Regulation and recovery of functions of Saccharomyces cerevisiae chaperone BiP/Kar2p after thermal insult.

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 degrees C, preconditioned at 37 degrees C, and treated briefly at 50 degrees C were shifted back to 24 degrees 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 degrees C to 24 degrees 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.

Upregulation of the Hsp104 chaperone at physiological temperature during recovery from thermal insult.

Thermal insult at 50 degrees C causes protein denaturation in yeast, but the cells survive if preconditioned at 37 degrees C. Survival depends on refolding of heat-denatured proteins. Refolding of cytoplasmic proteins requires Hsp104, the expression of which increases several-fold upon shift of the cells from physiological temperature 24 degrees C to 37 degrees C. We describe here a novel type of regulation of Hsp104, designated delayed upregulation (DUR). When Saccharomyces cerevisiae cells grown at 24 degrees C, preconditioned at 37 degrees C and treated briefly at 50 degrees C were shifted back to 24 degrees C, Hsp104 expression was negligible for 1 h, but increased then to a three to nine times higher level than that detected after growth at 24 degrees C, returning to normal after 5 h. A heat shock element (HSE) of the upstream sequence of HSP104 was necessary and sufficient for DUR, whereas stress response elements (STRE) were dispensable. Destruction of HSE plus all three STREs abolished Hsp104 expression, resulting in cell death after thermal insult. Deletion of MSN2/4, encoding transcription factors driving STRE-dependent gene expression, decreased DUR. Deletion of HOG1, encoding a heat-responsive and osmosensitive mitogen-activated protein kinase implicated to be functionally connected to Msn2/4p, abolished DUR. We suggest that DUR was regulated via HSE, required Hog1p and involved Msn2/4p-regulated gene products.