had the largest effect on cGMP binding. However, except for the C terminus, these sites were not involved in Pgamma inhibition of the cGMP hydrolytic activity of Palphabeta. In addition, the Pgamma concentration required for 50% stimulation of cGMP binding was much greater than that required for 50% inhibition of cGMP hydrolysis. These results suggest that the Palphabeta heterodimer contains two spatially and functionally distinct types of Pgamma-binding sites: one for inhibition of cGMP hydrolytic activity and the second for activation of cGMP binding to GAF domains. We propose a model for the Palphabeta-Pgamma interaction in which Pgamma, by binding to one of the two sites in Palphabeta, may preferentially act either as an inhibitor of catalytic activity or as an activator of cGMP binding to GAF domains in frog PDE.