A transforming p53 mutant, which binds DNA, transactivates and induces apoptosis reveals a nuclear:cytoplasmic shuttling defect.

The DG75 Burkitt lymphoma-derived human B cell line is heterozygous for p53, carrying wild type (WT) and mutant (Arg283His) alleles. The cells constitutively express high levels of both p53 proteins and also Mdm2. Arg283His transactivates the p21Waf1, Mdm2, bax, cyclin G and IGF-BP3 promoters in transient transfection assays equally as well as, if not better than WT p53. It also suppresses the outgrowth of SAOS-2 cells and specifically binds DNA like wild type protein. However, in primary rodent fibroblasts Arg283His fails to suppress transformation by HPV16-E7 and (Ha-)ras and even has modest transforming activity when transfected alone with (Ha-)ras. When Arg283His is transiently transfected into SAOS-2 cells it efficiently induces apoptosis, so - unlike mutants such as Arg175Pro - its behaviour in transformation assays does not clearly correlate with loss of the apoptosis function. Immunofluorescence staining of both REF transformants and transiently transfected SAOS-2 revealed that this unusual mutant becomes excluded from the nucleus and produces striking cytoplasmic fluorescence. The best correlation with transformation, therefore, appears to be the lack of nuclear retention of Arg283His. Since this mutation does not map to any known nuclear localization signal and its presence seems to result in aberrant exclusion from the nucleus, then it may prove very useful in exploring mechanisms involved in the nuclear:cytoplasmic shuttling of p53.