Aberrant cohesin function in Saccharomyces cerevisiae activates Mcd1 degradation to promote cell lethality

The cohesin complex is composed of core ring proteins (Smc1, Smc3 and Mcd1) and associated factors (Pds5, Scc3, and Rad61) that bind via Mcd1. Cohesin extrusion (looping from within a single DNA molecule) and cohesion (the tethering together of two different DNA molecules) underlie the many roles that cohesins play in chromosome segregation, gene transcription, DNA repair, chromosome condensation, replication fork progression, and genomic organization. While cohesin function flanks the activities of critical cell checkpoints (including spindle assembly and DNA damage checkpoints), the extent to which cells directly target cohesins in response to aberrant cohesin function remains unknown. Based on prior evidence that cells mutated for cohesin contain reduced Mcd1 protein, we tested whether loss of Mcd1 is based simply on cohesin instability. We find that Mcd1 loss persists even in rad61 cells, which contain elevated levels of stable chromosome-bound cohesins, contrary to a simple instability model. In fact, re-elevating Mcd1 levels suppressed the temperature-sensitive growth defects of all cohesin alleles tested, revealing that Mcd1 loss is a fundamental mechanism through which cohesins are inactivated to promote cell lethality. Our findings further reveal that cells that exhibit aberrant cohesin function employ E3 ligases to target Mcd1 for degradation. This mechanism of degradation appears unique in that Mcd1 is reduced during S phase, when Mcd1 levels typically peak and despite a dramatic upregulation in MCD1 transcription. We infer from these latter findings that cells contain a negative feedback mechanism used to maintain Mcd1 homeostasis.