The fork protection complex promotes symmetric histone recycling to maintain heterochromatin [ChIP-Seq]. The fork protection complex promotes symmetric histone recycling to maintain heterochromatin [ChIP-Seq]

The inheritance of parental histones across the replication fork is thought to mediate epigenetic memory. Here, we reveal that fission yeast Mrc1 (Claspin in humans) binds H3-H4 tetramers and operates as a central coordinator of symmetric parental histone inheritance. Mrc1 mutants in a key connector domain disrupted segregation of parental histones to the lagging strand comparable to Mcm2 histone-binding mutants. Both mutants showed clonal and asymmetric loss of H3K9me-mediated gene silencing. AlphaFold predicted co-chaperoning of H3-H4 tetramers by Mrc1 and Mcm2, with the Mrc1 connector domain bridging histone and Mcm2 binding. Biochemical and functional analysis validated this model and revealed a duality in Mrc1 function: disabling histone binding in the connector domain disrupted lagging strand recycling while another histone-binding mutation impaired leading strand recycling. We propose Mrc1 toggles histones between the lagging and leading strand recycling pathways, in part by intra-replisome co-chaperoning, to ensure epigenetic transmission to both daughter cells. Overall design: ChIP-Seq measuring chromatin occupancy for BrdU in WT and mrc1dHBS and H3K9me2 in WT, mrc1dHBS, mcm2-2A, and mrc1dHBSmcm2-2a cells. With corresponding Inputs and replicates

复制叉处亲本组蛋白的传递被认为是表观遗传记忆的介导途径。本研究揭示，裂殖酵母Mrc1（人类同源蛋白为Claspin）能够结合组蛋白H3-H4四聚体，并作为对称型亲本组蛋白遗传的核心协调因子。其关键连接结构域的Mrc1突变体，可使亲本组蛋白向滞后链的分配过程受到破坏，该缺陷程度与微染色体维持蛋白2（Mcm2）组蛋白结合突变体相当。两类突变体均出现了由组蛋白H3第9位赖氨酸甲基化（H3K9me）介导的基因沉默的克隆性与不对称性丢失。AlphaFold预测显示，Mrc1与微染色体维持蛋白2（Mcm2）可协同介导组蛋白H3-H4四聚体的分子伴侣功能，其中Mrc1的连接结构域可桥接组蛋白与Mcm2的结合。生化与功能实验验证了该模型，并揭示了Mrc1功能的双重性：连接结构域的组蛋白结合功能失活会破坏滞后链的组蛋白回收过程，而另一处组蛋白结合突变则会损伤前导链的组蛋白回收。我们提出，Mrc1可通过复制体内部的协同分子伴侣作用，在滞后链与前导链的组蛋白回收通路之间切换组蛋白，从而确保表观遗传信息向两个子代细胞传递。实验设计概况：针对野生型（WT）与mrc1dHBS细胞，开展针对5-溴-2'-脱氧尿苷（BrdU）染色质占据水平的ChIP-Seq测序；针对野生型、mrc1dHBS、mcm2-2A及mrc1dHBSmcm2-2a细胞，开展针对组蛋白H3第9位赖氨酸二甲基化（H3K9me2）的ChIP-Seq测序，并设置相应的输入对照文库与生物学重复。