CELLULAR-LOCALIZATION AND CELL-CYCLE REGULATION BY A TEMPERATURE-SENSITIVE P53-PROTEIN

Primary rat embryo fibroblasts were transformed by a p53 mutant (alanine to valine change at amino acid 135) plus ras. This p53val135 mutant is temperature sensitive for a conformational change detected by the binding of a monoclonal antibody, PAb246, which recognizes the wild-type protein or the great majority of p53val135 at 32.5-degrees-C. At 37-degrees-C, both mutant and wild-type p53 conformational forms co-exist in the cells, while at 39.5-degrees-C, the majority of the p53val135 in the cell is in a mutant conformation not recognized by PAb246 antibody. At 39.5-degrees-C, the mutant p53 is localized in the cytoplasm of the cell. At 32.5-degrees-C, the p53 protein enters the nucleus and stops the growth of these cells. At 37-degrees-C where there is a mixture of mutant and wild-type p53, the wild-type p53 protein is in a complex with hsc70 and mutant p53 protein in the cytoplasm of the cell during G1. This wild-type protein enters the nucleus as the cells enter the S-phase of the cell cycle. At 32.5-degrees-C, the cells stop replication and arrest at the G1/S border. After 48 hr at 32.5-degrees-C, 91% of the cells are in the G1 fraction of the cell cycle. The S-phase cells appear to be immune to the p53 negative regulation of growth until they enter the next G1 period. These data strongly suggest that mutant p53 proteins in transformed cells act to sequester the wild-type p53 protein in an hsc70-p53 complex, which resides in the cytoplasm during the stage of the cell cycle, G1, when nuclear wild-type p53 would normally act to regulate cell growth and progression through the cycle. In this way, mutant p53 proteins can act in a trans-dominant fashion to overcome growth regulation by the wild-type p53 allele and protein in a cell.