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Eukaryotic Cell, December 2005, p. 2008-2016, Vol. 4, No. 12
1535-9778/05/$08.00+0     doi:10.1128/EC.4.12.2008-2016.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

Regulation and Recovery of Functions of Saccharomyces cerevisiae Chaperone BiP/Kar2p after Thermal Insult

Program in Cellular Biotechnology, Institute of Biotechnology, P.O. Box 56, 00014 University of Helsinki, Finland,¹ Department of Applied Chemistry and Microbiology, P.O. Box 27, 00014 University of Helsinki, Finland²

Received 22 June 2005/ Accepted 20 September 2005

We described earlier a novel mode of regulation of Hsp104, a cytosolic chaperone directly involved in the refolding of heat-denatured proteins, and designated it delayed upregulation, or DUR. When Saccharomyces cerevisiae cells grown at the physiological temperature of 24°C, preconditioned at 37°C, and treated briefly at 50°C were shifted back to 24°C, Hsp104 expression was strongly induced after 2.5 h of recovery and returned back to normal after 5 h. Here we show that the endoplasmic reticulum (ER) chaperones BiP/Kar2p and Lhs1p and the mitochondrial chaperone Hsp78 were also upregulated at the physiological temperature during recovery from thermal insult. The heat shock element (HSE) in the KAR2 promoter was found to be sufficient to drive DUR. The unfolded protein element could also evoke DUR, albeit weakly, in the absence of a functional HSE. BiP/Kar2p functions in ER translocation and assists protein folding. Here we found that the synthesis of new BiP/Kar2p molecules was negligible for more than an hour after the shift of the cells from 50°C to 24°C. Concomitantly, ER translocation was blocked, suggesting that preexisting BiP/Kar2p molecules or other necessary proteins were not functioning. Translocation resumed concomitantly with enhanced synthesis of BiP/Kar2p after 3 h of recovery, after which ER exit and protein secretion also resumed. For a unicellular organism like S. cerevisiae, conformational repair of denatured proteins is the sole survival strategy. Chaperones that refold proteins in the cytosol, ER, and mitochondria of S. cerevisiae appear to be subject to DUR to ensure survival after thermal insults.

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