Shugoshin holds the potential to inhibit APC/C and thereby prevents separase activation

Accurate chromosome segregation during mitosis and meiosis depends on shugoshin protein which localizes at the centromere of chromosomes. Sgo1, a meiosis-specific shugoshin in fission yeast, collaborates with the PP2A phosphatase enzyme to remove phosphate groups from Rec8 cohesin, thereby preventing its cleavage by separase during meiosis. Here we demonstrate that ectopic expression of Sgo1 causes chromosome segregation errors during mitosis, especially in cut1-206 separase mutant cells. However, Sgo1 D-box mutant does not exhibit this effect in cut1-206 cells. Our molecular genetic analyses suggest that the Sgo1 D-box directly binds Slp1/CDC20, inhibiting APC/C-dependent separase activation and consequently preventing Rad21 cohesin cleavage. In contrast, during meiosis, the Sgo1 D-box mutant makes the cut1-206 mutant more difficult to cleave Rec8 cohesin. This is because the D-box mutation increases the amount of Sgo1 protein during meiosis, thereby enhancing its canonical protective ability against Rec8 cohesin. Thus, our studies using a separase mutant have revealed that Sgo1 has multiple faces to protect coheisn.