THE TRANSIENT RECEPTOR POTENTIAL PROTEIN (TRP), A PUTATIVE STORE-OPERATED CA2-MEDIATED PHOTORECEPTION, FORMS A SIGNALING COMPLEX WITH NORPA, INAC AND INAD( CHANNEL ESSENTIAL FOR PHOSPHOINOSITIDE)

The transient receptor potential protein (Trp) is a putative capacitat ive Ca2+ entry channel present in fly photoreceptors, which use the in ositol 1,4,5-trisphosphate (InsP(3)) signaling pathway for phototransd uction. By immunoprecipitation studies, we find that Trp is associated into a multiprotein complex with the norpA-encoded phospholipase C, a n eye-specific protein kinase C (InaC) and with the InaD protein (InaD ). InaD is a putative substrate of InaC and contains two PDZ repeats, putative protein-protein interaction domains. These proteins are prese nt in the photoreceptor membrane at about equimolar ratios. The Trp ho molog anal-zed here is isolated together, with NorpA, InaC and InaD fr om blowfly (Calliphora) photoreceptors. Compared to Drosophila Trp, th e Calliphora Trp homolog displays 77% amino acid identity. The highest sequence conservation is found in the region that contains the putati ve transmembrane domains S1-S6 (91% amino acid identity). As investiga ted by immunogold labeling with specific antibodies directed against T rp and InaD, the Trp signaling complex is located in the microvillar m embranes of the photoreceptor cells. The spatial distribution of the s ignaling complex argues against a direct conformational coupling of Tr p to an InsP(3) receptor supposed to be present in the membrane of int ernal photoreceptor Ca2+ stores. It is suggested that the organization of signal transducing proteins into a multiprotein complex provides t he structural basis for an efficient and fast activation and regulatio n of Ca2+ entry through the Trp channel.