Early intermediates in HIV-1 envelope glycoprotein-mediated fusion triggered by CD4 and co-receptor complexes

An early step in the process of HIV-1 entry into target cells is the activa tion of its envelope glycoprotein (GP120-GP41) to a fusogenic state upon bi nding to target cell CD4 and cognate co-receptor. Incubation of human immun odeficiency virus (HIV)-1 Env-expressing cells with an excess of CD4 and co -recepeptor-bearing target cells resulted in an influx of an impermeant nuc leic acid-staining fluorescent dye into the Env-expressing cells. The dye i nflux occurred concomitant with cell fusion. No influx of dye into target c ells was observed if they were incubated with an excess of Env-expressing c ells. The permeabilization of Env-expressing cells was also triggered by CD 4.co-receptor complexes attached to Protein G-Sepharose beads in the absenc e of target cells. The CD4 and co-receptor-induced permeabilization of Env- expressing cells occurred with the same specificity with respect to co-rece ptor usage as cell fusion. Natural ligands for the co-receptors and C-termi nal GP41 peptide inhibitors of HIV-1 fusion blocked this effect. Our result s indicate that the process of HIV-1 Env-mediated fusion is initiated by th e destabilization of HIV-1 Env-expressing membranes. Further elucidation of these early intermediates may help identify and develop potential inhibito rs of HIV-1 entry into cells.