Intermediate step of cohesin's ATPase cycle allows cohesin to entrap DNA.

Cohesin is a four-subunit ATPase in the family of structural maintenance of chromosomes (SMC). Cohesin promotes sister chromatid cohesion, chromosome condensation, DNA repair, and transcription regulation. Cohesin performs these functions as a DNA tether and potentially a DNA-based motor. At least one of its DNA binding activities involves entrapment of DNA within a lumen formed by its subunits. This activity can be reconstituted in vitro by incubating cohesin with DNA, ATP, and cohesin loader. Previously we showed that a mutant form of cohesin (DE-cohesin) possesses the ability to bind and tether DNA in vivo. Using in vitro reconstitution assays, we show that DE-cohesin can form stable complexes with DNA without ATP hydrolysis. We show that wild-type cohesin with ADP aluminum fluoride (cohesinADP/AlFx) can also form stable cohesin-DNA complexes. These results suggest that an intermediate nucleotide state of cohesin, likely cohesinADP-Pi, is capable of initially dissociating one interface between cohesin subunits to allow DNA entry into a cohesin lumen and subsequently interacting with the bound DNA to stabilize DNA entrapment. We also show that cohesinADP/AlFx binding to DNA is enhanced by cohesin loader, suggesting a function for loader other than stimulating the ATPase. Finally, we show that loader remains stably bound to cohesinADP/AlFx after DNA entrapment, potentially revealing a function for loader in tethering the second DNA substrate. These results provide important clues on how SMC complexes like cohesin can function as both DNA tethers and motors.

Mec1-dependent phosphorylation of Mms21 modulates its SUMO ligase activity.

The SUMO ligase Mms21, which is a subunit of the Smc5/6 complex, is required for DNA repair. Here we present results showing that Mms21 was phosophorylated during S-phase in a manner dependent on the DNA damage kinase Mec1. Phosphorylation of Mms21 occurred in unchallenged cells, but was more abundant in the presence of DNA damaging agents. Mass spectrometry identified five phosphorylated serines organized in two regions of Mms21, and two C-terminal serines, S260 and S261, formed part of a Mec1/Tel1 consensus motif. Nonphosphorylatable substitutions of the C-terminal serines, inactivation of Mec1 or removal of the Mms21 C-terminus all abolished Mms21 phosphorylation. Additionally, strains carrying Mms21 phosphoablative alleles displayed reduced SUMO ligase activity, sensitivity to MMS and an increased rate of chromosome loss in the presence of MMS. We propose that one function of S260 S261 phosphorylation is to positively regulate the SUMO ligase activity of Mms21 and thereby promote genomic stability.

The chromosomal association of the Smc5/6 complex depends on cohesion and predicts the level of sister chromatid entanglement.

The cohesin complex, which is essential for sister chromatid cohesion and chromosome segregation, also inhibits resolution of sister chromatid intertwinings (SCIs) by the topoisomerase Top2. The cohesin-related Smc5/6 complex (Smc5/6) instead accumulates on chromosomes after Top2 inactivation, known to lead to a buildup of unresolved SCIs. This suggests that cohesin can influence the chromosomal association of Smc5/6 via its role in SCI protection. Using high-resolution ChIP-sequencing, we show that the localization of budding yeast Smc5/6 to duplicated chromosomes indeed depends on sister chromatid cohesion in wild-type and top2-4 cells. Smc5/6 is found to be enriched at cohesin binding sites in the centromere-proximal regions in both cell types, but also along chromosome arms when replication has occurred under Top2-inhibiting conditions. Reactivation of Top2 after replication causes Smc5/6 to dissociate from chromosome arms, supporting the assumption that Smc5/6 associates with a Top2 substrate. It is also demonstrated that the amount of Smc5/6 on chromosomes positively correlates with the level of missegregation in top2-4, and that Smc5/6 promotes segregation of short chromosomes in the mutant. Altogether, this shows that the chromosomal localization of Smc5/6 predicts the presence of the chromatid segregation-inhibiting entities which accumulate in top2-4 mutated cells. These are most likely SCIs, and our results thus indicate that, at least when Top2 is inhibited, Smc5/6 facilitates their resolution.