Incorporation of [3 H]leucine, immunochemical analyses with a specific hamster HSP27 rabbit immunoserum, and [32 P]orthophosphate labeling were used to monitor synthesis, accumulation, and phosphorylation of HSP27 in Chinese hamster cells after induction of thermoresistance by arsenite, cycloheximide, A23187, and EGTA. In contrast to arsenite-induced thermotolerance, which develops in parallel to synthesis and accumulation of HSP27, enhanced thermoresistance observed immediately after incubating cells in the presence of cycloheximide, A23187, or EGTA is independent of HSP27 or other HSP accumulation. All these treatments, however, result in a rapid phosphorylation of preexisting HSP27. In view of previous results which indicated that HSP27 is involved in cell protection from thermal killing (J. Landry, P. Chrétien, H. Lambert, E. Hickey, and L. A. Weber, J. Cell Biol. 109, 7-15, 1989), it is proposed that activation of HSP27 through phosphorylation may be a key determinant in the regulation of cell thermosensitivity.

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