TRP, A PROTEIN ESSENTIAL FOR INOSITIDE-MEDIATED CA2+ INFLUX IS LOCALIZED ADJACENT TO THE CALCIUM STORES IN DROSOPHILA PHOTORECEPTORS

The Drosophila transient receptor potential (trp) gene product (TRP) s hows some structural similarity to vertebrate voltage-gated Ca2+ chann els. It appears to function as a novel Ca2+ channel responsible for li ght stimulated, inositol trisphosphate (InsP(3))-mediated Ca2+ entry i n the fly retina, The subcellular localization of TRP protein was dete rmined in this study using immunohistochemical staining with anti-TRP antibody (MAb83F6). TRP was localized to ii the base of the microvilli in a region adjacent to the presumed InsP(3)-sensitive Ca2+ stores. T his specific localization was Supported by measuring the magnitude of a TRP-dependent inward current that results from spontaneous activatio n of the light-sensitive channels during whole-cell recordings (the ru ndown current, RDC). We found that reduction of the microvilli area th rough genetic dissection with the opsin null mutant, ninaE(ora), was c orrelated with a pronounced enhancement of the TRP-dependent inward cu rrent relative to wild type, suggesting that the TRP-dependent current was not produced along the length of the microvilli. We suggest that the functional localization of the TRP protein is on the plasma membra ne loop found along the base of the rhabdomeric microvillus. Thus, the TRP channel may function in concert with the InsP(3)-sensitive Ca2+ s tores.