Nucleophosmin/B23 Negatively Regulates GCN5-dependent Histone Acetylation and Transactivation

Abstract

Nucleophosmin/B23 is a multifunctional phosphoprotein that is overexpressed in cancer cells and has been shown to be involved in both positive and negative regulation of transcription. In this study, we first identified GCN5 acetyltransferase as a B23-interacting protein by mass spectrometry, which was then confirmed by in vivo co-immunoprecipitation. An in vitro assay demonstrated that B23 bound the PCAF-N domain of GCN5 and inhibited GCN5-mediated acetylation of both free and mononucleosomal histones, probably through interfering with GCN5 and masking histones from being acetylated. Mitotic B23 exhibited higher inhibitory activity on GCN5-mediated histone acetylation than interphase B23. Immunodepletion experiments of mitotic extracts revealed that phosphorylation of B23 at Thr 199 enhanced the inhibition of GCN5-mediated histone acetylation. Moreover, luciferase reporter and microarray analyses suggested that B23 attenuated GCN5-mediated transactivation in vivo. Taken together, our studies suggest a molecular mechanism of B23 in the mitotic inhibition of GCN5-mediated histone acetylation and transactivation.The acetylation of nucleosomal histones by histone acetyltransferases (HATs) 5 has been known for several decades. Histone-modifying enzymes, such as GCN5 (general control of amino acid synthesis 5) in the Spt-Ada-Gcn5 acetyltransferase (SAGA) and Esa1p in the NuA4 complex, can acetylate specific lysine residues in histone N-terminal tails (1). According to the histone code hypothesis (2), site-specific acetylation of histone tails may trigger chromatin remodeling that leads to retention of effector proteins to active promoters and formation of transcriptionally active chromatin regions.GCN5 was originally identified as a transcriptional coactivator in yeast and was proposed to contribute to transcription by establishing interactions between certain activators and transcriptional complexes (3). GCN5 enhances transcription through its intrinsic acetyltransferase activity (4), which facilitates acetylation of histones and nonhistone substrates (3). Recruitment of the SAGA complex greatly increases transcriptional activation in vitro (5), and the requirement of GCN5 for chromatin remodeling has been demonstrated in vivo (6). However, the mechanism(s) that regulates GCN5 activity particularly in the context of histone acetylation has yet to be examined in detail. Potentially, GCN5 HAT activity can be regulated through posttranslational modifications, interacting with other proteins or at the level of substrate modifications. For example, phosphorylation by the Ku-DNA-dependent protein kinase or sumoylation may regulate GCN5-HAT activity (7,8). Also, interaction with the SANT domain of the Ada2 affects GCN5-HAT activity in yeast (9). Likewise, modification of substrates, such as phosphorylation of H3 serine 10, increases GCN5 acetylation activity toward lysine 14 of H3 (10, 11).Nucleophosmin/B23, also known as NPM1, NO38, or numatrin, has been identified as a phosphoprotein that possesses multiple cellular func...

Regulation of E2F1-induced Apoptosis by the Nucleolar Protein RRP1B

“…seen for both nucleophosmin and nucleolin, which recruit GCN5 and P/CAF respectively, to specific promoters for transcriptional regulation (51,58). Consistent with this hypothesis, RRP1B has been shown to bind acetylated lysine 5 of histone 4 and other nonubiquitous chromatin-binding proteins (68).…”

Knockdown of nucleophosmin induces S-phase arrest in HepG2 cells

“…NPM interacts with other proteins to function as an effector for several cell cycle regulators, including retinoblastoma protein[33], Myc[34], P53[35], P21[16], GCN5[36], MDM2, and ARF. NPM can also monitor the kinase activities of MEK[37], Cyclin E/CDK2[38], Cdk1, and Cdc25C[39].…”

NPM1/B23: A Multifunctional Chaperone in Ribosome Biogenesis and Chromatin Remodeling

“…Ac-NPM1 is preferentially found in the nucleoplasm in association with RNA polymerase II and the increase in Ac-NPM1 seen in cancer could be of mechanistic importance [139]. On the other hand, NPM1 binds GCN5 during mitosis to inhibit GCN5 mediated acetylation of free and mono-nucleosomal histones [70]. Fourth, NPM1 act as a corepressor or coactivator of transcription by binding to YY1, IRF1, p53, NF- κ B and other transcription factors (Table 1).…”