The SUMO deconjugating peptidase Smt4 contributes to the mechanism required for transition from sister chromatid arm cohesion to sister chromatid pericentromere separation

The pericentromere chromatin protrudes orthogonally from the sister-sister chromosome arm axis. Pericentric protrusions are organized in a series of loops with the centromere at the apex, maximizing its ability to interact with stochastically growing and shortening kinetochore microtubules. Each pericentromere loop is ∼50 kb in size and is organized further into secondary loops that are displaced from the primary spindle axis. Cohesin and condensin are integral to mechanisms of loop formation and generating resistance to outward forces from kinesin motors and anti-parallel spindle microtubules. A major unanswered question is how the boundary between chromosome arms and the pericentromere is established and maintained. We used sister chromatid separation and dynamics of LacO arrays distal to the pericentromere to address this issue. Perturbation of chromatin spring components results in 2 distinct phenotypes. In cohesin and condensin mutants sister pericentric LacO arrays separate a defined distance independent of spindle length. In the absence of Smt4, a peptidase that removes SUMO modifications from proteins, pericentric LacO arrays separate in proportion to spindle length increase. Deletion of Smt4, unlike depletion of cohesin and condensin, causes stretching of both proximal and distal pericentromere LacO arrays. The data suggest that the sumoylation state of chromatin topology adjusters, including cohesin, condensin, and topoisomerase II in the pericentromere, contribute to chromatin spring properties as well as the sister cohesion boundary.

着丝粒周边染色质（pericentromere）沿垂直于姐妹染色体臂共同轴的方向向外伸出。着丝粒周边突起组装为一系列环状结构，着丝粒位于环的顶端，最大化着丝粒与随机动态组装和解聚的动粒微管（kinetochore microtubule）相互作用的能力。

每个着丝粒周边环的大小约为50千碱基对（kb），并进一步形成脱离主纺锤体轴的次级环。黏连蛋白（cohesin）与凝缩蛋白（condensin）是环形成机制的必需组分，同时可介导对驱动蛋白（kinesin motor）及反平行纺锤体微管所施加向外作用力的抵抗。

目前尚未解决的核心科学问题之一，是染色体臂与着丝粒周边区之间的边界如何建立并维持。本研究借助姐妹染色单体分离及着丝粒周边区远端LacO阵列（LacO array）的动态变化，对该问题展开探究。

染色质弹簧组分的扰动会引发两种截然不同的表型：在黏连蛋白（cohesin）与凝缩蛋白（condensin）突变体中，姐妹着丝粒周边LacO阵列（LacO array）的分离距离固定，不受纺锤体长度影响；当缺失Smt4——一种可去除蛋白质上SUMO修饰的肽酶——时，着丝粒周边LacO阵列（LacO array）的分离距离随纺锤体长度增加呈正比变化。

与黏连蛋白、凝缩蛋白的耗竭不同，Smt4的缺失会导致着丝粒周边近端与远端的LacO阵列（LacO array）均发生拉伸。本研究数据表明，着丝粒周边区内包括黏连蛋白、凝缩蛋白及拓扑异构酶II（topoisomerase II）在内的染色质拓扑调控因子的SUMO化修饰状态，既影响染色质弹簧的特性，也参与维持姐妹染色单体黏连边界。