Independent anchoring and assembly mechanisms of INAD signaling complexes in Drosophila photoreceptors

In Drosophila photoreceptors the multivalent PDZ protein INAD organizes the phototransduction cascade into a macromolecular signaling complex containi ng the effector PLC, the light-activated TRP channels, and a regulatory PKC . Previously, we showed that the subcellular localization of INAD signaling complexes is critical for signaling. Now we have examined how INAD complex es are anchored and assembled in photoreceptor cells. We find that trp muta nts, or transgenic flies expressing inaD alleles that disrupt the interacti on between INAD and TRP, cause the mislocalization of the entire transducti on complex. The INAD-TRP interaction is not required for targeting but rath er for anchoring of complexes, because INAD and TRP can be targeted indepen dently of each other. We also show that, in addition to its scaffold role, INAD functions to preassemble transduction complexes. Preassembly of signal ing complexes helps to ensure that transduction complexes with the appropri ate composition end up in the proper location. This may be a general mechan ism used by cells to target different signaling machinery to the pertinent subcellular location.