title: Ubiquitin conjugation triggers misfolded protein sequestration into quality control foci when Hsp70 chaperone levels are limiting.
abstract: Ubiquitin accumulation in amyloid plaques is a pathological marker observed in the vast majority of neurodegenerative diseases, yet ubiquitin function in these inclusions is controversial. It has been suggested that ubiquitylated proteins are directed to inclusion bodies under stress conditions, when both chaperone-mediated refolding and proteasomal degradation are compromised or overwhelmed. Alternatively, ubiquitin and chaperones may be recruited to preformed inclusions to promote their elimination. We address this issue using a yeast model system, based on expression of several mildly misfolded degradation substrates in cells with altered chaperone content. We find that the heat shock protein 70 (Hsp70) chaperone pair Ssa1/Ssa2 and the Hsp40 cochaperone Sis1 are essential for degradation. Substrate ubiquitylation is strictly dependent on Sis1, whereas Ssa1 and Ssa2 are dispensable. Remarkably, in Ssa1/Ssa2-depleted cells, ubiquitylated substrates are sequestered into detergent-insoluble, Hsp42-positive inclusion bodies. Unexpectedly, sequestration is abolished by preventing substrate ubiquitylation. We conclude that Hsp40 is required for the targeting of misfolded proteins to the ubiquitylation machinery, whereas the decision to degrade or sequester ubiquitylated proteins is mediated by the Hsp70s. Accordingly, diminished Hsp70 levels, as observed in aging or certain pathological conditions, might be sufficient to trigger ubiquitin-dependent sequestration of partially misfolded proteins into inclusion bodies.
authors: Department of Biological Chemistry, Hebrew University of Jerusalem, Jerusalem 91904, Israel.
authors: Ayala
authors: A
authors: Shiber
authors: William
authors: W
authors: Breuer
authors: Michael
authors: M
authors: Brandeis
authors: Tommer
authors: T
authors: Ravid
mesh_terms: D000251:Adenosine Triphosphatases / Q000235:genetics* / Q000378:metabolism; D003593:Cytoplasm / Q000378:metabolism; D015966:Gene Expression Regulation, Fungal; D050956:HSP40 Heat-Shock Proteins / Q000235:genetics* / Q000378:metabolism; D018840:HSP70 Heat-Shock Proteins / Q000235:genetics* / Q000378:metabolism; D002479:Inclusion Bodies / Q000378:metabolism; D046988:Proteasome Endopeptidase Complex / Q000235:genetics* / Q000378:metabolism; D011485:Protein Binding; D017510:Protein Folding; D055550:Protein Stability; D059748:Proteolysis; D012441:Saccharomyces cerevisiae / Q000235:genetics* / Q000378:metabolism; D029701:Saccharomyces cerevisiae Proteins / Q000235:genetics* / Q000378:metabolism; D025801:Ubiquitin / Q000235:genetics* / Q000378:metabolism
chemical_list: D050956:HSP40 Heat-Shock Proteins; D018840:HSP70 Heat-Shock Proteins; C070320:SIS1 protein, S cerevisiae; C542288:SSA2 protein, S cerevisiae; D029701:Saccharomyces cerevisiae Proteins; D025801:Ubiquitin; D046988:Proteasome Endopeptidase Complex; D000251:Adenosine Triphosphatases; C095973:SSA1 protein, S cerevisiae
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