Towards a Structural Mechanism for Sister Chromatid Cohesion Establishment at the Eukaryotic Replication Fork

SIMPLE SUMMARY: Cohesion establishment between sister chromatids is essential for cell proliferation. We discuss in this review key observations that link sister chromatid cohesion establishment with DNA replication. Genetic, biochemical, and microscopy studies are critical to uncover that establishment factors associate with the replication machinery. Structural studies are starting to build a framework to explain how the replication machinery traverses cohesin and thereby transitions from one parental duplex DNA to two duplicated DNA filaments behind the fork. Sister chromatid cohesion establishment employs two pathways: one converts cohesin from parental to duplicated DNA, and the other pathway loads cohesin from the nucleoplasm behind the replication fork. We discuss how cryo-EM, combined with single-molecule imaging, could help explain cohesion establishment by discriminating between possible scenarios for cohesin bypass by the replication machinery. ABSTRACT: Cohesion between replicated chromosomes is essential for chromatin dynamics and equal segregation of duplicated genetic material. In the G1 phase, the ring-shaped cohesin complex is loaded onto duplex DNA, enriching at replication start sites, or “origins”. During the same phase of the cell cycle, and also at the origin sites, two MCM helicases are loaded as symmetric double hexamers around duplex DNA. During the S phase, and through the action of replication factors, cohesin switches from encircling one parental duplex DNA to topologically enclosing the two duplicated DNA filaments, which are known as sister chromatids. Despite its vital importance, the structural mechanism leading to sister chromatid cohesion establishment at the replication fork is mostly elusive. Here we review the current understanding of the molecular interactions between the replication machinery and cohesin, which support sister chromatid cohesion establishment and cohesin function. In particular, we discuss how cryo-EM is shedding light on the mechanisms of DNA replication and cohesin loading processes. We further expound how frontier cryo-EM approaches, combined with biochemistry and single-molecule fluorescence assays, can lead to understanding the molecular basis of sister chromatid cohesion establishment at the replication fork.