Degradation of a cohesin subunit by the N-end rule pathway is essential for chromosome stability

Cohesion between sister chromatids is established during DNA replication and depends on a protein complex called cohesin1,2,3,4,5,6,7. At the metaphase–anaphase transition in the yeast Saccharomyces cerevisiae, the ESP1-encoded protease separin cleaves SCC1, a subunit of cohesin with a relative molecular mass of 63,000 (Mr 63K)8. The resulting 33K carboxy-terminal fragment of SCC1 bears an amino-terminal arginine—a destabilizing residue in the N-end rule9. Here we show that the SCC1 fragment is short-lived (t1/2 ≈ 2 min), being degraded by the ubiquitin/proteasome-dependent N-end rule pathway. Overexpression of a long-lived derivative of the SCC1 fragment is lethal. In ubr1Δ cells, which lack the N-end rule pathway9, we found a highly increased frequency of chromosome loss. The bulk of increased chromosome loss in ubr1Δ cells is caused by metabolic stabilization of the ESP1-produced SCC1 fragment. This fragment is the first physiological substrate of the N-end rule pathway that is targeted through its N-terminal residue. A number of yeast proteins bear putative cleavage sites for the ESP1 separin, suggesting other physiological substrates and functions of the N-end rule pathway.