Overexpression of heat shock protein 70 restores the structural stability and functional defects of temperature-sensitive mutant of large T antigen at nonpermissive temperature

The effects of heat shock protein 70 (Hsp70), a molecular chaperone, on the degradation and functional alterations of a mutant large T antigen induced by a nonpermissive temperature were examined. In this study, mouse tracheal epithelial TM02-3 cells harboring temperature-sensitive simian virus 40 large T antigen and stable TM02-3 cells overexpressing human Hsp70 and/or Hsp40 were used. Although the temperature shift from 33°C (permissive temperature) to 39°C (nonpermissive temperature) induced increases in the endogenous chaperones including Hsp70 and Hsp40, degradation of the T antigen, activation of the p53-p21^waf1 pathway, and an arrest of cell growth were observed in the mock cells. In contrast, these changes induced by the temperature shift were partially but significantly prevented in stable cells overexpressing human Hsp70 and/or Hsp40. A combination of Hsp70 and Hsp40 was the most effective, suggesting that Hsp40 may cooperate with Hsp70. Moreover, immunocytochemical observation indicated that human Hsp70 was expressed in the cytoplasm at 33°C, but it colocalized with T antigen in the nucleus at 39°C. These results suggest that overexpressed Hsp70 translocates from the cytoplasm to nucleus, and significantly restores the structural stability and functional defects of mutant large T antigen in the cells.