Muto Shinsuke, Senda Miki, Akai Yusuke, Sato Lui, Suzuki Toru, Nagai Ryozo, Senda Toshiya, Horikoshi Masami
Laboratory of Developmental Biology, Institute of Molecular and Cellular Biosciences, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan.
Proc Natl Acad Sci U S A. 2007 Mar 13;104(11):4285-90. doi: 10.1073/pnas.0603762104. Epub 2007 Mar 6.
Histone chaperones assemble and disassemble nucleosomes in an ATP-independent manner and thus regulate the most fundamental step in the alteration of chromatin structure. The molecular mechanisms underlying histone chaperone activity remain unclear. To gain insights into these mechanisms, we solved the crystal structure of the functional domain of SET/TAF-Ibeta/INHAT at a resolution of 2.3 A. We found that SET/TAF-Ibeta/INHAT formed a dimer that assumed a "headphone"-like structure. Each subunit of the SET/TAF-Ibeta/INHAT dimer consisted of an N terminus, a backbone helix, and an "earmuff" domain. It resembles the structure of the related protein NAP-1. Comparison of the crystal structures of SET/TAF-Ibeta/INHAT and NAP-1 revealed that the two proteins were folded similarly except for an inserted helix. However, their backbone helices were shaped differently, and the relative dispositions of the backbone helix and the earmuff domain between the two proteins differed by approximately 40 degrees . Our biochemical analyses of mutants revealed that the region of SET/TAF-Ibeta/INHAT that is engaged in histone chaperone activity is the bottom surface of the earmuff domain, because this surface bound both core histones and double-stranded DNA. This overlap or closeness of the activity surface and the binding surfaces suggests that the specific association among SET/TAF-Ibeta/INHAT, core histones, and double-stranded DNA is requisite for histone chaperone activity. These findings provide insights into the possible mechanisms by which histone chaperones assemble and disassemble nucleosome structures.
组蛋白伴侣以不依赖ATP的方式组装和解聚核小体，从而调控染色质结构改变中最基本的步骤。组蛋白伴侣活性的分子机制仍不清楚。为了深入了解这些机制，我们以2.3埃的分辨率解析了SET/TAF-Iβ/INHAT功能结构域的晶体结构。我们发现SET/TAF-Iβ/INHAT形成了一个呈“耳机”状结构的二聚体。SET/TAF-Iβ/INHAT二聚体的每个亚基由一个N端、一个主链螺旋和一个“耳罩”结构域组成。它类似于相关蛋白NAP-1的结构。SET/TAF-Iβ/INHAT和NAP-1晶体结构的比较表明，除了一个插入的螺旋外，这两种蛋白质的折叠方式相似。然而，它们的主链螺旋形状不同，并且两种蛋白质之间主链螺旋和耳罩结构域的相对位置相差约40度。我们对突变体的生化分析表明，SET/TAF-Iβ/INHAT中参与组蛋白伴侣活性的区域是耳罩结构域的底面，因为该表面结合核心组蛋白和双链DNA。活性表面和结合表面的这种重叠或紧密性表明，SET/TAF-Iβ/INHAT、核心组蛋白和双链DNA之间的特异性结合对于组蛋白伴侣活性是必需的。这些发现为组蛋白伴侣组装和解聚核小体结构的可能机制提供了见解。
