Specific Acetylation of Chromosomal Protein HMG-17 by PCAF Alters Its Interaction with Nucleosomes

Abstract

Nonhistone chromosomal proteins HMG-14 and HMG-17 are closely related nucleosomal binding proteins that unfold the higher-order chromatin structure, thereby enhancing the transcription and replication potential of chromatin. Here we report that PCAF, a transcription coactivator with intrinsic histone acetyltransferase activity, specifically acetylates HMG-17 but not HMG-14. Using mass spectrum sequence analysis, we identified the lysine at position 2 as the predominant site acetylated by PCAF. Lysine 2 is a prominent acetylation site in vivo, suggesting that this PCAF-mediated acetylation is physiologically relevant. Experiments with HMG-17 deletion mutants and competition studies with various protein fragments indicate that the specific acetylation of HMG-17 is not determined solely by the primary sequence near the acetylation site. By equilibrium dialysis we demonstrated that acetylation reduces the affinity of HMG-17 to nucleosome cores. In addition, we found that the binding of HMG-14 and HMG-17 to nucleosome cores inhibits the PCAF-mediated acetylation of histone H3. Thus, the presence of HMG-14 and HMG-17 affects the ability of PCAF to acetylate chromatin, while the acetylation of HMG-17 reduces its binding affinity to chromatin. Conceivably, in HMG-17-containing chromatin, acetylation of HMG-17 precedes the acetylation of histones.Reversible acetylation of the N-terminal tails of histones plays a key role in the regulation of various nuclear activities such as chromatin assembly, replication, and transcription (2,19,29,39,49,51,52). The acetylation of lysine residues within nucleosomes weakens the interaction of the histone tails with the DNA and leads to chromatin decompaction (16,17). These structural transitions enhance the accessibility of the underlying DNA sequence to various factors, thereby reducing the repressive effect of the nucleosome on transcription and replication. The relationship between transcriptional regulation and histone acetylation has been strengthened considerably by the discovery that certain factors associated with transcriptional activation have intrinsic histone acetylase activity (7,20,30,31,44,53), while factors associated with transcriptional repression contain histone deacetylase activity (26,44). It is significant that in some cases this reversible acetylation is targeted and specific. For example, Tetrahymena GCN5 preferentially acetylates residues K8 and K16 of histone H4 and K14 of histone H3 (13, 24). In contrast, in Saccharomyces cerevisiae, transcriptional repression by UME6 involves the specific deacetylation of K5 in histone H4 by the deacetylase RPD3 (40). Furthermore, the pattern of H4 acetylation in heterochromatin is unique, suggesting that specific acetylation marks discrete functional states of chromatin structure (5, 32). Taken together with other findings, these results suggest that the reversible acetylation of histones is not merely a mechanism for indiscriminately unfolding chromatin but is a key step in the selective regulation of the expression of spe...

High Mobility Group Nucleosomal Binding Domain 2 (HMGN2) SUMOylation by the SUMO E3 Ligase PIAS1 Decreases the Binding Affinity to Nucleosome Core Particles

“…Post-translational modifications of HMGN2 have been shown to have a profound effect on its biochemical and biological functions. Phosphorylation of the serine residues located at the nucleosome binding domain could affect HMGN2 binding affinity with nucleosomes (17,36) and acetylation of HMGN2 by p300/CBP-associated factor (PCAF) also reduces the binding affinity with nucleosome (16), implying the possible role of SUMOylation in HMGN2.…”

“…The binding of HMGN pro-teins to nucleosomes can be altered by HMGN modification. Equilibrium dialysis experiments with reconstructed nucleosomes and non-acetylated HMGN2 showed that acetylated HMGN2 had lower binding affinity for nucleosomes, suggesting that acetylation may function to loosen the interaction between HMGN2 and nucleosomes (16). Phosphorylation of HMGN2 also serves to abolish the interaction of HMGN2 with its chromatin targets (17).…”

Transcriptional Regulation by the Acetylation of Nonhistone Proteins in Humans – A New Target for Therapeutics

“…Acetylation of HMGN2 leads to a weakening of its nucleosome binding ability (40,41). Binding of HMGN1/N2 to nucleosomal cores inhibits the PCAF mediated acetylation of histone H3 (41). It is however not known how the HMGNs get recruited in a specific gene.…”

“…Quite interestingly, though p300 can acetylate both HMGN1 and N2 at multiple sites (40), PCAF specifically acetylates HMGN2 (41). Acetylation of HMGN2 leads to a weakening of its nucleosome binding ability (40,41). Binding of HMGN1/N2 to nucleosomal cores inhibits the PCAF mediated acetylation of histone H3 (41).…”

“…Through transient interactions with the nucleosomes, they destabilize the higher ordered chromatin structure, increase the accessibility to the nucleosomal DNA, and impart flexibility to the chromatin fibre. Quite interestingly, though p300 can acetylate both HMGN1 and N2 at multiple sites (40), PCAF specifically acetylates HMGN2 (41). Acetylation of HMGN2 leads to a weakening of its nucleosome binding ability (40,41).…”