title: Crystal structures of beta- and gammaretrovirus fusion proteins reveal a role for electrostatic stapling in viral entry.
abstract: Membrane fusion is a key step in the life cycle of all envelope viruses, but this process is energetically unfavorable; the transmembrane fusion subunit (TM) of the virion-attached glycoprotein actively catalyzes the membrane merger process. Retroviral glycoproteins are the prototypical system to study pH-independent viral entry. In this study, we determined crystal structures of extramembrane regions of the TMs from Mason-Pfizer monkey virus (MPMV) and xenotropic murine leukemia virus-related virus (XMRV) at 1.7-Å and 2.2-Å resolution, respectively. The structures are comprised of a trimer of hairpins that is characteristic of class I viral fusion proteins and now completes a structural library of retroviral fusion proteins. Our results allowed us to identify a series of intra- and interchain electrostatic interactions in the heptad repeat and chain reversal regions. Mutagenesis reveals that charge-neutralizing salt bridge mutations significantly destabilize the postfusion six-helix bundle and abrogate retroviral infection, demonstrating that electrostatic stapling of the fusion subunit is essential for viral entry. Our data indicate that salt bridges are a major stabilizing force on the MPMV and XMRV retroviral TMs and likely provide the key energetics for viral and host membrane fusion.
authors.affiliation: Department of Laboratory Medicine and Pathobiology, Faculty of Medicine, University of Toronto, Toronto, ON, Canada.
authors.forename: Halil
authors.initials: H
authors.lastname: Aydin
authors.forename: Jonathan D
authors.initials: JD
authors.lastname: Cook
authors.forename: Jeffrey E
authors.initials: JE
authors.lastname: Lee
mesh_terms: D000595:Amino Acid Sequence; D030121:Betaretrovirus / Q000737:chemistry* / Q000502:physiology; D002942:Circular Dichroism; D003460:Crystallization; D017988:Gammaretrovirus / Q000737:chemistry* / Q000502:physiology; D057809:HEK293 Cells; D006801:Humans; D008561:Membrane Fusion; D008958:Models, Molecular; D008969:Molecular Sequence Data; D011487:Protein Conformation; D011993:Recombinant Fusion Proteins / Q000737:chemistry*; D017386:Sequence Homology, Amino Acid; D055672:Static Electricity; D014759:Viral Envelope Proteins / Q000737:chemistry*
chemical_list: D011993:Recombinant Fusion Proteins; D014759:Viral Envelope Proteins