SARS-CoV-2 encodes three putative ion channels: E, 8a, and 3a. In related SARS-CoV-1, 3a is implicated in viral release, inflammasome activation, and cell death and its deletion reduces viral titer and morbidity in animal models, suggesting 3a-targeted therapeutics could treat SARS and COVID-19. However, the structural basis for the function of 3a is unknown. Here, we show that SARS-CoV-2 forms large conductance cation channels and present cryo-EM structures of dimeric and tetrameric SARS-CoV-2 3a in lipid nanodiscs. 3a adopts a novel fold and is captured in a closed or inactivated state. A narrow bifurcated exterior pore precludes conduction and leads to a large polar cavity open to the cytosol. 3a function is conserved in a common variant among circulating SARS-CoV-2 that alters the channel pore. We identify 3a-like proteins in Alphaand Beta-coronaviruses that infect bats and humans, suggesting therapeutics targeting 3a could treat a range of coronaviral diseases.
In CFTR, the chloride ion channel mutated in cystic fibrosis (CF) patients, pore opening is coupled to ATP-binding-induced dimerization of two cytosolic nucleotide binding domains (NBDs) and closure to dimer disruption following ATP hydrolysis. CFTR opening rate, unusually slow because of its high-energy transition state, is further slowed by CF mutation ΔF508. Here, we exploit equilibrium gating of hydrolysis-deficient CFTR mutant D1370N and apply rate-equilibrium free-energy relationship analysis to estimate relative timing of opening movements in distinct protein regions. We find clear directionality of motion along the longitudinal protein axis and identify an opening transition-state structure with the NBD dimer formed but the pore still closed. Thus, strain at the NBD/pore-domain interface, the ΔF508 mutation locus, underlies the energetic barrier for opening. Our findings suggest a therapeutic opportunity to stabilize this transition-state structure pharmacologically in ΔF508-CFTR to correct its opening defect, an essential step toward restoring CFTR function.
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