title: HIV-1 envelope proteins complete their folding into six-helix bundles immediately after fusion pore formation.
abstract: Fusion proteins of many viruses, including HIV-1 envelope protein (Env), fold into six-helix bundle structures. Fusion between individual Env-expressing cells and target cells was studied by fluorescence microscopy, and a temperature jump technique, to determine whether folding of Env into a bundle is complete by the time fusion pores have formed. Lowering temperature to 4 degrees C immediately after a pore opened halted pore growth, which quickly resumed when temperature was raised again. HIV gp41-derived peptides that inhibit bundle formation (C34 or N36) caused the cold-arrested pore to quickly and irreversibly close, demonstrating that bundle formation is not complete by the time a pore has formed. In contrast, lowering the temperature to an intermediate value also halted pore growth, but the pore was not closed by the bundle-inhibiting peptides, and it enlarged when temperature was again elevated. This latter result shows that bundle formation is definitely required for the fusion process, but surprisingly, some (if not all) bundle formation occurs after a pore has formed. It is concluded that an essential function of the bundle is to stabilize the pore against collapse and ensure its growth.
authors.affiliation: Department of Molecular Biophysics and Physiology, Rush Medical College, Chicago, Illinois 60612, USA.
authors.forename: Ruben M
authors.initials: RM
authors.lastname: Markosyan
authors.forename: Fredric S
authors.initials: FS
authors.lastname: Cohen
authors.forename: Grigory B
authors.initials: GB
authors.lastname: Melikyan
mesh_terms: D000818:Animals; D015704:CD4 Antigens / Q000378:metabolism; D002460:Cell Line; D005456:Fluorescent Dyes / Q000378:metabolism; D015686:Gene Products, env / Q000737:chemistry / Q000378:metabolism*; D015700:HIV Envelope Protein gp41 / Q000737:chemistry / Q000378:metabolism*; D015497:HIV-1 / Q000378:metabolism*; D006801:Humans; D008561:Membrane Fusion / Q000502:physiology*; D010455:Peptides / Q000378:metabolism; D017510:Protein Folding; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D019718:Receptors, CXCR4 / Q000378:metabolism; D013696:Temperature
chemical_list: D015704:CD4 Antigens; D005456:Fluorescent Dyes; D015686:Gene Products, env; D015700:HIV Envelope Protein gp41; D010455:Peptides; D019718:Receptors, CXCR4