The candidate tumor suppressor p73 has a high sequence homology with p53 within the NH2-terminal transactivation domain, the sequence-specific DNA-binding region, and the oligomerization domain. However, p73 alpha, 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 p73 alpha or other p73 splicing variants. To examine the functional role of the p73 alpha COOH-terminal region, we generated a series of p73 alpha truncation mutants including p73 alpha(1-247) (retaining only a transactivation domain), p73 alpha(1-427) (lacking the most COOH-terminal region including a SAM domain), and p73 alpha(1-548) (deleting an extreme COOH-terminal region except a SAM domain). When transfected into COS cells, all of p73 alpha, p73 alpha(1-548), and p73 alpha(1-427) localized in the cellular nucleus, whereas p73 alpha(1-247) localized in both nucleus and cytoplasm. intriguingly, when compared with p73 alpha, both p73 alpha(1-427) and p73 alpha(1-548) showed a significant stimulation of the transcription of luciferase reporters harboring three p53-responsive promoters (p2(WafI), Mdm2, and Bax) in p53-deficient SAOS-2 cells. Gel retardation assays showed that DNA-binding activity of p73 alpha(1-427) and p73 alpha(1-548) was increased as compared with that of the full-length p73 alpha. However, the colony formation assays using SAOS-2 cells demonstrated that, contrary to p73 alpha, transfection of p73 alpha(1-427) or p73 alpha(1-548) resulted in no significant reduction of the number of colonies. These suggest that the distal COON-terminal region of p73 alpha is a cis- or trans-acting regulatory domain and regulates its functions diversely.
