Initial characterization of heat-induced excess nuclear proteins in HeLa cells

Exposure of mammalian cells to hyperthermia is known to cause protein aggregation in the nucleus. The presence of such aggregates has been detected as the relative increase in the protein mass that is associated with nuclei isolated from heated cells. We have characterized these excess nuclear proteins from the nuclei of heated HeLa cells by two-dimensional gel electrophoresis. The abundance of cytoskeletal elements which co-purify with the nuclei did not increase with exposure to hyperthermia, indicating that these proteins are not part of the excess nuclear proteins. In contrast, several specific polypeptides become newly bound or increase in abundance in nuclei isolated from heated cells. Members of the hsp 70 family were identified as a major component of the excess nuclear proteins. Among the other excess nuclear proteins we identified ten that had apparent molecular weights of 130, 95, 75, 58, 53, 48, 46, 37, 28, and 26 kilodaltons. Since hsp 70 is mainly cytoplasmic in non-heated cells, its association with nuclei in heated cells indicates that one mechanism accounting for the heat-induced excess nuclear proteins is the movement of cytoplasmic proteins to the nucleus. We also obtained evidence that increased binding of nuclear proteins is another mechanism for this effect. No overall increase or decrease in the phosphorylation of nuclear proteins was found to be associated with such altered binding or movement from the cytoplasm to the nucleus.