THE PHOSPHATIDYLINOSITOL TRANSFER PROTEIN DOMAIN OF DROSOPHILA RETINAL DEGENERATION B-PROTEIN IS ESSENTIAL FOR PHOTORECEPTOR CELL-SURVIVAL AND RECOVERY FROM LIGHT STIMULATION

The Drosophila retinal degeneration B (rdgB) gene encodes an integral membrane protein involved in phototransduction and prevention of retin al degeneration. RdgB represents a nonclassical phosphatidylinositol t ransfer protein (PITP) as all other known PITPs are soluble polypeptid es. Our data demonstrate roles for RdgB in proper termination of the p hototransduction light response and dark recovery of the photoreceptor cells. Expression of RdgB's PITP domain as a soluble protein (RdgB-PI TP) in rdgB(2) mutant flies is sufficient to completely restore the wi ld-type electrophysiological light response and prevent the degenerati on. However, introduction of the T59E mutation, which does not affect RdgB-PITP's phosphatidylinositol (PI) and phosphatidycholine (PC) tran sfer in vitro, into the soluble (RdgB-PITP-T59E) or full-length (RdgB- T59E) proteins eliminated rescue of retinal degeneration in rdgB(2) fl ies, while the light response was partially maintained. Substitution o f the rat brain PITP alpha, a classical PI transfer protein, for RdgB' s PITP domain (PITP alpha or PITP alpha-RdgB chimeric protein) neither restored the light response nor maintained retinal integrity when exp ressed in rdgB(2) flies. Therefore, the complete repertoire of essenti al RdgB functions resides in RdgB's PITP domain, but other PITPs posse ssing PI and/or PC transfer activity in vitro cannot supplant RdgB fun ction in vivo. Expression of either RdgB-T59E or PITP alpha-RdgB in rd gB(+) flies produced a dominant retinal degeneration phenotype, Wherea s RdgB-T59E functioned in a dominant manner to significantly reduce st eady-state levels of rhodopsin, PITP alpha-RdgB was defective in the a bility to recover from prolonged light stimulation and caused photorec eptor degeneration through an unknown mechanism, This in vivo analysis of PITP function in a metazoan system provides further insights into the links between PITP dysfunction and an inherited disease in a highe r eukaryote.