A LEUCINE ZIPPER-LIKE SEQUENCE FROM THE CYTOPLASMIC TAIL OF THE HIV-1ENVELOPE GLYCOPROTEIN BINDS AND PERTURBS LIPID BILAYERS

HIV-1 transmembrane envelope glycoprotein (gp41) has an unusually long cytoplasmic domain that has secondary associations with the inner lea flet of the membrane. Two highly amphiphatic alpha-helices in the cyto plasmic domain of gp41 have previously been shown to interact with lip id bilayers, We have detected a highly conserved leucine zipper-like s equence between the two alpha-helices. A peptide corresponding to this segment (residues 789-815, LLP-3) aggregates in aqueous solution, but spontaneously inserts into phospholipid membranes and dissociates int o alpha-helical monomers. The peptide perturbs the bilayer structure r esulting in the formation of micelles and other non-bilayer structures . Tryptophan fluorescence quenching experiments using brominated phosp holipids revealed that the peptide penetrates deeply into the hydropho bic milieu of the membrane bilayer. The peptide interacts equally with zwitterionic and negatively-charged phospholipid membranes and is pro tected from proteolytic digestion in its membrane-bound state. Polariz ed attenuated total reflection Fourier transform infrared (ATR-FTIR) s pectroscopy showed that the LLP-3 alpha-helix axis is about 70 degrees from the normal to the membrane plane, The ATR-FTIR CH2-stretching di chroic ratio increases when the peptide is incorporated into pure phos pholipid membranes, further indicating that the peptide can deeply pen etrate and perturb the bilayer structure. Integrating these data with what is already known about the membrane-associating features of adjac ent segments, we propose a revised structural model in which a large p ortion of the cytoplasmic tail of the HIV-1 envelope glycoprotein is a ssociated with the membrane.