Regulation of photoreceptor phosphodiesterase (PDE6) by the rod photoreceptor‐specific glutamic acid‐rich protein 2 (GARP2)

Abstract

As the central effector of the visual signaling pathway, photoreceptor phosphodiesterase (PDE6) is precisely regulated to control the sensitivity, speed and amplitude of the light response. This study seeks to define the regulatory role of the glutamic acid‐rich protein 2 (GARP2) that binds to PDE6 in rod photoreceptors. GARP2 is a splice variant of the rod photoreceptor cGMP‐gated channel β‐subunit. Using interaction assays, we identified the inhibitory γ‐subunit (Pγ) of the PDE6 holoenzyme (αβγγ) as the site of interaction with GARP2. Using Pγ truncation mutants, we found that GARP2 binds primarily to the N‐terminal region (a.a 1–30) of Pγ. The stoichiometry of GARP2 binding to PDE6 holoenzyme was determined to be 1.0 ± 0.2 mol GARP2 per mol PDE6 holoenzyme. In addition to suppressing PDE6 basal catalytic activity, GARP2 also accelerates cGMP dissociation from noncatalytic cGMP binding sites in the regulatory domain of PDE6. Together, these results indicate that GARP2 binding to the N‐terminal region of the inhibitory Pγ subunit can lower PDE6 catalytic activity in the dark‐adapted state, effectively reducing photoreceptor dark noise. In addition, GARP2 modulation of cGMP binding affinity to the PDE6 regulatory domains is consistent with a negative feedback role for GARP2 in PDE6 regulation following light activation of the phototransduction cascade. Supported by NIH grant EY‐05798 (to RHC).