hSSB1 regulates both the stability and the transcriptional activity of p53.

The tumor suppressor p53 is essential for several cellular processes that are involved in the response to diverse genotoxic stress, including cell cycle arrest, DNA repair, apoptosis and senescence. Studies of the regulation of p53 have mostly focused on its stability and transactivation; however, new regulatory molecules for p53 have also been frequently identified. Here, we report that human ssDNA binding protein SSB1 (hSSB1), a novel DNA damage-associated protein, can interact with p53 and protect p53 from ubiquitin-mediated degradation. Furthermore, hSSB1 also associates with the acetyltransferase p300 and is required for efficient transcriptional activation of the p53 target gene p21 by affecting the acetylation of p53 at lysine382. Functionally, the hSSB1 knockdown-induced abrogation of the G2/M checkpoint is partially dependent on p53 or p300. Collectively, our results indicate that hSSB1 may regulate DNA damage checkpoints by positively modulating p53 and its downstream target p21.