Alternative Splicing of a Protein Domain Indispensable for Function of Transient Receptor Potential Melastatin 3 (TRPM3) Ion Channels

TRPM3 channels form ionotropic steroid receptors in the plasma membrane of pancreatic beta and dorsal root ganglion cells and link steroid hormone signaling to insulin release and pain perception, respectively. We identified and compared the function of a number of TRPM3 splice variants present in mouse, rat and human tissues. Wefound that variants lacking a region of 18 amino acid residues display neither Ca2+ entry nor ionic currents when expressed alone. Hence, splicing removes a region that is indispensable for channel function, which is called the ICF region. TRPM3 variants devoid of this region (TRPM3 Delta ICF), are ubiquitously present in different tissues and cell types where their transcripts constitute up to 15% of the TRPM3 isoforms. The ICF region is conserved throughout the TRPM family, and its presence in TRPM8 proteins is also necessary for function. Within the ICF region, 10 amino acid residues form a domain essential for the formation of operative TRPM3 channels. TRPM3 Delta ICF variants showed reduced interaction with other TRPM3 isoforms, and their occurrence at the cell membrane was diminished. Correspondingly, coexpression of Delta ICF proteins with functional TRPM3 subunits not only reduced the number of channels but also impaired TRPM3-mediated Ca2+ entry. We conclude that TRPM3 Delta ICF variants are regulatory channel subunits fine-tuning TRPM3 channel activity.