title: Cysteine residues in a yeast viral A/B toxin crucially control host cell killing via pH-triggered disulfide rearrangements.
abstract: K28 is a viral A/B protein toxin that intoxicates yeast and fungal cells by endocytosis and retrograde transport to the endoplasmic reticulum (ER). Although toxin translocation into the cytosol occurs on the oxidized α/β heterodimer, the precise mechanism of how the toxin crosses the ER membrane is unknown. Here we identify pH-triggered, toxin-intrinsic thiol rearrangements that crucially control toxin conformation and host cell killing. In the natural habitat and low-pH environment of toxin-secreting killer yeasts, K28 is structurally stable and biologically active as a disulfide-bonded heterodimer, whereas it forms inactive disulfide-bonded oligomers at neutral pH that are caused by activation and thiol deprotonation of β-subunit cysteines. Because such pH increase reflects the pH gradient during compartmental transport within target cells, potential K28 oligomerization in the ER lumen is prevented by protein disulfide isomerase. In addition, we show that pH-triggered thiol rearrangements in K28 can cause the release of cytotoxic α monomers, suggesting a toxin-intrinsic mechanism of disulfide bond reduction and α/β heterodimer dissociation in the cytosol.
authors.affiliation: Molecular and Cell Biology, Department of Biosciences, and Center of Human and Molecular Biology (ZHMB), Saarland University, D-66123 Saarbruecken, Germany.
authors.forename: Yutaka
authors.initials: Y
authors.lastname: Suzuki
authors.affiliation: Molecular and Cell Biology, Department of Biosciences, and Center of Human and Molecular Biology (ZHMB), Saarland University, D-66123 Saarbruecken, Germany.
authors.forename: Sara L
authors.initials: SL
authors.lastname: Schwartz
authors.affiliation: Molecular and Cell Biology, Department of Biosciences, and Center of Human and Molecular Biology (ZHMB), Saarland University, D-66123 Saarbruecken, Germany.
authors.forename: Nina C
authors.initials: NC
authors.lastname: Mueller
authors.affiliation: Molecular and Cell Biology, Department of Biosciences, and Center of Human and Molecular Biology (ZHMB), Saarland University, D-66123 Saarbruecken, Germany mjs@microbiol.uni-sb.de.
authors.forename: Manfred J
authors.initials: MJ
authors.lastname: Schmitt
mesh_terms: D001692:Biological Transport; D003545:Cysteine / Q000378:metabolism*; D003600:Cytosol / Q000378:metabolism; D004220:Disulfides / Q000378:metabolism*; D004705:Endocytosis; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D055646:Killer Factors, Yeast / Q000235:genetics / Q000378:metabolism*; D019704:Protein Disulfide-Isomerases / Q000378:metabolism; D017510:Protein Folding; D011499:Protein Processing, Post-Translational; D021381:Protein Transport; D012441:Saccharomyces cerevisiae / Q000378:metabolism; D029701:Saccharomyces cerevisiae Proteins / Q000378:metabolism
chemical_list: D004220:Disulfides; D055646:Killer Factors, Yeast; D029701:Saccharomyces cerevisiae Proteins; D019704:Protein Disulfide-Isomerases; D003545:Cysteine