Structures of TRPM5 channel elucidate mechanism of activation and inhibition
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Abstract
The Ca 2+ -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 Ca 2+ concentration 4 , yet the mechanism remains elusive. Here we report cryo-electron microscopy structures of the zebrafish TRPM5 in an apo closed state, a Ca 2+ -bound open state, and an antagonist-bound inhibited state, at resolutions up to 2.3 Å. We defined two novel ligand binding sites: a Ca 2+ 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 release 5 . The Ca ICD site is unique to TRPM5 and has two roles: shifting the voltage dependence toward negative membrane potential, and promoting Ca 2+ binding to the Ca TMD site that is conserved throughout Ca 2+ -sensitive TRPM channels 6 . Replacing glutamate 337 in the Ca ICD site with an alanine not only abolished Ca 2+ binding to Ca ICD but also reduced Ca 2+ 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 Ca 2+ 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.