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

Background: TRPM3 proteins form Ca2+ permeable ion channels involved in insulin secretion and pain perception. Results: A domain indispensable for TRPM3 channel function (ICF) is subject to alternative splicing. Conclusion: This domain contributes essentially to the formation of TRPM channels and removing it by splicing modulates TRPM3-mediated Ca2+ signaling. Significance: Alternative splicing of the ICF domain regulates biological functions attributed to TRPM3. TRPM3 channels form ionotropic steroid receptors in the plasma membrane of pancreatic β 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. We found 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 (TRPM3ΔICF), 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. TRPM3ΔICF variants showed reduced interaction with other TRPM3 isoforms, and their occurrence at the cell membrane was diminished. Correspondingly, coexpression of ΔICF proteins with functional TRPM3 subunits not only reduced the number of channels but also impaired TRPM3-mediated Ca2+ entry. We conclude that TRPM3ΔICF variants are regulatory channel subunits fine-tuning TRPM3 channel activity.