Regulation of p53 and p73 function by PCAF and adenovirus E1B 55-kDa oncoprotein

Abstract

The E1B-55kDa (E1B) oncoprotein of human adenovirus binds to and inactivates p53, preventing the transcription of p53-responsive genes.The functions of p53 are regulated mainly at the level of post-translational modification. In order to address the mechanism by which E1B inactivate p53, we studied whether E1B interferes with p53 acetylation by PCAF. We show that E1B specifically inhibits p53 acetylation by PCAF in vivo and in vitro, while acetylation of histones and PCAF autoacetylation is not affected. Furthermore, the DNA-binding activity of p53 is diminished in cells expressing the E1B proteins.The central domain of PCAF, containing the HAT region, binds to a region near the C-terminus of p53 encompassing Lys-320, the specific PCAF acetylation site. It also binds to the C-terminus of E1B through its bromodomain. We further show that E1B interferes with the interaction between p53 and PCAF, thus suggesting that E1B might specifically inhibit PCAF acetylase function on p53 by preventing the enzyme-substrate interaction. Recently, a homologue of p53, called p73, was identified. It resembles p53 both in structure and function. Previous studies have shown that some cellular and viral proteins involved in the p53 network, such as Mdm2, p300/CBP and E4orf6, are also involved in the p73 network. These proteins regulate p53 and p73 in similar but distinct ways. On the other hand, other proteins including the three main viral oncoproteins (SV40 large T antigen, HPV E6 and Ad E1B) which inactivate p53, fail to inactivate p73. In order to shed light on the p73 network and compare the similarities and differences between the p53 and p73 pathways, we studied whether PCAF also activates p73. We show that PCAF binds to a central domain of p73 that is shared by all p73 isoforms, and a small C-terminal region that is unique to p73[alpha]. This region of p73[alpha] is acetylated by PCAF in vitro at lysine 623. p73[alpha], but not p73[bêta], is acetylated in vivo. We further show that PCAF significantly stimulates both p73[alpha] and p73[bêta] transactivation function.The HAT domain of PCAF is required for stimulation of p73[bêta]-mediated transcription. We also demonstrate that mutation of p73[alpha] K623 to R leads to a reduction in its transcription activity. These data suggest that PCAF serves as a transcriptional coactivator of p73. Consistent with other published results, we show that E1B failed to inactivate p73[alpha]- and p73[bêta]-dependent transcription or to inhibit PCAF function on p73.--Résumé abrégé par UMI.