We have identified a region within the ectodomain of the fusogenic human immunodeficiency virus type 1 (HIV-1) gp41, different from the fusion peptide, that interacts strongly with membranes. This conserved sequence, which immediately precedes the transmembrane anchor, is not highly hydrophobic according to the Kyte-Doolittle hydropathy prediction algorithm, yet it shows a high tendency to partition into the membrane interface, as revealed by the Wimley-White interfacial hydrophobicity scale. We have investigated here the membrane effects induced by NH(2)-DKWASLWNWFNITNWLWYIK-CONH(2) (HIV(c)), the membrane interface-partitioning region at the C terminus of the gp41 ectodomain, in comparison to those caused by NH(2)-AVGIGALFLGFLGAAGSTMGARS-CONH(2) (HIV(n)), the fusion peptide at the N terminus of the subunit. Both HIV(c) and HIV(n) were seen to induce membrane fusion and permeabilization, although lower doses of HIV(c) were required for comparable effects to be detected. Experiments in which equimolar mixtures of HIV(c) and HIV(n) were used indicated that both peptides may act in a cooperative way. Peptide-membrane and peptide-peptide interactions underlying those effects were further confirmed by analyzing 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 the HIV(c) ability to induce membrane fusion or form complexes with HIV(n) but not its ability to associate with vesicles. Hydropathy analysis indicated that the presence of two membrane-partitioning stretches separated by a collapsible intervening sequence is a common structural motif among other viral envelope proteins. Moreover, sequences with membrane surface-residing residues preceding the transmembrane anchor appeared to be a common feature in viral fusion proteins of several virus families. According to our experimental results, such a feature might be related to their fusogenic function.