TRP, a protein essential for inpsitide-mediated Ca²⁺ influx is localized adjacent to the calcium stores in Drosophila photoreceptors

The Drosophila transient receptor potential (trp) gene product (TRP) shows some structural similarity to vertebrate voltage-gated Ca²⁺ channels. It appears to function as a novel Ca²⁺ channel responsible for light stimulated, inositol trisphosphate (InsP₃)-mediated Ca²⁺ entry in the fly retina. The subcellular localization of TRP protein was determined in this study using immunohistochemical staining with anti-TRP antibody (MAb83F6). TRP was localized to the base of the microvilli in a region adjacent to the presumed InsP₃-sensitive Ca²⁺ stores. This specific localization was supported by measuring the magnitude of a TRP-dependent inward current that results from spontaneous activation of the light-sensitive channels during whole-cell recordings (the rundown current, RDC). We found that reduction of the microvilli area through genetic dissection with the opsin null mutant, ninaE^ora, was correlated with a pronounced enhancement of the TRP-dependent inward current 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 membrane loop found along the base of the rhabdomeric microvillus. Thus, the TRP channel may function in concert with the InsP₃-sensitive Ca²⁺ stores.