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The p53 Isoform p53 Lacks Intrinsic Transcriptional Activity and Reveals the Critical Role of Nuclear Import in Dominant-Negative Activity

Department of Biochemistry, Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong

Requests for reprints: Randy Y.C. Poon, Department of Biochemistry, Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong. Phone: 852-2358-8703; Fax: 852-2358-1552; E-mail: bcrandy{at}ust.hk.

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, 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 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, p53 was not imported into the nucleus. Engineering of nuclear localization signals to p53 restored nuclear accumulation. However, the nuclear-targeting p53 remained inactive, indicating that the lack of intrinsic activity of p53 was not simply due to subcellular localization but to its incomplete DNA binding domain. Similar to p53, p53 was subjected to MDM2-mediated ubiquitination/proteolysis. The cytoplasmic localization of p53 correlated with the instability of the protein because forcing p53 into the nucleus increased its stability. Although p53 could form a complex with p53 and stimulated the cytoplasmic retention of p53, it was not a robust inhibitor of p53. Targeting p53 into the nucleus enhanced the dominant-negative activity of p53. These observations underscore the critical role of subcellular localization in the dominant-negative action of p53. [Cancer Res 2007;67(5):1959–69]