Schematic representation of Env mutants used in this study and the proposed topology models. (A) The points of truncation of gp41 were indicated together with a schematic diagram of the gp41 subunit. ED: ectodomain, MSD: membrane-spanning domain, CT: cytoplasmic tail, LLP: lentiviral lytic peptide. The numbering of the amino acid is based on that of the HXB2 strain. The vertical dashed line shows the position of the N-terminus of the gp41 used for the analysis in the bacterial system. The numbers and letters on the right indicate the position and the amino acid residue of the C-terminus. (B and C) Proposed topology models. The grey numbered arrowheads indicate the truncation points of gp41, the numbers and colors correspond to (A).