Dual roles of HSP70 chaperone HSPA1 in quality control of nascent and newly synthesized proteins.

Exposure to heat stress triggers a well-defined acute response marked by HSF1-dependent transcriptional upregulation of heat shock proteins. Cells allowed to recover acquire thermotolerance, but this adaptation is poorly understood. By quantitative proteomics, we discovered selective upregulation of HSP70-family chaperone HSPA1 and its co-factors, HSPH1 and DNAJB1, in MCF7 breast cancer cells acquiring thermotolerance. HSPA1 was found to have dual function during heat stress response: (i) During acute stress, it promotes the recruitment of the 26S proteasome to translating ribosomes, thus poising cells for rapid protein degradation and resumption of protein synthesis upon recovery; (ii) during thermotolerance, HSPA1 together with HSPH1 maintains ubiquitylated nascent/newly synthesized proteins in a soluble state required for their efficient proteasomal clearance. Consistently, deletion of HSPH1 impedes thermotolerance and esophageal tumor growth in mice, thus providing a potential explanation for the poor prognosis of digestive tract cancers with high HSPH1 and nominating HSPH1 as a cancer drug target. We propose dual roles of HSPA1 either alone or in complex with HSPH1 and DNAJB1 in promoting quality control of nascent/newly synthesized proteins and cellular thermotolerance.