title: Characterization of the infectious salmon anemia virus fusion protein.
abstract: Infectious salmon anemia virus (ISAV) is an orthomyxovirus causing serious disease in Atlantic salmon (Salmo salar L.). This study presents the characterization of the ISAV 50-kDa glycoprotein encoded by segment 5, here termed the viral membrane fusion protein (F). This is the first description of a separate orthomyxovirus F protein, and to our knowledge, the first pH-dependent separate viral F protein described. The ISAV F protein is synthesized as a precursor protein, F0, that is proteolytically cleaved to F1 and F2, which are held together by disulfide bridges. The cleaved protein is in a metastable, fusion-activated state that can be triggered by low pH, high temperature, or a high concentration of urea. Cell-cell fusion can be initiated by treatment with trypsin and low pH of ISAV-infected cells and of transfected cells expressing F, although the coexpression of ISAV HE significantly improves fusion. Fusion is initiated at pH 5.4 to 5.6, and the fusion process is coincident with the trimerization of the F protein, or most likely a stabilization of the trimer, suggesting that it represents the formation of the fusogenic structure. Exposure to trypsin and a low pH prior to infection inactivated the virus, demonstrating the nonreversibility of this conformational change. Sequence analyses identified a potential coiled coil and a fusion peptide. Size estimates of F1 and F2 and the localization of the putative fusion peptide and theoretical trypsin cleavage sites suggest that the proteolytic cleavage site is after residue K276 in the protein sequence.
authors.affiliation: Department of Biology, University of Bergen, Thormøhlensgate 55, 5020 Bergen, Norway. Vidar.Aspehaug@bio.uib.no
authors.forename: Vidar
authors.initials: V
authors.lastname: Aspehaug
authors.forename: Aase B
authors.initials: AB
authors.lastname: Mikalsen
authors.forename: Michael
authors.initials: M
authors.lastname: Snow
authors.forename: Eirik
authors.initials: E
authors.lastname: Biering
authors.forename: Stéphane
authors.initials: S
authors.lastname: Villoing
mesh_terms: D020816:Amino Acid Motifs; D000595:Amino Acid Sequence; D000818:Animals; D002459:Cell Fusion; D002460:Cell Line; D006382:Hemadsorption; D006358:Hot Temperature; D006863:Hydrogen-Ion Concentration; D046829:Isavirus; D008969:Molecular Sequence Data; D011499:Protein Processing, Post-Translational; D020317:Salmo salar; D017422:Sequence Analysis, DNA; D017385:Sequence Homology; D014162:Transfection; D014357:Trypsin / Q000378:metabolism; D014508:Urea; D014760:Viral Fusion Proteins / Q000737:chemistry* / Q000235:genetics / Q000502:physiology*
chemical_list: D014760:Viral Fusion Proteins; D014508:Urea; D014357:Trypsin