Eberharter A, Ferrari S, Längst G, Straub T, Imhof A, Varga-Weisz P, Wilm M, Becker P B
Adolf-Butenandt-Institut, Molekularbiologie, Schillerstrasse 44, D-80336 München, Germany.
EMBO J. 2001 Jul 16;20(14):3781-8. doi: 10.1093/emboj/20.14.3781.
The chromatin accessibility complex (CHRAC) was originally defined biochemically as an ATP-dependent 'nucleosome remodelling' activity. Central to its activity is the ATPase ISWI, which catalyses the transfer of histone octamers between DNA segments in cis. In addition to ISWI, four other potential subunits were observed consistently in active CHRAC fractions. We have now identified the p175 subunit of CHRAC as Acf1, a protein known to associate with ISWI in the ACF complex. Interaction of Acf1 with ISWI enhances the efficiency of nucleosome sliding by an order of magnitude. Remarkably, it also modulates the nucleosome remodelling activity of ISWI qualitatively by altering the directionality of nucleosome movements and the histone 'tail' requirements of the reaction. The Acf1-ISWI heteromer tightly interacts with the two recently identified small histone fold proteins CHRAC-14 and CHRAC-16. Whether topoisomerase II is an integral subunit has been controversial. Refined analyses now suggest that topoisomerase II should not be considered a stable subunit of CHRAC. Accordingly, CHRAC can be molecularly defined as a complex consisting of ISWI, Acf1, CHRAC-14 and CHRAC-16.
染色质可及性复合体（CHRAC）最初在生化层面被定义为一种依赖ATP的“核小体重塑”活性。其活性的核心是ATP酶ISWI，它催化组蛋白八聚体在顺式DNA片段之间的转移。除了ISWI之外，在活性CHRAC组分中还始终观察到其他四个潜在亚基。我们现已确定CHRAC的p175亚基为Acf1，这是一种已知在ACF复合体中与ISWI结合的蛋白质。Acf1与ISWI的相互作用将核小体滑动的效率提高了一个数量级。值得注意的是，它还通过改变核小体移动的方向性以及反应对组蛋白“尾巴”的需求，从质的方面调节ISWI的核小体重塑活性。Acf1-ISWI异源二聚体与最近鉴定出的两种小的组蛋白折叠蛋白CHRAC-14和CHRAC-16紧密相互作用。拓扑异构酶II是否为一个完整亚基一直存在争议。现在的精细分析表明，不应将拓扑异构酶II视为CHRAC的稳定亚基。因此，CHRAC在分子层面可被定义为一个由ISWI、Acf1、CHRAC-14和CHRAC-16组成的复合体。
