“We knew before that mutants in Swi6 lost chromosomes at a high rate and displayed a high frequency of lagging chromosomes on anaphase spindles,” says Allshire. But the explanation for this defect remained unclear. Was Swi6 helping to establish kinetochore structure? Allshire says this remains a possibility, but for now he has established another explanation. Mutants lacking Swi6 maintain cohesion on chromosome arms during a mitotic block but lose cohesion at the centromeres thanks to the loss of a cohesin subunit.
Swi6 is also required for binding of cohesin near telomeres and the silent mating locus—the other heterochromatic regions in fission yeast. This suggests that kinetochore components are not necessary for Swi6-dependent cohesin loading. Only future work will show whether a direct interaction between Swi6 and cohesin is responsible for loading cohesin.▪