Membrane Interface-Interacting Sequences within the Ectodomain of the Human Immunodeficiency Virus Type 1 Envelope Glycoprotein: Putative Role during Viral Fusion

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Abstract
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.