The p53 isoform Δp53 lacks intrinsic transcriptional activity and reveals the critical role of nuclear import in dominant-negative activity

The transcription factor p53 is one of the most frequently mutated tumor suppressors. Recent progress has unraveled several novel isoforms of p53. Intriguingly, one of the p53 isoform, Delta p53, which lacks part of the DNA binding domain, was reported to be transcriptionally active toward some p53 target genes and is critical for the intra-S phase checkpoint. Here, we show that, in contrast to full-length p53, ectopically expressed Delta p53 neither transactivated the promoters of p21 (CIP1/WAF1) or murine double minute-2 (MDM2) nor repressed the cyclin B1 promoter in unstressed H1299 cells. Due to the deletion of a nuclear localization signal, Delta p53 was not imported into the nucleus. Engineering of nuclear localization signals to Delta p53 restored nuclear accumulation. However, the nuclear-targeting Delta p53 remained inactive, indicating that the lack of intrinsic activity of Delta p53 was not simply due to subcellular localization but to its incomplete DNA binding domain. Similar to p53, Ap53 was subjected to MDM2-mediated ubiquitination/proteolysis. The cytoplasmic localization of Ap53 correlated with the instability of the protein because forcing Ap53 into the nucleus increased its stability. Although Delta p53 could form a complex with p53 and stimulated the cytoplasmic retention of p53, it was not a robust inhibitor of p53. Targeting Delta p53 into the nucleus enhanced the dominant-negative activity of Delta p53. These observations underscore the critical role of subcellular localization in the dominant-negative action of p53.