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 an d depends on a protein complex called cohesin(1-7). At the metaphase-anapha se transition in the yeast Saccharomyces cerevisiae, the ESP1-encoded prote ase separin cleaves SCC1, a subunit of cohesin with a relative molecular ma ss of 63,000 (M-r 63K)(8). The resulting 33K carboxy-terminal fragment of S CC1 bears an amino-terminal arginine-a destabilizing residue in the N-end r ule(9). Here we show that the SCC1 fragment is short-lived (t(1/2) approxim ate to 2 min), being degraded by the ubiquitin/proteasome-dependent N-end r ule pathway. Overexpression of a long-lived derivative of the SCC1 fragment is lethal. In ubr1 Delta cells, which lack the N-end rule pathway(9), we f ound a highly increased frequency of chromosome loss. The bulk of increased chromosome loss in ubr1 Delta cells is caused by metabolic stabilization o f the ESP1-produced SCC1 fragment. This fragment is the first physiological substrate of the N-end rule pathway that is targeted through its N-termina l residue. A number of yeast proteins bear putative cleavage sites for the ESP1 separin, suggesting other physiological substrates and functions of th e N-end rule pathway.