The Amphotericin B–Ergosterol Complex Spans a Lipid Bilayer as a Single-Length Assembly

Amphotericin B (AmB) is a polyene macrolide antibiotic clinically used as an antifungal drug. Its preferential complexation with ergosterol (Erg), the major sterol of fungal membranes, leads to the formation of a barrel-stave-like ion channel across a lipid bilayer. To gain a better understanding of the mechanism of action, the mode of lipid bilayer spanning provides essential information. However, because of the lack of methodologies to observe it directly, it has not been revealed for the Erg-containing channel assembly for many years. In this study, we disclosed that the AmB–Erg complex spans a lipid bilayer with a single-molecule length, using solid-state nuclear magnetic resonance (NMR) experiments. Paramagnetic relaxation enhancement by Mn²⁺ residing near the surface of lipid bilayers induced the depth-dependent decay of ¹³C NMR signals for individual carbon atoms of AmB. We found that both terminal segments, the 41-COOH group and C38–C40 methyl groups, come close to the lipid bilayer surfaces, suggesting that the AmB–Erg complex spans a palmitoyloleoylphosphatidylcholine (POPC) bilayer with a single-molecule length. Molecular dynamics simulation experiments further confirmed the stabilization of the AmB–Erg complex as a single-length spanning complex. These results provide experimental evidence of the single-length complex incorporated in the membrane by making thinner a POPC–Erg bilayer that mimics fungal membranes.