Fig. 4: Sequential interactions with gateway residues as an extracted POPC molecule translocates from the external transmembrane cavity to the annulus orifice.

A–C Salt-bridge formation of acidic amino acid residues D575, E584, and D585 with the N(CH)₃⁺ headgroup moiety of POPC as the lipid molecule moves from the monolayer toward the annulus during MD simulation. Gateway, green cartoon with attached stick amino acid residues (aromatic residues, magenta); annulus, cyan space-filling; annulus orifice, white space-filling; extracted POPC, yellow space-filling; salt bridged amino acid residues, space-filling; location of outer monolayer, space-filling phosphorus atoms. D–F Changes in the conformation of the Y573/W574 aromatic cluster shield the PO₄⁻ and acyl side chains of POPC during translocation of the lipid molecule from the monolayer toward the annulus during MD simulation. G, H Van der Waals contacts of POPC upon its approach to the annulus orifice during SMD simulation of the translocation of POPC. G Protein in cartoon mode. H Protein in space-filling mode. Color code: annulus orifice, white; gateway—aromatic residues, magenta; basic residues, blue; acidic residues, red; hydrophobic residues, orange; other residues, green.