LOCALIZATION OF DROSOPHILA-RETINAL-DEGENERATION-B, A MEMBRANE-ASSOCIATED PHOSPHATIDYLINOSITOL TRANSFER PROTEIN

The Drosophila retinal degeneration B (rdgB) mutation causes abnormal photoreceptor response and light-enhanced retinal degeneration. Immuno blots using polyclonal anti-rdgB serum showed that rdgB is a 160-kD me mbrane protein. The antiserum localized the rdgB protein in photorecep tors, antennae, and regions of the Drosophila brain, indicating that t he rdgB protein functions in many sensory and neuronal cells. In photo receptors, the protein localized adjacent to the rhabdomeres, in the v icinity of the subrhabdomeric cisternae. The rdgB protein's amino-term inal 281 residues are >40% identical to the rat brain phosphatidylinos itol transfer protein (PI-TP). A truncated rdgB protein, which contain s only this amino-terminal domain, possesses a phosphatidylinositol tr ansfer activity in vitro. The remaining 773 carboxyl terminal amino ac ids have additional functional domains. Nitrocellulose overlay experim ents reveal that an acidic amino acid domain, adjacent to the PI trans fer domain, binds Ca-45(+2). Six hydrophobic segments are found in the middle of the putative translation product and likely function as mem brane spanning domains. These results suggest that the rdgB protein, u nlike the small soluble PI-TPs, is a membrane-associated PI-TP, which may be directly regulated by light-induced changes in intracellular ca lcium.