NUCLEAR ACCUMULATION OF P53 PROTEIN IS MEDIATED BY SEVERAL NUCLEAR-LOCALIZATION SIGNALS AND PLAYS A ROLE IN TUMORIGENESIS

The basic carboxy terminus of p53 plays an important role in directing the protein into the nuclear compartment. The C terminus of the p53 molecule contains a cluster of several nuclear localization signals (NLSs) that mediate the migration of the protein into the cell nucleus. NLSI, the most active domain, is highly conserved in genetically diverged species and shares perfect homology with consesus NLS sequences found in other nuclear proteins. The other two NLSs, II and III, appear to be less effective and less conserved. Although nuclear localization is dictated primarily by the NLSs inherent in the primary amino acid sequence, the actual nuclear homing can be modified by interactions with other proteins expressed in the cell. Comparison between wild-type p53 and naturally occurring mutant p53 showed that both protein categories could migrate into the nucleus of rat primary embryonic fibroblasts by essentially similar mechanisms. Nuclear localization of both proteins was totally dependent on the existence of functional NLS domains. In COS cells, however, we found that NLS-deprived wild-type p53 molecules could migrate into the nucleus by complexing with another nuclear protein, simian virus 40 large-T antigen. Wild-type and mutant p53 proteins differentially complexed with viral or cellular proteins, which may significantly affect the ultimate compartmentalization of p53 in the cell; this finding suggests that the actual subcellular compartmentalization of may differ in various cell type milieux and may largely be affected by the ability of these proteins to complex with other proteins expressed in the cell. Experiments designed to test the physiological significance of p53 subcellular localization indicated that nuclear localization of mutant p53 is essential for this protein to enhance the process of malignant transformation of partially transformed cells, suggesting that p53 functions within the cell nucleus.