Interaction of retinal guanylate cyclase with the α subunit of transducin: potential role in transducin localization

Vertebrate phototransduction is mediated by cGMP, which is generated by retGC (retinal guanylate cyclase) and degraded by cGMP phosphodiesterase. Light stimulates cGMP hydrolysis via the G-protein transducin, which directly binds to and activates phosphodiesterase. Bright light also causes relocalization of transducin from the OS (outer segments) of the rod cells to the inner compartments. In the present study, we show experimental evidence for a previously unknown interaction between G_αt (the transducin α subunit) and retGC. G_αt co-immunoprecipitates with retGC from the retina or from co-transfected COS-7 cells. The retGC–G_αt complex is also present in cones. The interaction also occurs in mice lacking RGS9 (regulator of G-protein signalling 9), a protein previously shown to associate with both G_αt and retGC. The G_αt–retGC interaction is mediated primarily by the kinase homology domain of retGC, which binds GDP-bound G_αt stronger than the GTP[S] (GTPγS; guanosine 5′-[γ-thio]triphosphate) form. Neither G_αt nor G_βγ affect retGC-mediated cGMP synthesis, regardless of the presence of GCAP (guanylate cyclase activating protein) and Ca²⁺. The rate of light-dependent transducin redistribution from the OS to the inner segments is markedly accelerated in the retGC-1-knockout mice, while the migration of transducin to the OS after the onset of darkness is delayed. Supplementation of permeabilized photoreceptors with cGMP does not affect transducin translocation. Taken together, these results suggest that the protein–protein interaction between G_αt and retGC represents a novel mechanism regulating light-dependent translocation of transducin in rod photoreceptors.