The Ca2+-activated TRPM5 channel plays an essential role in the perception of sweet, bitter, and umami stimuli in type II taste cells and in insulin secretion by pancreatic beta cells. Interestingly, the voltage dependence of TRPM5 in taste bud cells depends on the intracellular Ca2+ concentration, yet the mechanism remains elusive. In this talk, I will share our new findings on the structure and function of the TRPM5 channel. We determined cryo-electron microscopy structures of the zebrafish TRPM5 in an apo closed state, a Ca2+-bound open state, and an antagonist-bound inhibited state, at resolutions up to 2.3 Å. We defined two novel ligand binding sites: a Ca2+ binding site (CaICD) in the intracellular domain (ICD), and a binding site in the transmembrane domain (TMD) for a highly potent antagonist. The CaICD site is unique to TRPM5 and has two roles: shifting the voltage dependence toward negative membrane potential, and promoting Ca2+ binding to the CaTMD site that is conserved throughout Ca2+-sensitive TRPM channels. Replacing glutamate 337 in the CaICD site with an alanine not only abolished Ca2+ binding to CaICD but also reduced Ca2+ binding affinity to CaTMD, suggesting a cooperativity between the two sites. We have defined mechanisms underlying channel activation and inhibition. Conformational changes initialized from both Ca2+ sites, 70 Å apart, are propagated to the ICD–TMD interface and cooperatively open the ion-conducting pore. The antagonist wedges into the space between the S1-S4 domain and pore domain, stabilizing the TMD in an apo-like closed state. Our results lay the foundation for understanding the voltage-dependent TRPM channels and developing new therapeutic agents to treat metabolic disorders.

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