ENHANCED PHOSPHORYLATION OF A 65 KDA PROTEIN IS ASSOCIATED WITH RAPID INDUCTION OF STRESS PROTEINS IN 9L RAT-BRAIN TUMOR-CELLS

Induction of heat-shock proteins and glucose-regulated proteins in 9L rat brain tumor cells can be differentially elicited by sodium arsenite, cadmium chloride, zinc chloride, copper sulfate, sodium fluoride, and L-azetidine-2-carboxylic acid. The kinds of stress protein induced by the above chemicals varied considerably, mainly determined by the nature and the concentration of the chemicals, as well as the treatment protocols. In addition, at the concentrations where stress proteins can be induced, the above chemicals were able to suppress general protein synthesis and were cytotoxic. Enhanced phosphorylation of a protein with an apparent molecular weight of 65 kDa was detected during the induction of stress proteins except in azetidine treatments during which uptake of phosphate by the cells was impaired after prolonged incubation. The phosphate moiety on the 65 kDa phosphoprotein appeared to be alkaline-stable and two-dimensional gel electrophoresis revealed that the phosphoprotein resolved into four isoforms with isoelectric points ranging from 5.1 to 5.6. Enhanced phosphorylation of the same protein was also detected in heat-shocked and withangulatin A-treated 9L cells in which stress proteins were induced. It is suggested that this phosphoprotein may be a common target for heat stress response-stimulated phosphorylation and important in the further metabolic responses of the cell to stress.