8.A.135.  The Trp Cation Channel Auxiliary Subunit, InaF (InaF) Family 

TRP channels function in many types of sensory receptor cells. Auxiliary subunits could influence localization, trafficking, and/or function of TRP channels. Using Drosophila melanogaster, Chen and Montell 2020 found a previously unknown TRP interacting protein, INAF-C, which is expressed exclusively in ultraviolet-sensing R7 photoreceptor cells. INAF-C is encoded by an unusual locus comprised of four distinct coding regions, which give rise to four unique single transmembrane containing proteins. Both INAF-B and INAF-C are required for TRP stability and localization in R7 cells, but loss of just INAF-B greatly reduced TRP from other types of photoreceptor cells, but not R7. The requirements for TRP and INAF are reciprocal, since loss of TRP decreased the concentrations of both INAF-B and INAF-C. INAF-A, which is not normally expressed in photoreceptor cells, can functionally substitute for INAF-B, indicating that it is a third TRP auxiliary protein. Reminiscent of the structural requirements between Kv channels and KCNE auxiliary subunits, the co-dependencies of TRP and INAF depended on several TMSs in TRP, and the TMS and  C-terminus of INAF-B (Chen and Montell 2020).