Abstract
During mating of Saccharomyces cerevisiae, two nuclei fuse to produce a single diploid nucleus. Two genes, KAR7 and KAR8, were previously identified by mutations that cause defects in nuclear membrane fusion. KAR7 is allelic to SEC71, a gene involved in protein translocation into the endoplasmic reticulum. Two other translocation mutants, sec63-1 and sec72Delta, also exhibited moderate karyogamy defects. Membranes from kar7/sec71Delta and sec72Delta, but not sec63-1, exhibited reduced membrane fusion in vitro, but only at elevated temperatures. Genetic interactions between kar7 and kar5 mutations were suggestive of protein-protein interactions. Moreover, in sec71 mutants, Kar5p was absent from the SPB and was not detected by Western blot or immunoprecipitation of pulse-labeled protein. KAR8 is allelic to JEMI, encoding an endoplasmic reticulum resident DnaJ protein required for nuclear fusion. Overexpression of KAR8/JEM1 (but not SEC63) strongly suppressed the mating defect of kar2-1, suggesting that Kar2p interacts with Kar8/Jem1p for nuclear fusion. Electron microscopy analysis of kar8 mutant zygotes revealed a nuclear fusion defect different from kar2, kar5, and kar7/sec71 mutants. Analysis of double mutants suggested that Kar5p acts before Kar8/Jem1p. We propose the existence of a nuclear envelope fusion chaperone complex in which Kar2p, Kar5p, and Kar8/Jem1p are key components and Sec71p and Sec72p play auxiliary roles.
Publication types
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Research Support, Non-U.S. Gov't
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Research Support, U.S. Gov't, P.H.S.
MeSH terms
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Alleles
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Biological Transport
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Cell Nucleus / genetics*
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Endoplasmic Reticulum / metabolism
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Fungal Proteins / genetics*
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Fungal Proteins / metabolism
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Gene Dosage
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Gene Expression Regulation, Fungal
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HSP40 Heat-Shock Proteins
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HSP70 Heat-Shock Proteins / genetics
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HSP70 Heat-Shock Proteins / metabolism
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Heat-Shock Proteins / genetics
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Heat-Shock Proteins / metabolism
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Membrane Fusion / genetics
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Membrane Glycoproteins / genetics*
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Membrane Glycoproteins / metabolism
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Membrane Proteins / genetics*
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Membrane Proteins / metabolism
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Membrane Transport Proteins*
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Microscopy, Electron
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Molecular Chaperones
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Mutation
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Nuclear Envelope / genetics
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Nuclear Proteins / genetics*
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Nuclear Proteins / metabolism
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SEC Translocation Channels
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Saccharomyces cerevisiae / genetics*
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Saccharomyces cerevisiae / physiology
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Saccharomyces cerevisiae / ultrastructure
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Saccharomyces cerevisiae Proteins*
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Suppression, Genetic
Substances
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Fungal Proteins
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HSP40 Heat-Shock Proteins
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HSP70 Heat-Shock Proteins
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Heat-Shock Proteins
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JEM1 protein, S cerevisiae
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KAR2 protein, yeast
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KAR5 protein, S cerevisiae
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Membrane Glycoproteins
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Membrane Proteins
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Membrane Transport Proteins
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Molecular Chaperones
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Nuclear Proteins
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SCJ1 protein, S cerevisiae
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SEC Translocation Channels
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SEC61 protein, S cerevisiae
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SEC63 protein, S cerevisiae
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SEC66 protein, S cerevisiae
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SEC72 protein, S cerevisiae
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Saccharomyces cerevisiae Proteins