Hake Sandra B, Garcia Benjamin A, Duncan Elizabeth M, Kauer Monika, Dellaire Graham, Shabanowitz Jeffrey, Bazett-Jones David P, Allis C David, Hunt Donald F
Laboratory of Chromatin Biology, The Rockefeller University, New York, New York 10021, USA.
J Biol Chem. 2006 Jan 6;281(1):559-68. doi: 10.1074/jbc.M509266200. Epub 2005 Nov 2.
Covalent histone modifications and the incorporation of histone variants bring about changes in chromatin structure that in turn alter gene expression. Interest in non-allelic histone variants has been renewed, in part because of recent work on H3 (and other) histone variants. However, only in mammals do three non-centromeric H3 variants (H3.1, H3.2, and H3.3) exist. Here, we show that mammalian cell lines can be separated into two different groups based on their expression of H3.1, H3.2, and H3.3 at both mRNA and protein levels. Additionally, the ratio of these variants changes slightly during neuronal differentiation of murine ES cells. This difference in H3 variant expression between cell lines could not be explained by changes in growth rate, cell cycle stages, or chromosomal ploidy, but rather suggests other possibilities, such as changes in H3 variant incorporation during differentiation and tissue- or species-specific H3 variant expression. Moreover, quantitative mass spectrometry analysis of human H3.1, H3.2, and H3.3 showed modification differences between these three H3 variants, suggesting that they may have different biological functions. Specifically, H3.3 contains marks associated with transcriptionally active chromatin, whereas H3.2, in contrast, contains mostly silencing modifications that have been associated with facultative heterochromatin. Interestingly, H3.1 is enriched in both active and repressive marks, although the latter marks are different from those observed in H3.2. Although the biological significance as to why mammalian cells differentially employ three highly similar H3 variants remains unclear, our results underscore potential functional differences between them and reinforce the general view that H3.1 and H3.2 in mammalian cells should not be treated as equivalent proteins.
共价组蛋白修饰和组蛋白变体的掺入导致染色质结构发生变化,进而改变基因表达。对非等位组蛋白变体的兴趣再度兴起,部分原因是最近关于H3(及其他)组蛋白变体的研究。然而,仅在哺乳动物中存在三种非着丝粒H3变体(H3.1、H3.2和H3.3)。在此,我们表明,基于哺乳动物细胞系在mRNA和蛋白质水平上H3.1、H3.2和H3.3的表达情况,可将其分为两个不同的组。此外,在小鼠胚胎干细胞的神经元分化过程中,这些变体的比例略有变化。细胞系之间H3变体表达的这种差异无法用生长速率、细胞周期阶段或染色体倍性的变化来解释,而是暗示了其他可能性,例如分化过程中H3变体掺入的变化以及组织或物种特异性的H3变体表达。此外,对人H3.1、H3.2和H3.3的定量质谱分析表明,这三种H3变体之间存在修饰差异,表明它们可能具有不同的生物学功能。具体而言,H3.3含有与转录活性染色质相关的标记,而相比之下,H3.2主要含有与兼性异染色质相关的沉默修饰。有趣的是,H3.1在活性和抑制性标记中均富集,尽管后者的标记与在H3.2中观察到的不同。尽管哺乳动物细胞为何差异使用三种高度相似的H3变体的生物学意义尚不清楚,但我们的结果强调了它们之间潜在的功能差异,并强化了一种普遍观点,即哺乳动物细胞中的H3.1和H3.2不应被视为等效蛋白质。