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Endoplasmic reticulum-associated degradation is required for cold adaptation and regulation of sterol biosynthesis in the yeast Saccharomyces cerevisiae

Department of Chemistry, Seattle University, Seattle, WI 98122, USA.
Eukaryotic Cell (Impact Factor: 3.18). 05/2006; 5(4):712-22. DOI: 10.1128/EC.5.4.712-722.2006
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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.

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    • "The changes in sterol composition observed in doa10Δ mutants are expected to affect the membrane properties in these cells. Accordingly, we noticed that mutations in components of the Doa10 complex have a mild growth phenotype when grown at low temperatures (10°C), as previously reported (Loertscher et al., 2006 and data not shown). Altogether, the data presented so far indicate that sterol-dependent degradation of Erg1 by Doa10 is part of a feedback system essential for sterol homeostasis. "
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    ABSTRACT: Sterol homeostasis is essential for the function of cellular membranes and requires feedback inhibition of HMGR, a rate-limiting enzyme of the mevalonate pathway. As HMGR acts at the beginning of the pathway, its regulation affects the synthesis of sterols and of other essential mevalonate-derived metabolites, such as ubiquinone or dolichol. Here, we describe a novel, evolutionarily conserved feedback system operating at a sterol-specific step of the mevalonate pathway. This involves the sterol-dependent degradation of squalene monooxygenase mediated by the yeast Doa10 or mammalian Teb4, a ubiquitin ligase implicated in a branch of the endoplasmic reticulum (ER)-associated protein degradation (ERAD) pathway. Since the other branch of ERAD is required for HMGR regulation, our results reveal a fundamental role for ERAD in sterol homeostasis, with the two branches of this pathway acting together to control sterol biosynthesis at different levels and thereby allowing independent regulation of multiple products of the mevalonate pathway. DOI: http://dx.doi.org/10.7554/eLife.00953.001
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    • "Some KDEL-tagged recombinant protein can be also misfolded and delivered for degradation through an ERdependent mechanism named ''unfolded protein response'' or UPR (Sitia and Backaaman 2003). This pathway is functional either for endogenous or heterologous proteins (Loertscher et al. 2006). The K/HDEL system is common to all eukaryotes but zeins, in maize, use a different ER localization system consisting of specialized organelles called protein bodies (PB), which stably accumulate within the ER. "
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    • "Although some of the cold-responsive proteins might be essential for the cells to adapt and resume growth under the new unfavourable environmental conditions, some evidence suggests that the main aim of other common cold-shock responses is to protect cells against freeze injury (Fig. 1). For example, the stability of Ole1p, the only fatty-acid desaturase known in S. cerevisiae (Stukey et al., 1989, 1990), seems to be important for cold growth (Loertscher et al., 2006); however, overexpression of OLE1 does not confer growth advantages at low temperatures (Kajiwara et al., 2000). By contrast, production of recombinant desaturases increased the unsaturation index and fluidity of the yeast membrane, and positively influenced freeze tolerance of baker's yeast cells (Rodríguez-Vargas et al., 2007). "
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