Protein inhibitor of retinal membrane guanylyl cyclase suppresses cGMP synthesis in photoreceptors

Retinal membrane guanylyl cyclase (RetGC) regulated by RetGC activating proteins (GCAPs) imparts light sensitivity to rods and cones by producing cyclic GMP (cGMP) to open cGMP-gated channels in the photoreceptor outer segments. However, excessive cGMP synthesis by deregulated RetGC:GCAP complex provokes cone-rod degeneration and causes congenital blindness. We developed the first to date specific protein inhibitor of retinal guanylyl cyclase (PIGCY) capable of reducing RetGC activity in photoreceptor outer segment. PIGCY was constructed by modifying GCAP1 to eliminate its ability to activate RetGC while increasing its affinity for RetGC. PIGCY uncouples RetGC1:GCAP1 complex in vitro, reducing V_max and increasing K_mGTP of RetGC activity. PIGCY inhibits both basal and GCAP-stimulated RetGC activity from wildtype, GCAP1^-/-, GCAP2^-/-, and GCAPs^-/- mouse retinas homogenates and in isolated rod outer segments. When expressed in mouse rods, PIGCY accumulates in outer segments and decelerates endogenous GCAP-stimulated RetGC activity in PIGCY^Tg retina.