Heat-induced protein dephosphorylation in Chinese hamster ovary cells

A brief heat shock of 10 min at 45 degrees C caused the selective dephosphorylation of a protein with Mr 50 kDa in Chinese hamster ovary (CHO) cells. Dephosphorylation was observed in the cytosolic, but not in the insoluble particulate cell fraction. Neither cycloheximide (10 micrograms/ml), actinomycin D (5 micrograms/ml), nor NaF (10 mM) reduced the loss of label from the 50 kDa band during hyperthermia. Alkali stability of the label suggests that the bonds in the 50 kDa phosphoprotein are of the phosphoserine or phosphothreonine type. The 50 kDa phosphoprotein band reappeared by 5 h post-hyperthermia, at a time when thermotolerance is known to be expressed. Progressive development of thermotolerance was accompanied by the phosphorylation of 2 major proteins with Mr 28 and 89 kDa. The 89 kDa band was clearly visible at 5 and 14 h posthyperthermia, but not at 23 h, when thermotolerance had begun to decay; the 28 kDa remained heavily labeled even at 23 h. The data show that hyperthermia induces major perturbations in cellular protein phosphorylation both during and after heating. Rapid dephosphorylation of the 50 kDa phosphoprotein during 10 min of heating may play a role in the initiation and regulation of subsequent protein glycosylation and thermotolerance expression.