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.