Chromatin Modification by Histone Ubiquitination

描述(由申请人提供)：通过组蛋白的共价修饰来改变染色质是基因表达调控的中心。乙酰化是组蛋白最具特征的共价修饰，对基因表达有广泛的影响。组蛋白的磷酸化对于转录激活、有丝分裂和减数分裂过程中染色体的凝聚以及细胞分裂的调节都是重要的。组蛋白甲基化和单泛素化在基因表达调控中的作用最近被揭示。 几年来，我们一直在研究MLL蛋白在人类白血病发展中的作用。酿酒酵母SetL蛋白与人MLL密切相关。我们和其他人已经提纯了一种含有SET1的多蛋白复合体，我们称之为COMPASS。我们已经证明了COMPASS是组蛋白H3氨基末端赖氨酸4甲基化所必需的，而这种组蛋白修饰是沉默染色体端粒附近基因表达所必需的。我们发现COMPASS的募集优先发生在启动子和早期延伸的聚合酶上。在对酵母基因组中组蛋白H3甲基化所需基因产物的调查中，我们发现泛素结合酶Rad6及其可能的E3连接酶Bre1和Paf1复合体是组蛋白H3赖氨酸4和赖氨酸79甲基化所必需的。由于RAD6催化组蛋白H_2B的赖氨酸123的单偶联素化，因此组蛋白H_2B的这种修饰可能是一个指示COMPAS催化组蛋白H_3的K4甲基化的信号。我们希望确定组蛋白被Rad6、Brel和Paf1复合体泛素化的机制，以及它们在通过单核苷酸修饰组蛋白来调节基因表达中的作用。我们的具体目标是： 目的1：了解组蛋白H_2B的赖氨酸123如何被Rad6/Bre1限制在基因的启动子区域，并了解这一事件对基因调控的影响。 目的2：了解组蛋白H 2B的单素化是否作为组蛋白甲基转移酶指南针招募和/或激活的信号。 目的3：明确历史H_2B脱泛素化的分子机制。 这些研究有望：i)提高我们对组蛋白单泛素化在转录调控中的作用的理解；ii)揭示组蛋白泛素化如何以及为什么优先发生在启动子上；iii)揭示组蛋白在启动子上的泛素化是否、为什么和如何传递组蛋白甲基化信号；以及iv)揭示酵母Set1和人类MLL中的保守SET结构域在血液系统恶性肿瘤的发生和发病中的作用。

DESCRIPTION (provided by applicant): Alteration of chromatin by covalent modification of histone proteins is central to regulation of gene expression. Acetylation, the best-characterized covalent modification of histones, has wide-ranging effects on gene expression. Histone phosphorylation is important for transcriptional activation, condensation of chromosomes during mitosis and meiosis, and regulation of cell division. Roles for histone methylation and monoubiquitination in the regulation of gene expression have recently been revealed. For several years we have been studying the role of the MLL protein in the development of human leukemia. Setl protein of S. cerevisiae is closely related to human MLL. We and others have purified a Setl containing multiprotein complex that we call COMPASS. We have demonstrated that COMPASS is required for methylation of lysine 4 in the amino-terminal tail of histone H3, and that this histone modification is required for silencing of expression of genes located near chromosome telomeres. We found that recruitment of COMPASS occurs preferentially at the promoter and with the early elongating polymerase. In a survey of the yeast genome for gene products necessary for histone H3 methylation, we discovered that the ubiquitin-conjugating enzyme Rad6, together with its likely E3 ligase, Bre1, and the Paf1 complex are required for methylation of lysine 4 and lysine 79 of histone H3. Since Rad6 catalyzes monoubiquitination of lysine 123 of histone H2B, this modification of histone H2B may be a signal that directs COMPASS to catalyze methylation of K4 of histone H3. We wish to define the mechanism of histone ubiquitination by Rad6, Brel, and the Pafl complex and their roles in regulation of gene expression via histone modifications by monoubiqutination. Our specific aims are: Aim 1: To understand how monoubiquitination of lysine 123 of histone H2B by Rad6/Bre1 is limited to the promoter regions of genes, and learn about the consequences of this event for gene regulation. Aim 2: To learn if monoubiquitination of histone H2B serves as a signal for the recruitment and/or activation of the histone methyltransferase COMPASS. Aim 3: To define the molecular mechanism ofdeubiquitination of historic H2B. These studies promise to: i) improve our understanding of the role of histone monoubiquitination in regulation of transcription, ii) shed light on how and why ubiquitination occurs preferentially at promoters, iii) reveal if, why, and how ubiquitination of histones at promoters signals histone methylation, and iv) shed light on the role of the conserved Set domain in yeast Setl and human MLL in development and pathogenesis of hematological malignancies.