Inhibits Induction of Apoptosis in Mammalian Cells Its Transactivation Function and DNA-binding Activity

The candidate tumor suppressor p73 has a high sequence homology with p53 within the NH2-terminal transactivation domain, the sequencespecific DNA-binding region, and the oligomerization domain. However, p73a, which is most abundantly expressed in many tissues and cells among the alternatively spliced forms of p73, has an additional long COOH-terminal tail that might distinguish the function of p53 and p73a or other p73 splicing variants. To examine the functional role of the p73a COOH-terminal region, we generated a series of p73a truncation mutants including p73a(1–247) (retaining only a transactivation domain), p73a(1– 427) (lacking the most COOH-terminal region including a SAM domain), and p73a(1–548) (deleting an extreme COOH-terminal region except a SAM domain). When transfected into COS cells, all of p73a, p73a(1–548), and p73a(1–427) localized in the cellular nucleus, whereas p73a(1–247) localized in both nucleus and cytoplasm. Intriguingly, when compared with p73a, both p73a(1–427) and p73a(1–548) showed a significant stimulation of the transcription of luciferase reporters harboring three p53responsive promoters (p21, Mdm2, and Bax) in p53-deficient SAOS-2 cells. Gel retardation assays showed that DNA-binding activity of p73a(1– 427) and p73a(1–548) was increased as compared with that of the fulllength p73a. However, the colony formation assays using SAOS-2 cells demonstrated that, contrary to p73a, transfection of p73a(1–427) or p73a(1–548) resulted in no significant reduction of the number of colonies. These suggest that the distal COOH-terminal region of p73a is a cisor trans-acting regulatory domain and regulates its functions diversely.