The p300-CBP-associated factor (PCAF) is a histone acetyltransferase (HAT) involved in the reversible acetylation of various transcriptional regulators, including the tumour suppressor p53. It is implicated in many cellular processes, such as transcription, differentiation, proliferation and apoptosis. We observed that knockdown of PCAF expression in HeLa or U2OS cell lines induces stabilization of the oncoprotein Hdm2, a RING finger E3 ligase primarily known for its role in controlling p53 stability. To investigate the molecular basis of this effect, we examined whether PCAF is involved in Hdm2 ubiquitination. Here, we show that PCAF, in addition to its acetyltransferase activity, possesses an intrinsic ubiquitination activity that is critical for controlling Hdm2 expression levels, and thus p53 functions. Our data highlight a regulatory crosstalk between PCAF and Hdm2 activities, which is likely to have a central role in the subtle control of p53 activity after DNA damage.

The rate of HIV-1 gene expression is a key step that determines the kinetics of virus spread and AIDS progression. Viral entry and gene expression were described to be the key determinants for cell permissiveness to HIV. Recent reports highlighted the involvement of miRNA in regulating HIV-1 replication post-transcriptionally. In this study we explored the role of cellular factors required for miRNA-mediated mRNA translational inhibition in regulating HIV-1 gene expression. Here we show that HIV-1 mRNAs associate and co-localize with components of the RNA Induced Silencing Complex (RISC), and we characterize some of the proteins required for miRNA-mediated silencing (miRNA effectors). RCK/p54, GW182, LSm-1 and XRN1 negatively regulate HIV-1 gene expression by preventing viral mRNA association with polysomes. Interestingly, knockdown of RCK/p54 or DGCR8 resulted in virus reactivation in PBMCs isolated from HIV infected patients treated with suppressive HAART.

Although the proteasome facilitates transcription from several yeast promoters, it is unclear if its role is proteolytic or which subunits are involved. We show that the proteasome regulates the HIV-1 promoter in both proteolytic and nonproteolytic modes. In the absence of transcription factor, Tat, proteasome was associated with promoter and coding regions, and its proteolytic activity regulated the level of basal transcription emanating from the promoter. Tat switched the proteasome to a nonproteolytic mode by recruiting a proteasome-associated protein, PAAF1, which favors proteasome dissociation into 19S and 20S particles. Gel filtration chromatography showed that expression of both Tat and PAAF1 enhanced the abundance of a 19S-like complex in nuclear extracts. 19S, but not 20S, subunits were strongly recruited to the promoter in the presence of Tat and PAAF1 and coactivated Tat-dependent transcription. 19S components facilitated transcriptional elongation and may be involved in clearance of paused transcriptional elongation complexes from the promoter.