cGMP binding to noncatalytic sites on mammalian rod photoreceptor phosphodiesterase is regulated by binding of its gamma and delta subunits

The binding of cGMP to the noncatalytic sites on two isoforms of the phosph odiesterase (PDE) from mammalian rod outer segments has been characterized to evaluate their role in regulating PDE during phototransduction. Nonactiv ated, membrane-associated PDE (PDE-M, alpha beta gamma(2)) has one exchange able site for cGMP binding; endogenous cGMP remains nonexchangeable at the second site, Non-activated, soluble PDE (PDE-S, alpha beta gamma(2)delta) c an release and bind cGMP at both noncatalytic sites; the delta subunit is l ikely responsible for this difference in cGMP exchange rates. Removal of th e delta and/or gamma subunits yields a catalytic cup dimer with identical c atalytic and binding properties for both PDE-M and PDE-S as follows: high a ffinity cGMP binding is abolished at one site (K-D > mu M); cGMP binding af finity at the second site (K-D similar to 60 nM) is reduced 3-4-fold compar ed with the nonactivated enzyme; the kinetics of cGMP exchange to activated PDE-M and PDE-S are accelerated to similar extents. The properties of nona ctivated PDE can be restored upon addition of gamma subunit. Occupancy of t he noncatalytic sites by cGMP may modulate the interaction of the gamma sub unit with the alpha beta dimer and thereby regulate cytoplasmic cGMP concen tration and the lifetime of activated PDE during visual transduction in pho toreceptor cells.