Quinone-transporting filaments expand bioenergetic capacity in Gram-positive Bacillota.

Cellular respiration depends on transferring electrons to hydrophobic quinones in membrane bilayers, meaning bioenergetic capacity is constrained by available membrane surface area. While Gram-negative bacteria expand this capacity through internal membrane invaginations and eukaryotes use membrane-bound organelles, whether Gram-positive bacteria have alternative capacity-generating mechanisms is unknown. Here we show that Bacillus subtilis forms a quinone-transporting pseudomembrane composed of filaments of the NADH dehydrogenase Ndh and the quinone-transporting protein Ncp. Cryo-EM, lipidomics and molecular dynamics reveal that Ndh and Ncp co-assemble with lipids into a 4:4 stoichiometric complex with a solvent-excluded hydrophobic lumen containing phospholipids, which sequesters quinones. These complexes further assemble into filaments, linking chambers into a continuous conduit that amplifies quinone reduction. Phylogenetic analysis suggests that this capacity is widespread in Bacillota. Quinone-transporting filaments thus reveal a strategy to expand the quinone pool and bioenergetic capacity while occupying minimal membrane space.