Glutamine 5 serotonylation on histone H3 is a permissive modification that functions combinatorialy with H3K4me3 to potentiate TFIID interactions

Chemical modifications of histone proteins, along with the ?writers,? ?erasers? and ?readers? of these modifications, are capable of mediating a diverse set of DNA-templated processes including gene transcription1-7. Here, we provide evidence for a new class of histone posttranslational modification (PTM), serotonylation of glutamine, which occurs at position 5 (Q5ser) on histone H3 in serotonin (5-hydroxytryptamine, 5-HT) producing organisms. We demonstrate that the tissue Transglutaminase 2 (TGM2) enzyme, a transamidase that is both necessary and sufficient to deposit the mark, can serotonylate H3 on lysine 4 tri-methylated (H3K4me3) nucleosomes resulting in the presence of combinatorial H3K4me3Q5ser in vivo. H3K4me3Q5ser displays a ubiquitous pattern of tissue expression in mammals, with predicted enrichment observed in brain and gut, two organ systems responsible for the bulk of 5-HT production. Genome-wide analyses of its enrichment in human serotonergic neurons, developing mouse brain and cultured serotonergic cells, using an H3K4me3Q5ser specific antibody, indicate that the mark is enriched in euchromatin, is sensitive to cellular differentiation and correlates with permissive gene expression, phenomena that are linked to the mark?s potentiation of TFIID8-10 interactions with H3K4me3. Cells ectopically expressing an H3 mutant that cannot be serotonylated display significantly altered expression of H3K4me3Q5ser target loci leading to deficits in differentiation. Taken together, these data identify a direct role for 5-HT, independent from its contributions to neurotransmission and cellular signaling, in the mediation of permissive gene expression in mammalian cells, and further define its biophysical activities as a putative co-regulator of TFIID recruitment to H3K4me3 marked chromatin.组蛋白（histone）的化学修饰，以及这类修饰的书写器（writers）、擦除器（erasers）与阅读器（readers），能够介导一系列以DNA为模板的生物学过程，包括基因转录1-7。本研究报道了一类新型组蛋白翻译后修饰（posttranslational modification, PTM）：谷氨酰胺的5-羟色胺酰化（serotonylation），该修饰发生于能够产生5-羟色胺（5-hydroxytryptamine, 5-HT）的生物体的组蛋白H3第5位谷氨酰胺残基（Q5ser）。我们证实，组织型转谷氨酰胺酶2（Transglutaminase 2, TGM2）作为一种介导该修饰沉积所必需且充分的转酰胺酶（transamidase），可对三甲基化赖氨酸4的组蛋白H3（H3K4me3）核小体（nucleosome）进行5-羟色胺酰化，从而在体内形成组合修饰H3K4me3Q5ser。H3K4me3Q5ser在哺乳动物组织中呈现广谱表达模式，在脑与肠道这两大5-HT主要产生器官系统中预测富集程度更高。利用靶向H3K4me3Q5ser的特异性抗体，我们对人血清素能神经元（serotonergic neurons）、发育中小鼠脑及体外培养的血清素能细胞开展全基因组富集分析，结果显示该修饰富集于常染色质（euchromatin），对细胞分化敏感，且与许可性基因表达呈正相关；这一现象与该修饰增强转录因子IID（TFIID）8-10与H3K4me3的相互作用密切相关。异位表达无法发生5-羟色胺酰化的H3突变体的细胞，其H3K4me3Q5ser靶位点的基因表达发生显著改变，进而引发分化缺陷。综上，本研究证实5-HT可独立于其在神经递质传递与细胞信号转导中的功能，直接介导哺乳动物细胞的许可性基因表达；同时进一步明确了其作为转录因子IID（TFIID）招募至H3K4me3标记染色质的潜在共调控因子的生物物理活性。