GlcNAcylation of histone H2B facilitates its monoubiquitination.

Chromatin reorganization is governed by multiple post-translational modifications of chromosomal proteins and DNA. These histone modifications are reversible, dynamic events that can regulate DNA-driven cellular processes. However, the molecular mechanisms that coordinate histone modification patterns remain largely unknown. In metazoans, reversible protein modification by O-linked N-acetylglucosamine (GlcNAc) is catalysed by two enzymes, O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA). However, the significance of GlcNAcylation in chromatin reorganization remains elusive. Here we report that histone H2B is GlcNAcylated at residue S112 by OGT in vitro and in living cells. Histone GlcNAcylation fluctuated in response to extracellular glucose through the hexosamine biosynthesis pathway (HBP). H2B S112 GlcNAcylation promotes K120 monoubiquitination, in which the GlcNAc moiety can serve as an anchor for a histone H2B ubiquitin ligase. H2B S112 GlcNAc was localized to euchromatic areas on fly polytene chromosomes. In a genome-wide analysis, H2B S112 GlcNAcylation sites were observed widely distributed over chromosomes including transcribed gene loci, with some sites co-localizing with H2B K120 monoubiquitination. These findings suggest that H2B S112 GlcNAcylation is a histone modification that facilitates H2BK120 monoubiquitination, presumably for transcriptional activation.

Multiple post-translational modifications in hepatocyte nuclear factor 4α.

To investigate the role of post-translational modifications (PTMs) in the hepatocyte nuclear factor 4α (HNF4α)-mediated transcription, we took a comprehensive survey of PTMs in HNF4α protein by mass-spectrometry and identified totally 8 PTM sites including newly identified ubiquitilation and acetylation sites. To assess the impact of identified PTMs in HNF4α-function, we introduced point mutations at the identified PTM sites and, tested transcriptional activity of the HNF4α. Among the point-mutations, an acetylation site at lysine 458 was found significant in the HNF4α-mediated transcriptional control. An acetylation negative mutant at lysine 458 showed an increased transcriptional activity by about 2-fold, while an acetylation mimic mutant had a lowered transcriptional activation. Furthermore, this acetylation appeared to be fluctuated in response to extracellular nutrient conditions. Thus, by applying an comprehensive analysis of PTMs, multiple PTMs were newly identified in HNF4α and unexpected role of an HNF4α acetylation could be uncovered.

KIAA1718 is a histone demethylase that erases repressive histone methyl marks.

The methylation states of histone lysine residues are regarded as significant epigenetic marks governing transcriptional regulation. A number of histone demethylases containing a jumonji C (JmjC) domain have been recognized; however, their properties remain to be investigated. Here, we show that KIAA1718, a PHF2/PHF8 subfamily member, possesses histone demethylase activity specific for H3K9 and H3K27, transcriptionally repressive histone marks. Biochemical purification of the KIAA1718 interactants reveals that KIAA1718 forms complexes with several factors including KAP1, a transcriptional co-activator. Consistent with these findings, KIAA1718 shows a transcriptional activation function in the chromatin context. Thus, our study identifies KIAA1718 as a histone demethylase for repressive methyl marks and shows that it is involved in transcriptional activation.

Glucose-induced expression of MIP-1 genes requires O-GlcNAc transferase in monocytes.

O-glycosylation has emerged as an important modification of nuclear proteins, and it appears to be involved in gene regulation. Recently, we have shown that one of the histone methyl transferases (MLL5) is activated through O-glycosylation by O-GlcNAc transferase (OGT). Addition of this monosaccharide is essential for forming a functional complex. However, in spite of the abundance of OGT in the nucleus, the impact of nuclear O-glycosylation by OGT remains largely unclear. To address this issue, the present study was undertaken to test the impact of nuclear O-glycosylation in a monocytic cell line, THP-1. Using a cytokine array, MIP-1alpha and -1beta genes were found to be regulated by nuclear O-glycosylation. Biochemical purification of the OGT interactants from THP-1 revealed that OGT is an associating partner for distinct co-regulatory complexes. OGT recruitment and protein O-glycosylation were observed at the MIP-1alpha gene promoter; however, the known OGT partner (HCF-1) was absent when the MIP-1alpha gene promoter was not activated. From these findings, we suggest that OGT could be a co-regulatory subunit shared by functionally distinct complexes supporting epigenetic regulation.

GlcNAcylation of a histone methyltransferase in retinoic-acid-induced granulopoiesis.

The post-translational modifications of histone tails generate a 'histone code' that defines local and global chromatin states. The resultant regulation of gene function is thought to govern cell fate, proliferation and differentiation. Reversible histone modifications such as methylation are under mutual controls to organize chromosomal events. Among the histone modifications, methylation of specific lysine and arginine residues seems to be critical for chromatin configuration and control of gene expression. Methylation of histone H3 lysine 4 (H3K4) changes chromatin into a transcriptionally active state. Reversible modification of proteins by beta-N-acetylglucosamine (O-GlcNAc) in response to serum glucose levels regulates diverse cellular processes. However, the epigenetic impact of protein GlcNAcylation is unknown. Here we report that nuclear GlcNAcylation of a histone lysine methyltransferase (HKMT), MLL5, by O-GlcNAc transferase facilitates retinoic-acid-induced granulopoiesis in human HL60 promyelocytes through methylation of H3K4. MLL5 is biochemically identified in a GlcNAcylation-dependent multi-subunit complex associating with nuclear retinoic acid receptor RARalpha (also known as RARA), serving as a mono- and di-methyl transferase to H3K4. GlcNAcylation at Thr 440 in the MLL5 SET domain evokes its H3K4 HKMT activity and co-activates RARalpha in target gene promoters. Increased nuclear GlcNAcylation by means of O-GlcNAc transferase potentiates retinoic-acid-induced HL60 granulopoiesis and restores the retinoic acid response in the retinoic-acid-resistant HL60-R2 cell line. Thus, nuclear MLL5 GlcNAcylation triggers cell lineage determination of HL60 through activation of its HKMT activity.