Phosducin Regulates the Level of Transducin Expression in Rod Photoreceptors

Abstract

Abstract: : Purpose:Phosducin (Pd) is putative regulator of G protein–mediated signaling, which is expressed in vertebrate rods and cones. The purpose of this study was to determine the function of Pd in the rod phototransduction cascade. Methods:Quantitative western blotting and single cell recordings from Pd–/– mice and wild type mice were conducted according to standard protocols. Pd–/– Gα_t–/– double knockout mice were generated by crossing Pd–/– with previously described rod transducin α subunit knockout mouse(Gα_t–/–). Results: Pd–/– and Pd–/– Gα_t–/– mice develop morphologically unaltered, functional rods that express normal amount of rhodopsin. However, the rods of Pd–/– mice contained ∼40% of the normal amount of transducin ßγ subunits, whereas the transducin α subunit content was normal. Even lower transducin ßγ subunit level, ∼10% of normal, was detected in the retinas of Pd–/– Gα_t–/– double knockout mice. Single photon responses of dark–adapted Pd–/– rods were more than two–fold smaller than those of wild type rods, consistent with the reduced gain of transduction resulting from the lowered transducin heterotrimer levels. All measures of light adaptation in Pd–/– rods were indistinguishable from those of wild type rods. Conclusions:Our data indicate that Pd is required for normal expression of transducin ßγ subunit in rods, which content determines the amplitude of the rod photoresponse. The regulation of transducin ßγ by Pd in in vitro studies appears to have no physiological correlate for the outer segment photoresponse in either dark– or light–adapted conditions.

Keywords: photoreceptors • protein structure/function • electrophysiology: non-clinical