Dimers and complexes with p53 are the prevalent oligomeric forms of a transforming nonkaryophilic T antigen of simian virus 40

The emerging picture of p53

1. The cellular phosphoprotein p53 is a negative regulator of the cell growth. A great majority of human malignancies expresses a mutated p53 that represents an oncogenic version of the protein. 2. However, in the meantime many tumors were identified containing a p53 protein without any mutation. Here also other events than genomic alterations of p53 might be implicated in the process of cell transformation. 3. The expression of wild-type or mutant conformation is not exclusively defined by the p53 DNA sequence but also influenced by the subcellular environment and the interaction of cellular proteins with p53. 4. In particular, the mdm-2 gene product appears to be an important partner of p53 somehow involved in these complex regulatory processes. 5. Recent findings supported a role for p53 in transcriptional regulation, perhaps by reducing the expression of genes that are needed for ongoing cell proliferation. 6. This property may be based upon the ability of p53 to bind DNA as well as different proteins from viral or cellular origin. 7. Especially transcription factors or further cellular proteins connected in any way with the regulation of cell proliferation are possible candidates. 8. Thus, it is not surprising that p53 is implicated in the regulation of the cell cycle and in the decision of a cell to replicate DNA or to go into apoptosis.

Cell-dependent efficiency of reiterated nuclear signals in a mutant simian virus 40 oncoprotein targeted to the nucleus

We investigated the requisites for, and functional consequences of, the relocation to the nucleus of a transforming nonkaryophilic mutant of the simian virus 40 large T antigen (a natural deletion mutant lacking an internal large-T-antigen domain that includes the signal for nuclear transport). Synthetic oligonucleotides were used to obtain gene variants with one or more copies of the signal-specifying sequence inserted near the gene 3' end, in a region dispensable for the main large-T-antigen functions. The analysis of stable transfectant populations showed that mouse NIH 3T3 cells, rat embryo fibroblasts, and simian CS cells (a subclone of CV1 cells) differed considerably in their ability to localize some variant molecules into the nucleus. CS cells were always the most efficient, and NIH 3T3 cells were the least efficient. The nuclear localization improved either with reiteration of the signal or with a left-flank modification of the signal amino acid context. Three signals appeared to be necessary and sufficient, even in NIH 3T3 cells, to obtain a nuclear accumulation comparable to that of wild-type simian virus 40 large T antigen; other signal-cell combinations caused a large variability in subcellular localization among cells of the same population, as if the nuclear uptake of some molecules depended on individual cell states. The effect of the modified location on the competence of the protein to alter cell growth was examined by comparing the activity of variants containing either the normal signal or a signal with a mutation (corresponding to large-T-antigen amino acid 128) that prevented nuclear transport. It was found that the nuclear variant was slightly more active than the cytoplasmic variants in rat embryo fibroblasts and NIH 3T3 cells and was notably less active in CS cells.

Cell-dependent efficiency of reiterated nuclear signals in a mutant simian virus 40 oncoprotein targeted to the nucleus

We investigated the requisites for, and functional consequences of, the relocation to the nucleus of a transforming nonkaryophilic mutant of the simian virus 40 large T antigen (a natural deletion mutant lacking an internal large-T-antigen domain that includes the signal for nuclear transport). Synthetic oligonucleotides were used to obtain gene variants with one or more copies of the signal-specifying sequence inserted near the gene 3' end, in a region dispensable for the main large-T-antigen functions. The analysis of stable transfectant populations showed that mouse NIH 3T3 cells, rat embryo fibroblasts, and simian CS cells (a subclone of CV1 cells) differed considerably in their ability to localize some variant molecules into the nucleus. CS cells were always the most efficient, and NIH 3T3 cells were the least efficient. The nuclear localization improved either with reiteration of the signal or with a left-flank modification of the signal amino acid context. Three signals appeared to be necessary and sufficient, even in NIH 3T3 cells, to obtain a nuclear accumulation comparable to that of wild-type simian virus 40 large T antigen; other signal-cell combinations caused a large variability in subcellular localization among cells of the same population, as if the nuclear uptake of some molecules depended on individual cell states. The effect of the modified location on the competence of the protein to alter cell growth was examined by comparing the activity of variants containing either the normal signal or a signal with a mutation (corresponding to large-T-antigen amino acid 128) that prevented nuclear transport. It was found that the nuclear variant was slightly more active than the cytoplasmic variants in rat embryo fibroblasts and NIH 3T3 cells and was notably less active in CS cells.