title: Endoplasmic reticulum-associated degradation is required for cold adaptation and regulation of sterol biosynthesis in the yeast Saccharomyces cerevisiae.
abstract: Endoplasmic reticulum-associated degradation (ERAD) mediates the turnover of short-lived and misfolded proteins in the ER membrane or lumen. In spite of its important role, only subtle growth phenotypes have been associated with defects in ERAD. We have discovered that the ERAD proteins Ubc7 (Qri8), Cue1, and Doa10 (Ssm4) are required for growth of yeast that express high levels of the sterol biosynthetic enzyme, 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGR). Interestingly, the observed growth defect was exacerbated at low temperatures, producing an HMGR-dependent cold sensitivity. Yeast strains lacking UBC7, CUE1, or DOA10 also assembled aberrant karmellae (ordered arrays of membranes surrounding the nucleus that assemble when HMGR is expressed at high levels). However, rather than reflecting the accumulation of abnormal karmellae, the cold sensitivity of these ERAD mutants was due to increased HMGR catalytic activity. Mutations that compromise proteasomal function also resulted in cold-sensitive growth of yeast with elevated HMGR, suggesting that improper degradation of ERAD targets might be responsible for the observed cold-sensitive phenotype. However, the essential ERAD targets were not the yeast HMGR enzymes themselves. The sterol metabolite profile of ubc7Delta cells was altered relative to that of wild-type cells. Since sterol levels are known to regulate membrane fluidity, the viability of ERAD mutants expressing normal levels of HMGR was examined at low temperatures. Cells lacking UBC7, CUE1, or DOA10 were cold sensitive, suggesting that these ERAD proteins have a role in cold adaptation, perhaps through effects on sterol biosynthesis.
authors.affiliation: Department of Chemistry, Seattle University, Seattle, WI 98122, USA.
authors.forename: Jennifer
authors.initials: J
authors.lastname: Loertscher
authors.forename: Lynnelle L
authors.initials: LL
authors.lastname: Larson
authors.forename: Clinton K
authors.initials: CK
authors.lastname: Matson
authors.forename: Mark L
authors.initials: ML
authors.lastname: Parrish
authors.forename: Alicia
authors.initials: A
authors.lastname: Felthauser
authors.forename: Aaron
authors.initials: A
authors.lastname: Sturm
authors.forename: Christine
authors.initials: C
authors.lastname: Tachibana
authors.forename: Martin
authors.initials: M
authors.lastname: Bard
authors.forename: Robin
authors.initials: R
authors.lastname: Wright
mesh_terms: D000064:Acclimatization / Q000502:physiology*; D002352:Carrier Proteins / Q000235:genetics / Q000502:physiology*; D003080:Cold Temperature; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D017353:Gene Deletion; D008565:Membrane Proteins / Q000235:genetics / Q000502:physiology*; D010749:Phosphoprotein Phosphatases / Q000235:genetics / Q000378:metabolism*; D046988:Proteasome Endopeptidase Complex / Q000502:physiology; D012441:Saccharomyces cerevisiae / Q000502:physiology*; D029701:Saccharomyces cerevisiae Proteins / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D015398:Signal Transduction; D013261:Sterols / Q000032:analysis / Q000096:biosynthesis*; D044764:Ubiquitin-Activating Enzymes / Q000378:metabolism; D044763:Ubiquitin-Conjugating Enzymes / Q000235:genetics / Q000502:physiology*; D044767:Ubiquitin-Protein Ligases / Q000235:genetics / Q000502:physiology*; D015854:Up-Regulation
chemical_list: C109280:CUE1 protein, S cerevisiae; D002352:Carrier Proteins; D008565:Membrane Proteins; D029701:Saccharomyces cerevisiae Proteins; D013261:Sterols; C077072:UBC7 protein, S cerevisiae; D044763:Ubiquitin-Conjugating Enzymes; C091362:SSM4 protein, S cerevisiae; D044767:Ubiquitin-Protein Ligases; C024938:HMG-CoA reductase phosphatase; D010749:Phosphoprotein Phosphatases; D046988:Proteasome Endopeptidase Complex; D044764:Ubiquitin-Activating Enzymes