Photoreceptor Phosphodiesterase: Interaction of Inhibitory γ Subunit and Cyclic GMP with Specific Binding Sites on Catalytic Subunits

Abstract

The photoreceptor phosphodiesterase (PDE6) is the central effector enzyme in the phototransduction cascade of photoreceptor cells. It is the only known PDE isoform the activity of which is regulated by interaction with a heterotrimeric G protein. The rod PDE6 holoenzyme is a tetrameric protein consisting of two large catalytic α and β subunits and two small γ subunits, which serve as potent inhibitors of PDE6. In dark-adapted photoreceptors, the γ subunits maintain PDE6 activity at a low level. When exposed to light the visual pigment rhodopsin activates the retinal G protein, transducin, leading to release of the inhibitory action of the γ subunits. In addition to the active sites where cGMP is hydrolyzed, the α and β catalytic subunits have high-affinity, noncatalytic cGMP binding sites. These noncatalytic sites do not directly regulate cGMP catalysis at the active site, but rather can modulate the affinity with which the γ subunits bind to the catalytic subunits. This article describes a number of experimental approaches that have recently been developed for studying the interactions between catalytic and inhibitory subunits of PDE6, as well as the dynamics of cGMP binding to and dissociation from the PDE6 noncatalytic sites.