Fitting of secondary structural elements in the cryo-EM density map of VCC . (A) Side view of heptameric bilayer–bound VCC fitted with the atomic model. Enlarged views show a part of the β-trefoil region and the entire transmembrane channel. (B) Magnified image of the core region viewed from the top of the β-barrel. Representation of the narrowest aperture of the pore marked by seven W318 residues observed in the cryo-EM density.