Katrine Landvad:
The role of the CRL4^Cdt2 target Spd1 in chromosome segregation in fission yeast

Ddb1, a component of the E3 ubiquitin ligase CRL4^Cdt2, is needed for proper chromosome segregation in fission yeast as ddb1 deleted cells show unequal distribution of DNA to daughter cells and sensitivity to the microtubule destabilising drug TBZ. In this study we show that Δddb1 cells have increased chromosome loss rates and that the CRL4 receptor protein Cdt2 is regulating Ddb1 activity in chromosome segregation. Furthermore, we show that ddb1 deficiency results in premature disjunction of the sister chromatids and cause a decrease in the levels of cohesion establishment factor Eso1. Concomitant deletion of spd1, a known target of CRL4^Cdt2, substantially reduces the observed defects of the ddb1 single mutant, indicating that degradation of Spd1 is important to ensure proper chromosome segregation. Spd1 is degraded on proliferating cell nuclear antigen (PCNA) and we propose that accumulation of Spd1 reduces access of Eso1 to PCNA, causing the observed segregation defects.