SUMO Localizes to the Central Element of Synaptonemal Complex and Is Required for the Full Synapsis of Meiotic Chromosomes in Budding Yeast

The synaptonemal complex (SC) is a widely conserved structure that mediates the intimate alignment of homologous chromosomes during meiotic prophase and is required for proper homolog segregation at meiosis I. However, fundamental details of SC architecture and assembly remain poorly understood. The coiled-coil protein, Zip1, is the only component whose arrangement within the mature SC of budding yeast has been extensively characterized. It has been proposed that the Small Ubiquitin-like MOdifier, SUMO, plays a role in SC assembly by linking chromosome axes with Zip1's C termini. The role of SUMO in SC structure has not been directly tested, however, because cells lacking SUMO are inviable. Here, we provide direct evidence for SUMO's function in SC assembly. A meiotic smt3 reduction-of-function strain displays reduced sporulation, abnormal levels of crossover recombination, and diminished SC assembly. SC structures are nearly absent when induced at later meiotic time points in the smt3 reduction-of-function background. Using Structured Illumination Microscopy we furthermore determine the position of SUMO within budding yeast SC structure. In contrast to previous models that positioned SUMO near Zip1's C termini, we demonstrate that SUMO lies at the midline of SC central region proximal to Zip1's N termini, within a subdomain called the “central element”. The recently identified SUMOylated SC component, Ecm11, also localizes to the SC central element. Finally, we show that SUMO, Ecm11, and even unSUMOylatable Ecm11 exhibit Zip1-like ongoing incorporation into previously established SCs during meiotic prophase and that the relative abundance of SUMO and Ecm11 correlates with Zip1's abundance within SCs of varying Zip1 content. We discuss a model in which central element proteins are core building blocks that stabilize the architecture of SC near Zip1's N termini, and where SUMOylation may occur subsequent to the incorporation of components like Ecm11 into an SC precursor structure.联会复合体（synaptonemal complex, SC）是一类高度保守的结构，可介导减数分裂前期同源染色体的紧密配对，且对减数第一次分裂时同源染色体的正确分离不可或缺。然而，SC的结构架构与组装过程的核心细节仍有待阐明。卷曲螺旋蛋白Zip1是目前唯一在酿酒酵母成熟SC中被广泛解析其分子排布的组分。此前有研究提出，小泛素样修饰因子（Small Ubiquitin-like MOdifier, SUMO）可通过连接染色体轴与Zip1的C端，在SC组装中发挥作用，但由于SUMO缺失的细胞无法存活，SUMO在SC结构中的功能尚未得到直接验证。本研究为SUMO在SC组装中的功能提供了直接实验证据：减数分裂时期的smt3功能减弱菌株表现出孢子形成能力下降、交换重组水平异常以及SC组装受损的表型；若在该菌株背景下于减数分裂后期诱导SC形成，则SC结构几乎完全缺失。本研究借助结构照明显微镜（Structured Illumination Microscopy, SIM）进一步确定了SUMO在酿酒酵母SC结构中的具体位置：与此前将SUMO定位于Zip1 C端附近的模型不同，我们证实SUMO位于SC中央区的中线处，靠近Zip1的N端，处于被称为“中央元件（central element）”的亚结构域中。近期被鉴定出的SUMO化SC组分Ecm11同样定位于SC中央元件。最后，我们发现SUMO、Ecm11甚至无法被SUMO化的Ecm11，均呈现出类似Zip1的动态掺入模式——在减数分裂前期持续整合到已形成的SC中；且SUMO与Ecm11的相对丰度与不同Zip1含量的SC内Zip1的丰度呈显著相关。我们据此提出了一个模型：中央元件蛋白是稳定SC靠近Zip1 N端区域结构的核心构建模块，而SUMO化修饰可能发生在Ecm11等组分掺入SC前体结构之后。