Effects of heat shock on nuclear and nucleolar protein phosphorylation in Chinese hamster ovary cells

Distribution and 32P labeling of nuclear and nucleolar phosphoproteins were studied in Chinese hamster ovary cells incubated at supranormal temperature (1 h at 43 degrees C). The heat shock induced the phosphorylation of a nucleolar protein with a molecular weight of 95,000. Similarly, in the non-nucleolar fraction of the nucleus, phosphorylation of a 54,000-Mr protein was induced while a protein with a molecular weight of 35,000 was rapidly dephosphorylated. Except for these definite changes, the labeling pattern of proteins that were 32P labeled prior to the heat shock was not affected. During recovery at 37 degrees C, the efficiency of labeling was reduced (10-50% of control values accoring to the subcellular fraction) and certain changes in labeling of phosphoproteins were detected. The nucleolar protein of 95,000 Mr was no longer phosphorylated. Phosphorylation of the 54,000-Mr protein, induced by heat shock, continued during the first 4 h of recovery at 36 degrees C and then stopped. The 35,000-Mr protein was slightly phosphorylated during the first hours at 37 degrees C and progressively reached its control level by 7 h. These findings,along with previous results on the effects of heat shock on RNA synthesis, suggest that the phosphorylation level of the three phosphoproteins may play a role in the regulation of transcription and in RNA processing.