Abstract
The cohesin subunits Smc1, Smc3 and Scc1 form large tripartite rings which mediate sister chromatid cohesion and chromatin structure. These are thought to entrap DNA with the help of the associated proteins SA1/2 and Pds5A/B. Structural information is available for parts of cohesin, but analyses of entire cohesin complexes are limited by their flexibility. Here we generated a more rigid ‘bonsai' cohesin by truncating the coiled coils of Smc1 and Smc3 and used single-particle electron microscopy, chemical crosslinking-mass spectrometry and in silico modelling to generate three-dimensional models of cohesin bound to Pds5B. The HEAT-repeat protein Pds5B forms a curved structure around the nucleotide-binding domains of Smc1 and Smc3 and bridges the Smc3-Scc1 and SA1-Scc1 interfaces. These results indicate that Pds5B forms an integral part of the cohesin ring by contacting all other cohesin subunits, a property that may reflect the complex role of Pds5 proteins in controlling cohesin–DNA interactions.
Highlights
The cohesin subunits Smc[1], Smc[3] and Scc[1] form large tripartite rings which mediate sister chromatid cohesion and chromatin structure
Structural information has been obtained for the three ringforming subunit interfaces of cohesin (Smc1–Smc[3], Smc1–Scc[1], Scc1–Smc3)[14,17,36], for parts of Scc[1], SA2 and Wapl and structures have been predicted for the Huntingtin-Elongation factor 3-A subunit-TOR (HEAT) repeats of Pds[5] proteins[13]
Compared with cohesin tetramers (Fig. 1c), an additional density was observed in Pds5B-containing complexes in close vicinity to the nucleotide-binding domain (NBD) of Smc[1] and Smc[3]
Summary
The cohesin subunits Smc[1], Smc[3] and Scc[1] form large tripartite rings which mediate sister chromatid cohesion and chromatin structure. We obtained low-resolution three-dimensional (3D) structures of bonsai trimers (Smc1B, Smc3B, Scc1) bound to either SA1, or Pds5B or both, and used these together with crosslinking-mass spectrometry to analyse how SA1 and Pds5B interact with cohesin The results of these experiments imply that Pds5B is not associated with the cohesin ring only via binding to Scc[1], but forms an integral part of the ring by forming contacts with Smc[1], Smc[3] and SA1. This structural information may help to explain the numerous functions that have been attributed to Pds[5] proteins
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