Structures of TRPM5 channel elucidate mechanism of activation and inhibition

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^1–3. Interestingly, the voltage dependence of TRPM5 in taste bud cells depends on the intracellular Ca²⁺concentration⁴, yet the mechanism remains elusive. Here we report 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 (Ca_ICD) in the intracellular domain (ICD), and an antagonist binding site in the transmembrane domain (TMD) for a drug (NDNA) that regulates insulin and GLP-1 release⁵. The Ca_ICDsite is unique to TRPM5 and has two roles: shifting the voltage dependence toward negative membrane potential, and promoting Ca²⁺binding to the Ca_TMDsite that is conserved throughout Ca²⁺-sensitive TRPM channels⁶. Replacing glutamate 337 in the Ca_ICDsite with an alanine not only abolished Ca²⁺binding to Ca_ICDbut also reduced Ca²⁺binding affinity to Ca_TMD, 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 NDNA 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.