Institut Curie, PSL Research University, CNRS, UMR3244, Paris, France.
Université Pierre et Marie Curie (UPMC), Paris, France.
PLoS Genet. 2018 Feb 14;14(2):e1007223. doi: 10.1371/journal.pgen.1007223. eCollection 2018 Feb.
Histone H3K4 methylation is a feature of meiotic recombination hotspots shared by many organisms including plants and mammals. Meiotic recombination is initiated by programmed double-strand break (DSB) formation that in budding yeast takes place in gene promoters and is promoted by histone H3K4 di/trimethylation. This histone modification is recognized by Spp1, a PHD finger containing protein that belongs to the conserved histone H3K4 methyltransferase Set1 complex. During meiosis, Spp1 binds H3K4me3 and interacts with a DSB protein, Mer2, to promote DSB formation close to gene promoters. How Set1 complex- and Mer2- related functions of Spp1 are connected is not clear. Here, combining genome-wide localization analyses, biochemical approaches and the use of separation of function mutants, we show that Spp1 is present within two distinct complexes in meiotic cells, the Set1 and the Mer2 complexes. Disrupting the Spp1-Set1 interaction mildly decreases H3K4me3 levels and does not affect meiotic recombination initiation. Conversely, the Spp1-Mer2 interaction is required for normal meiotic recombination initiation, but dispensable for Set1 complex-mediated histone H3K4 methylation. Finally, we provide evidence that Spp1 preserves normal H3K4me3 levels independently of the Set1 complex. We propose a model where Spp1 works in three ways to promote recombination initiation: first by depositing histone H3K4 methylation (Set1 complex), next by "reading" and protecting histone H3K4 methylation, and finally by making the link with the chromosome axis (Mer2-Spp1 complex). This work deciphers the precise roles of Spp1 in meiotic recombination and opens perspectives to study its functions in other organisms where H3K4me3 is also present at recombination hotspots.
组蛋白 H3K4 甲基化是许多生物（包括植物和哺乳动物）减数分裂重组热点的特征。减数分裂重组由程序性双链断裂（DSB）的形成引发，在芽殖酵母中，DSB 发生在基因启动子中，并由组蛋白 H3K4 二/三甲基化促进。这种组蛋白修饰被 Spp1 识别，Spp1 是一种含有 PHD 指结构域的蛋白质，属于保守的组蛋白 H3K4 甲基转移酶 Set1 复合物。在减数分裂过程中，Spp1 结合 H3K4me3，并与 DSB 蛋白 Mer2 相互作用，以促进靠近基因启动子的 DSB 形成。Set1 复合物和 Mer2 相关的 Spp1 功能是如何连接的尚不清楚。在这里，我们结合全基因组定位分析、生化方法和功能分离突变体的使用，表明 Spp1 在减数分裂细胞中存在于两个不同的复合物中，即 Set1 复合物和 Mer2 复合物。破坏 Spp1-Set1 相互作用轻度降低 H3K4me3 水平，但不影响减数分裂重组起始。相反，Spp1-Mer2 相互作用是正常减数分裂重组起始所必需的，但不需要 Set1 复合物介导的组蛋白 H3K4 甲基化。最后，我们提供了证据表明 Spp1 独立于 Set1 复合物维持正常的 H3K4me3 水平。我们提出了一个模型，其中 Spp1 通过三种方式促进重组起始：首先通过沉积组蛋白 H3K4 甲基化（Set1 复合物），其次通过“读取”和保护组蛋白 H3K4 甲基化，最后通过与染色体轴建立联系（Mer2-Spp1 复合物）。这项工作阐明了 Spp1 在减数分裂重组中的精确作用，并为研究其在其他存在 H3K4me3 的重组热点的生物体中的功能提供了新的视角。
