Effects of Heat Shock on Nuclear and Nucleolar Protein Phosphorylation in Chinese Hamster Ovary Cells

Abstract

Distribution and ³²P labeling of nuclear and nucleolar phosphoproteins were studied in Chinese hamster ovary cells incubated at supranormal temperature (1 h at 43°C). The heat shock induced the phosphorylation of a nucleolar protein with a molecular weight of 95000. Similarly, in the non-nucleolar fraction of the nucleus, phosphorylation of a 54000–M_r protein was induced while a protein with a molecular weight of 35000 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°C, the efficiency of labeling was reduced (10–50% of control values according to the subcellular fraction) and certain changes in labeling of phosphoproteins were detected. The nucleolar protein of 95000 M_r was no longer phosphorylated. Phosphorylation of the 54000-M_r protein, induced by heat shock, continued during the first 4 h of recovery at 37°C and then stopped. The 35000-M_r protein was slightly phosphorylated during the first hours at 37°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.