title: Membrane fusion triggering: three modules with different structure and function in the upper half of the measles virus attachment protein stalk.
abstract: The measles virus (MV) fusion apparatus consists of a fusion protein and an attachment protein named hemagglutinin (H). After receptor-binding through its cuboidal head, the H-protein transmits the fusion-triggering signal through its stalk to the fusion protein. However, the structural basis of signal transmission is unclear because only structures of H-heads without their stalk have been solved. On the other hand, the entire ectodomain structure of the hemagglutinin-neuraminidase protein of another Paramyxovirus revealed a four-helix bundle stalk. To probe the structure of the 95-residue MV H-stalk we individually substituted head-proximal residues (positions 103-153) with cysteine, and biochemically and functionally characterized the resultant proteins. Our results indicate that most residues in the central segment (positions 103-117) can be cross-linked by engineered disulfide bonds, and thus may be engaged in a tetrameric structure. While covalent tetramerization disrupts fusion triggering function, disulfide bond reduction restores it in most positions except Asp-113. The next stalk segment (residues 123-138) also has high propensity to form covalent tetramers, but since these cross-links have little or no effect on function, it can conduct the fusion-triggering signal while remaining in a stabilized tetrameric configuration. This segment may act as a spacer, maintaining H-heads at an optimal height. Finally, the head-proximal segment (residues 139-154) has very limited propensity to trap tetramers, suggesting bifurcation into two flexible linkers clamped by inter-subunit covalent links formed by natural Cys-139 and Cys-154. We discuss the modular structure of the MV H-stalk in the context of membrane fusion triggering and cell entry by Paramyxoviruses.
authors.affiliation: Department of Molecular Medicine, Mayo Clinic and Virology and Gene Therapy Track, Mayo Graduate School, Rochester, Minnesota 55905, USA.
authors.forename: Chanakha K
authors.initials: CK
authors.lastname: Navaratnarajah
authors.forename: Surendra
authors.initials: S
authors.lastname: Negi
authors.forename: Werner
authors.initials: W
authors.lastname: Braun
authors.forename: Roberto
authors.initials: R
authors.lastname: Cattaneo
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D002522:Chlorocebus aethiops; D003545:Cysteine / Q000737:chemistry; D004220:Disulfides / Q000737:chemistry; D006388:Hemagglutinins / Q000737:chemistry; D008459:Measles virus / Q000378:metabolism*; D008561:Membrane Fusion; D008969:Molecular Sequence Data; D009154:Mutation; D011485:Protein Binding; D011487:Protein Conformation; D017434:Protein Structure, Tertiary; D014162:Transfection; D014709:Vero Cells; D014760:Viral Fusion Proteins / Q000737:chemistry; D053585:Virus Attachment; D053586:Virus Internalization
chemical_list: D004220:Disulfides; D006388:Hemagglutinins; D014760:Viral Fusion Proteins; D003545:Cysteine