Analyses of protein complexes are facilitated by methods that enable the generation of recombinant complexes via coexpression of their subunits from multigene DNA constructs. However, low experimental throughput limits the generation of such constructs in parallel. Here we describe a method that allows up to 25 cDNAs to be assembled into a single baculoviral expression vector in only two steps. This method, called biGBac, uses computationally optimized DNA linker sequences that enable the efficient assembly of linear DNA fragments, using reactions developed by Gibson for the generation of synthetic genomes. The biGBac method uses a flexible and modular “mix and match” approach and enables the generation of baculoviruses from DNA constructs at any assembly stage. Importantly, it is simple, efficient, and fast enough to allow the manual generation of many multigene expression constructs in parallel. We have used this method to generate and characterize recombinant forms of the anaphase-promoting complex/cyclosome, cohesin, and kinetochore complexes.

The spatial organization, correct expression, repair, and segregation of eukaryotic genomes depend on cohesin, ring‐shaped protein complexes that are thought to function by entrapping DNA. It has been proposed that cohesin is recruited to specific genomic locations from distal loading sites by an unknown mechanism, which depends on transcription, and it has been speculated that cohesin movements along DNA could create three‐dimensional genomic organization by loop extrusion. However, whether cohesin can translocate along DNA is unknown. Here, we used single‐molecule imaging to show that cohesin can diffuse rapidly on DNA in a manner consistent with topological entrapment and can pass over some DNA‐bound proteins and nucleosomes but is constrained in its movement by transcription and DNA‐bound CCCTC‐binding factor (CTCF). These results indicate that cohesin can be positioned in the genome by moving along DNA, that transcription can provide directionality to these movements, that CTCF functions as a boundary element for moving cohesin, and they are consistent with the hypothesis that cohesin spatially organizes the genome via loop extrusion.

Chromosome segregation depends on sister chromatid cohesion mediated by cohesin. The cohesin subunits Smc1, Smc3, and Scc1 form tripartite rings that are thought to open at distinct sites to allow entry and exit of DNA. However, direct evidence for the existence of open forms of cohesin is lacking. We found that cohesin's proposed DNA exit gate is formed by interactions between Scc1 and the coiled-coil region of Smc3. Mutation of this interface abolished cohesin's ability to stably associate with chromatin and to mediate cohesion. Electron microscopy revealed that weakening of the Smc3-Scc1 interface resulted in opening of cohesin rings, as did proteolytic cleavage of Scc1. These open forms may resemble intermediate states of cohesin normally generated by the release factor Wapl and the protease separase, respectively.