Regulation of mammalian photoreceptor phosphodiesterase (PDE) by its noncatalytic GAF domains and its two small subunits, gamma and delta
cGMP phosphodiesterase (PDE) is the central effector of visual transduction in retinal rod photoreceptors. The overall goal of this work was to investigate the possible regulatory mechanisms of PDE in mammalian rod photoreceptors by its noncatalytic cGMP binding sites on the PDE catalytic dimer (Palphabeta), and by the interactions between the inhibitory Pgamma subunit and Palphabeta in their phosphorylated and non-phosphorylated states. First, the binding of cGMP to the noncatalytic sites on membrane-associated PDE (PDE-M) and soluble PDE (PDE-S) was characterized. We found that PDE-M holoenzyme contains 2 non-identical high-affinity cGMP binding sites; one of the cGMP binding sites is functionally nonexchangeable. In contrast, PDE-S can release and bind cGMP at both noncatalytic sites. Activation of PDE reduces more than 100-fold its cGMP binding affinity at one site, while the second cGMP binding site exhibits a 3-fold reduction in binding affinity. We postulate that cGMP dissociation from these two non-identical binding sites might contribute differently to controlling the activation and inactivation kinetics of PDE6 during visual transduction in rod photoreceptors. We also studied interactions between Pgamma and the mammalian Palphabeta catalytic dimer. It was found that Pgamma binds to two distinct sites on the catalytic Palphabeta dimer. cGMP occupancy at the noncatalytic binding sites is responsible for this binding heterogeneity. Two major domains on Pgamma interact with Palphabeta independently. The N-terminal half of Pgamma functions to restore and stabilize cGMP binding at the GAF domain with a binding affinity 50-fold greater than its C-terminal, inhibitory region. Phosphorylation is an important regulatory mechanism in the visual transduction pathway. Our results showed that both Pgamma and Palphabeta could be stoichiometrically phosphorylated in vitro. Pgamma phosphorylation has modest effects on its binding affinity to Palphabeta and its overall inhibitory potency. However, phosphorylation at Thr22 mildly decreases the ability of the central region of Pgamma to bind to Palphabeta and to stabilize cGMP binding at the GAF domains. Similar to PDE5, phosphorylation of photoreceptor Palphabeta regulates cGMP binding at the GAF domains. These results suggest a potential role of Palphabeta phosphorylation in regulating PDE by regulating cGMP levels in photoreceptor cells.
Biology, Molecular, Neuroscience, Engineering, Biomedical
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