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Title: Mechanism of Protein Quality Control in the Cytosol in Budding Yeast
Keywords: Protein quality control, CytoQC, Chaperones, Proteasome, E3 ubiquitin ligase, Protein degradation
Issue Date: 2-Dec-2010
Citation: RUPALI PRASAD (2010-12-02). Mechanism of Protein Quality Control in the Cytosol in Budding Yeast. ScholarBank@NUS Repository.
Abstract: Intracellular quality control systems monitor protein conformational states. Irreversibly misfolded proteins are cleared through specialized degradation pathways. Their importance is underscored by numerous pathologies caused by aberrant proteins. In the cytosol, where most proteins are synthesized, quality control remains poorly understood. Stress-inducible chaperones and the 26S proteasome are known mediators but how their activities are linked is unclear. In this thesis, I have used Saccharomyces cerevisiae as a model organism to study the quality control of cytosolic misfolded proteins. To better understand quality control of cytosolic proteins in chapter 3 and 4 of this thesis, a panel of model misfolded substrates was analyzed in detail. Surprisingly, their degradation occurs not in the cytosol but in the nucleus (Prasad et al., 2010). Degradation is dependent on the E3 ubiquitin ligase San1p, known previously to direct the turnover of damaged nuclear proteins (Gardner et al., 2005). San1p, however, is not required for nuclear import of substrates. Two reasons can account for nuclear trafficking of misfolded cytosolic proteins. First, in S. cerevisie, nucleus accounts for over 80 % of proteasomes at steady state throughout the cell cycle, suggesting the requirement of nuclear import of misfolded cytosolic proteins. Second, by trafficking misfolded proteins in the nucleus, cells provide enough time for newly synthesized proteins to fold in proper conformation. One view asserts that a key strategy of protein quality control is the integration of timing devices to permit folding (Helenius and Aebi, 2004). As such, proteins failing to fold within a set window are targeted for degradation. Experimental precedence comes from ERAD studies where a sophisticated timing mechanism utilizes a series of glycosidases to set a time limit for folding (Clerc et al., 2009). Proteins still unfolded after the final trimming step by Htm1p are detected by the Yos9p ERAD factor (OS-9 in mammals), which binds the resulting glycan signal (Quan et al., 2008). In CytoQC, nuclear import of substrate can provide an analogous function. The detailed analyses of cytosolic substrates have provided a clue that the Hsp70 family proteins Ssa1p and Ssa2p and its co-chaperone Ydj1p are needed for efficient import and degradation. In chapter 5 of this thesis, I have described a genome wide genetic screen to identify the genes involved and to decipher the mechanism for quality control of cytosolic protein. Among the genes identified, there are genes that encodes for proteasomal subunits (RPN7 and RPN11) and UMP1, a chaperone required for assembly of 26S proteasome. Together all our data reveal a new function of the nucleus as a compartment central to the quality control of cytosolic proteins.
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