Membrane interface-interacting sequences within the ectodomain of the human immunodeficiency virus type 1 envelope glycoprotein: Putative role duringviral fusion

We have identified a region within the ectodomain of the fusogenic human im munodeficiency virus type 1 (HIV-1) gp41. different from the fusion peptide , that interacts strongly with membranes. This conserved sequence, which im mediately precedes the transmembrane anchor, is not highly hydrophobic acco rding to the Kyte-Doolittle hydropathy prediction algorithm, Set it shows a high tendency to partition into the membrane interface, as revealed by the Wimley-White interfacial hydrophobicity scale. We have investigated here t he membrane effects induced by NH2-DKWASLWNWFNITNWLWYIK-CONH2 (HIVc) the me mbrane interface-partitioning region at the C terminus of the gp41 ectodoma in, in comparison to those caused by NH2-AVGIGALFLGFLGAAGSTMGARS-CONH2 (HIV n), the fusion peptide at the N terminus of the subunit. Both HIVc and HIVn were seen to induce membrane fusion and permeabilization, although lower d oses of HIVc were required for comparable effects to be detected. Experimen ts in which equimolar mixtures of HIVc and HIVn were used indicated that bo th peptides may act in a cooperative way. Peptide-membrane and peptide-pept ide interactions underlying those effects were further confirmed by analyzi ng the changes in fluorescence of peptide Trp residues. Replacement of the first three Trp residues by Ala, known to render a defective gp41 phenotype unable to mediate both cell-cell fusion and virus entry, also abrogated th e HIVc ability to induce membrane fusion or form complexes with HIVn but no t its ability to associate with vesicles. Hydropathy analysis indicated tha t the presence of two membrane-partitioning stretches separated by a collap sible intervening sequence is a common structural motif among other viral e nvelope proteins. Moreover, sequences with membrane surface-residing residu es preceding the transmembrane anchor appeared to be a common feature in vi ral fusion proteins of several virus families. According to our experimenta l results, such a feature might be related to their fusogenic function.