Abstract
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.
Highlights
Chromosomes must form enduring attachments with their homologous partners in order to successfully orient and segregate during the first meiotic division
Chromosome ploidy is reduced during meiosis by virtue of prior associations established between homologous chromosomes
Such associations, which are secured by crossover recombination events, allow homologs to achieve an opposing orientation and segregate from one another at meiosis I
Summary
Chromosomes must form enduring attachments with their homologous partners in order to successfully orient and segregate during the first meiotic division. Such pair-wise chromosomal attachments are generated by interhomolog crossover recombination events, which occur through the repair of programmed, double-stranded DNA breaks using the homologous partner chromosome [1,2,3]. Ultrastructural studies in several different organisms led to the description of at least three substructures that define SC [5,6]: first, a synapsed pair of chromosomes exhibit two electron dense structures, termed lateral elements, that lie in parallel to one another. Two distinct substructures within the SC central region itself are visible: transverse filaments are oriented perpendicular to lateral elements and span the central region, while a structure called the central element is oriented in parallel to lateral elements at the midline of the SC central region
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