ASH1, a Drosophila trithorax group protein, is required for methylation of lysine 4 residues on histone H3

Abstract

Covalent modifications of histone tails modulate gene expression via chromatin organization. As examples, methylation of lysine 9 residues of histone H3 (H3) (H3-K9) is believed to repress transcription by compacting chromatin, whereas methylation of lysine 4 residues of H3 (H3-K4) is believed to activate transcription by relaxing chromatin. The Drosophila trithorax group protein absent, small, or homeotic discs 1 (ASH1) is involved in maintaining active transcription of many genes. Here we report that in extreme ash1 mutants, no H3-K4 methylation is detectable. Within the limits of our assays, this lack of detectable H3-K4 methylation implies that ASH1 is required for essentially all H3-K4 methylation that occurs in vivo. We report further that the 149-aa SET domain of ASH1 is sufficient for H3-K4 methylation in vitro. These findings support a model in which ASH1 is directly involved in maintaining active transcription by conferring a relaxed chromatin structure. C hromatin serves as the template for processing genetic information. The dynamic DNA-protein structure of chromatin is influenced by epigenetic modifications on both the DNA and nucleosomal histones. Chromatin can exist in alternate states: a relaxed state (euchromatin) where underlying DNA becomes accessible to transcription factors, and a condensed state (heterochromatin) where underlying DNA is transcriptionally restricted. The functional organization of chromatin structure is in part the result of targeted covalent modifications on the N-terminal tails of the core histones. The histone code hypothesis suggests that combinations of histone modifications format the chromatin for proper gene regulation (1).Although histone methylation was first described in 1964 (2), its significance for regulating chromatin structure and transcription remained unclear until the discovery of the first histone methyltransferase (HMTase), SUV39H (3), a SET domaincontaining protein. Lysine-directed histone methylation is intriguing, because it is involved in mediating both compacted and relaxed chromatin. Histone H3 (H3) can be methylated at lysine residues 4, 9, 27, and 36. Methylation of lysine 9 modulates gene silencing (3), whereas methylation of lysine 4 appears to function as a signal for gene activation (4). It has been proposed that the combination of methylation on the lysine 4 and 9 residues of histone H3 with methylation on the lysine 20 residue of histone H4 may also serve as a signal for gene activation (5).The SET domain is present in transcriptional regulators that have opposing functions. For example, consider the three Drosophila proteins that originally defined the SET domain: suppressor of variegation 3-9 [SU(VAR)3-9] is involved in heterochromatin-dependent gene silencing (6, 7), enhancer of zeste, is involved in maintaining both activated and repressed transcriptional states (8), and trithorax (TRX) is involved in maintaining gene activation (9). The HMTase function of the SET domain has been implicated in conferring both active and repressed transc...

H3K36 Methylation Antagonizes PRC2-mediated H3K27 Methylation

“…5C), indicating that Ash1 is an H3K36-specific dimethylase. Our results are in agreement with one previous report (20) but disagree with three other reports (19,21,22).…”

“…Thus, we decided to turn to the controversial histone methyltransferase Ash1, which is a well known Trithorax group protein that antagonizes Polycomb silencing (15,16). Although several groups reported that Ash1 is specific for H3K4 (19,21,22) or even H3K9 and H4K20 (19), one independent study reported that Ash1 is a H3K36-specific methyltransferase (20). Moreover, the Set domain of Ash1 is much more closely related to H3K36-specific methyltransferases (including HYPB and Mes4) than H3K4-specific methyltransferases such as Set1 or Trithorax (supplemental Fig.…”

“…Ash1 was also reported to be a histone methyltransferase specific for H3K36 (20). In addition, two other independent studies reported Ash1 to be a histone H3K4-specific methyltransferase (21,22). Therefore, the exact chromatin modification that Ash1 generates and thereby potentially antagonizes PRC2-mediated H3K27 methylation remains controversial.…”

O-GlcNAc and the Epigenetic Regulation of Gene Expression

“…5 and 26). The trithorax group is associated with "activating histone modifications" such as histone H3 Lys-4 and Lys-36 methylation (59,60) and may involve O-GlcNAcylation as part of the regulatory network (26). In general terms, the trithorax group acts as an antirepressor that counters the action of the polycomb repressive complex.…”

Plant SET domain-containing proteins: Structure, function and regulation

“…Repression of homeotic gene expression is mediated by a Polycomb group (PcG) protein complex, containing Enhancer of zeste [E(Z)] and extra sex combs (ESC), that targets H3K27 and H3K9 methylation [34,35]. In contrast, the trithorax group (trxG) proteins, ASH1 [36] and Trx [37], function in targeting H3K4 methylation [38][39][40] and hence in maintaining an active state of homeotic genes.…”