A Constitutively Closed Pannexin1 Channel in Lipid Bilayer Nanodiscs Assembles as a Large-Pore Heptamer

Pannexin 1 (Panx1) channels are widely expressed and play important roles in apoptotic cell clearance, inflammation, blood pressure regulation, neurological disorders, opiate withdrawal, and cancer progression and metastasis. We performed (1) physicochemical analysis on a constitutively closed Panx1 channel (designated fPanx1{Delta}C) to examine the entire population of particles to detect multiple oligomeric states and (2) cryoEM in the membrane mimetics amphipol A8-35 and lipid bilayer nanodiscs. Our results reveal that the dominant if not exclusive oligomeric state of fPanx1{Delta}C is a heptamer, in solution and by cryoEM. The Panx1 heptamer provides further structural diversity within the family of large-pore channels, including hexameric LRRC8 (SWELL1) channels and connexin hemichannels, octameric CALHM1 channels and innexin hemichannels, and undecameric CALHM2 channels. Conserved structural themes are a large cytoplasmic vestibule with a diameter that corresponds roughly with the oligomeric state and a 4-helix bundle protomer, albeit with noncanonical helical packing for CALHM1 and CALHM2. In BriefThe 4-helix bundle protomer of a constitutively closed pannexin1 channel assembles as a heptamer in solution and by cryoEM.