Functions as a histone acetyltransferase (HAT) to promote transcriptional activation. Has significant histone acetyltransferase activity with core histones (H3 and H4); and also with nucleosome core particles. Also acetylates non-histone proteins; such as ACLY. Inhibits cell-cycle progression and counteracts the mitogenic activity of the adenoviral oncoprotein E1A. In case of HIV-1 infection; it is recruited by the viral protein Tat. Regulates Tat's transactivating activity and may help inducing chromatin remodeling of proviral genes. Acts as a circadian transcriptional coactivator which enhances the activity of the circadian transcriptional activators: NPAS2-ARNTL/BMAL1 and CLOCK-ARNTL/BMAL1 heterodimers. {ECO:0000269|PubMed:10675335; ECO:0000269|PubMed:14645221; ECO:0000269|PubMed:23932781; ECO:0000269|PubMed:8684459; ECO:0000269|PubMed:9707565}.