Browsing by Author "McClellan, Amie J."
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Item Diverse Cellular Functions of the Hsp90 Molecular Chaperone Uncovered Using Systems Approaches(Elsevier, 2007-10) McClellan, Amie J.; Xia, Yu; Deutschbauer, Adam M.; Davis, Ron W.; Gerstein, Mark; Frydman, JudithA comprehensive understanding of the cellular functions of the Hsp90 molecular chaperone has remained elusive. Although Hsp90 is essential, highly abundant under normal conditions, and further induced by environmental stress, only a limited number of Hsp90 ‘‘clients’’ have been identified. To define Hsp90 function, a panelofgenome-wide chemical-genetic screens in Saccharomyces cerevisiae were combined with bioinformatic analyses. This approach identified several unanticipated functions of Hsp90 under normal conditions and in response to stress. Under normal growth conditions, Hsp90 plays a major role in various aspects of the secretory pathway and cellular transport; during environmental stress, Hsp90 is required for the cell cycle, meiosis, and cytokinesis. Importantly, biochemical and cell biological analyses validated several of these Hsp90-dependent functions, highlighting the potential of our integrated global approach to uncover chaperone functions in the cell.Item Folding and Quality Control of the VHL Tumor Suppressor Proceed through Distinct Chaperone Pathways(Elsevier, 2005-06) McClellan, Amie J.; Scott, Melissa D.; Frydman, JudithThe mechanisms by which molecular chaperones assist quality control of cytosolic proteins are poorly understood. Analysis of the chaperone requirements for degradation of misfolded variants of a cytosolic protein, the VHL tumor suppressor, reveals that distinct chaperone pathways mediate its folding and quality control. While both folding and degradation of VHL require Hsp70, the chaperon in TRiC is essential for folding but is dispensable for degradation. Conversely, the chaperone Hsp90 neither participates in VHL folding nor is required to maintain misfolded VHL solubility but is essential for its degradation. The cochaperone HOP/Sti1p also participates in VHL quality control and may direct the triage decision by bridging the Hsp70-Hsp90 interaction. Our finding that a distinct chaperone complex is uniquely required for quality control provides evidence for active and specific chaperone participation in triage decisions and suggests that a hierarchy of chaperone interactions can control the alternate fates of a cytosolic protein.Item Heterozygous Yeast Deletion Collection Screens Reveal Essential Targets of Hsp90(Public Library of Science, 2011-11) Franzosa, Eric A.; Albanèse, Véronique; Frydman, Judith; Xia, Yu; McClellan, Amie J.Hsp90 is an essential eukaryotic chaperone with a role in folding specific ‘‘client’’ proteins such as kinases and hormone receptors. Previously performed homozygous diploid yeast deletion collection screens uncovered broad requirements for Hsp90 in cellular transport and cell cycle progression. These screens also revealed that the requisite cellular functions of Hsp90 change with growth temperature. We present here for the first time the results of heterozygous deletion collection screens conducted at the hypothermic stress temperature of 15°C. Extensive bioinformatic analyses were performed on the resulting data in combination with data from homozygous and heterozygous screens previously conducted at normal (30°C) and hyperthermic stress (37°C) growth temperatures. Our resulting meta-analysis uncovered extensive connections between Hsp90 and (1) general transcription, (2) ribosome biogenesis and (3) GTP binding proteins. Predictions from bioinformatic analyses were tested experimentally, supporting a role for Hsp90 in ribosome stability. Importantly, the integrated analysis of the 15°C heterozygous deletion pool screen with previously conducted 30°C and 37°C screens allows for essential genetic targets of Hsp90 to emerge. Altogether, these novel contributions enable a more complete picture of essential Hsp90 functions.Item Protein Quality Control: Chaperones Culling Corrupt Conformations(Nature Publishing Group, 2005-08) McClellan, Amie J.; Tam, Stephen; Kaganovich, Daniel; Frydman, JudithAchieving the correct balance between folding and degradation of misfolded proteins is critical for cell viability. The importance of defining the mechanisms and factors that mediate cytoplasmic quality control is underscored by the growing list of diseases associated with protein misfolding and aggregation. Molecular chaperones assist protein folding and also facilitate degradation of misfolded polypeptides by the ubiquitin–proteasome system. Here we discuss emerging links between folding and degradation machineries and highlight challenges for future research.Item Quality Control of Protein Folding in the Cytosol(Wilvey, 2012-05) McClellan, Amie J.In order to function properly, newly synthesised proteins must rapidly and efficiently attain their native conformations. If they fail to do so, the cell may be adversely affected due to loss of function or toxic gain of function effects of misfolded polypeptides. Effective quality control mechanisms to recognise and eliminate misfolded proteins are thus critical for cell viability.The primary means by which misfolded proteins are selectively removed from the cell is via the ubiquitin–proteasome system. Although much is known about regulated proteolysis, how any given protein,which could potentially misfold, is recognised and targeted for proteasome-mediated degradation has been challenging to decipher. Recent progress,much of it in yeast, has identified specific E3 ligases involved in this process,clarified or added to our knowledge of the roles of molecular chaperones,and identified multiple cellular locations where degradation, or failing that, aggregation, occurs.