Effect of heat shock on the intracellular distribution of proteins

Exposure of cells to heat induces thermotolerance, a transient resistance to subsequent heat challenges. It has been shown that thermotolerance is correlated in time with the enhanced synthesis of heat shock proteins. In this study, the association of induced heat shock proteins with various cellular fractions was investigated and the heat-induced changes in skeletal protein composition in thermotolerant and control cells was compared. All three major heat shock proteins induced in Chinese hamster fibroblasts after a 46 degrees C, 4-min heat treatment (70, 87, and 110 kDa) were purified with the cytoplasmic fraction, whereas only the 70-kDa protein was also found in other cell fractions, including that containing the cellular skeleton. Immediately after a second heat treatment at 45 degrees C for 45 min, the 110-kDa protein from thermotolerant cells also purified extensively with the cellular skeletal fraction. In this regard, the 110-kDa protein behaved similarly to many other cellular proteins, since we observed an overall temperature-dependent increase in the total labeled protein content of the high-salt-resistant cellular skeletal fraction after heat shock. Pulse-chase studies demonstrated that this increased protein content gradually returned to normal levels after a 3-hr incubation at 37 degrees C. The alteration or recovery kinetics of the total labeled protein content of the cellular skeletal fraction after heat shock did not correlate with the dramatic increase in survival observed in thermotolerant cells. The relationship between heat shock proteins and thermotolerance, therefore, does not correlate directly with changes in the heat-induced cellular alterations leading to differences in protein fractionation.