The effect of salts on the stability of the H2A-H2B histone dimer
The effect of salts on the stability of the H2A-H2B histone dimer
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DOI：
10.1021/bi026282s
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发表时间：
2002-12-17
期刊：
影响因子：
2.9
通讯作者：
Placek, BJ
中科院分区：
文献类型：
--
作者：
Gloss, LM;Placek, BJ
核心核小体由一个H3-H4四聚体和两个H2A-H2B二聚体组成，它不是一个静态的DNA包装结构。核小体是一种动态的蛋白质- dna复合物，其结构的调控是转录调控的重要组成部分。为了开始了解核小体动力学的分子细节，我们研究了分离的H2A-H2B二聚体的稳定性。利用远紫外圆二色性和本征酪氨酸荧光检测了尿素诱导的异源二聚体的平衡反应。两种光谱探针产生了一致的跃迁，尿素诱导的可逆展开的全局拟合进一步证明了H2A-H2B的展开是一种双态平衡响应。在生理离子强度下，H2A-H2B在无尿素条件下展开的自由能为11.8 +/- 0.3 kcal mol(-1)，二聚体的稳定性为26.4 kDa。m值，即展开对尿素的敏感性为2.9 +/- 0.1 kcal mol(-1) m -1。这个值明显大于二聚化基序单独展开时的预测值（类似于2 kcal mol(-1) M-1），这表明n端尾部可能采用了一种塌陷的、不含溶剂的结构，经历了展开转变。考察了几种钾盐和三种氯化盐对H2A-H2B二聚体的稳定作用。H2A-H2B二聚体的盐依赖性稳定表明Hofmeister效应是主要的稳定模式。然而，采用多种盐的研究表明，在高碱性异二聚体中，必须通过筛选静电排斥来产生稳定成分。二聚体中电荷最高的区域是n端尾部，这是翻译后修饰的位点，如乙酰化和磷酸化。这些修饰改变了尾部的电荷密度，参与了核小体动力学的调节。
The core nucleosome, which comprises an H3-H4 tetramer and two H2A-H2B dimers, is not a static DNA packaging structure. The nucleosome is a dynamic protein-DNA complex, and the modulation of its structure is an important component of transcriptional regulation. To begin to understand the molecular details of nucleosome dynamics, we have investigated the stability of the isolated H2A-H2B dimer. The urea-induced equilibrium responses of the heterodimer have been examined by far-UV circular dichroism and intrinsic tyrosine fluorescence. The two spectroscopic probes yielded coincident transitions, and global fitting of the reversible urea-induced unfolding further demonstrated that H2A-H2B unfolds by a two-state equilibrium response. At physiological ionic strengths, the free energy of unfolding in the absence of urea of H2A-H2B is 11.8 +/- 0.3 kcal mol(-1), moderate stability for a dimer of 26.4 kDa. The m value, or sensitivity of the unfolding to urea, is 2.9 +/- 0.1 kcal mol(-1) M-1. This value is significantly larger than would be predicted for the unfolding of the dimerization motif alone (similar to2 kcal mol(-1) M-1), suggesting that the N-terminal tails may adopt a collapsed, solvent-excluding structure that undergoes an unfolding transition.. The efficacies of several potassium salts and three chloride salts to stabilize the H2A-H2B dimer were determined. The salt-dependent stabilization of the H2A-H2B dimer shows that the Hofmeister effect is the predominant mode of stabilization. However, studies employing multiple salts suggest that there is a component of stabilization that must arise from screening of electrostatic repulsion in the highly basic heterodimer. The most highly charged regions of the dimer are the N-terminal tails, sites of posttranslational modifications such as acetylation and phosphorylation. These modifications, which alter the charge density of the tails, are involved in regulation of nucleosome dynamics.