Laboratory of Biochemistry and Molecular Biology, National Cancer Institute, National Institutes of Health, Bethesda, MD 20892, USA.
Cell. 2010 Nov 24;143(5):725-36. doi: 10.1016/j.cell.2010.10.019.
Histone variant H2A.Z-containing nucleosomes are incorporated at most eukaryotic promoters. This incorporation is mediated by the conserved SWR1 complex, which replaces histone H2A in canonical nucleosomes with H2A.Z in an ATP-dependent manner. Here, we show that promoter-proximal nucleosomes are highly heterogeneous for H2A.Z in Saccharomyces cerevisiae, with substantial representation of nucleosomes containing one, two, or zero H2A.Z molecules. SWR1-catalyzed H2A.Z replacement in vitro occurs in a stepwise and unidirectional fashion, one H2A.Z-H2B dimer at a time, producing heterotypic nucleosomes as intermediates and homotypic H2A.Z nucleosomes as end products. The ATPase activity of SWR1 is specifically stimulated by H2A-containing nucleosomes without ensuing histone H2A eviction. Remarkably, further addition of free H2A.Z-H2B dimer leads to hyperstimulation of ATPase activity, eviction of nucleosomal H2A-H2B, and deposition of H2A.Z-H2B. These results suggest that the combination of H2A-containing nucleosome and free H2A.Z-H2B dimer acting as both effector and substrate for SWR1 governs the specificity and outcome of the replacement reaction.
组蛋白变体 H2A.Z 包含的核小体存在于大多数真核生物启动子中。这种掺入是由保守的 SWR1 复合物介导的，该复合物以 ATP 依赖的方式将经典核小体中的组蛋白 H2A 替换为 H2A.Z。在这里，我们表明在酿酒酵母中，启动子近端核小体中 H2A.Z 的高度不均一，存在大量含有一个、两个或零个 H2A.Z 分子的核小体。体外 SWR1 催化的 H2A.Z 替换以逐步和单向的方式发生，一次一个 H2A.Z-H2B 二聚体，产生异质核小体作为中间体和同质 H2A.Z 核小体作为终产物。SWR1 的 ATP 酶活性特异性地被含有 H2A 的核小体刺激，而不会随之驱逐组蛋白 H2A。值得注意的是，进一步添加游离的 H2A.Z-H2B 二聚体导致 ATP 酶活性的超刺激、核小体 H2A-H2B 的驱逐以及 H2A.Z-H2B 的沉积。这些结果表明，含有 H2A 的核小体和游离的 H2A.Z-H2B 二聚体的组合，既作为 SWR1 的效应物又作为底物，控制着替换反应的特异性和结果。
