Control of mitotic spindle disassembly through SUMO-targeted ubiquitylation of yeast kinesin-5

Faithful cell division requires precise control of spindle assembly and disassembly. In budding yeast, spindle breakdown at mitotic exit depends on Cdk1 inactivation and ubiquitin-mediated degradation of spindle stabilizing factors, including the kinesin-5 Cin8 and the crosslinker Ase1/PRC1. Timely degradation of Cin8 by APC ^Cdh1 prevents spindle over-elongation and deformation. We have identified SUMOylation and the SUMO-targeted ubiquitin ligase Slx5 as regulators of spindle disassembly. Cin8 is SUMOylated and interacts with Slx5, suggesting that it is a substrate of Slx5. Deletion of the SLX5 gene or expression of a SUMOylation-defective Cin8 mutant cause spindle disassembly defects that resemble those of degradation-resistant Cin8 mutants. Reduction of Cin8 SUMOylation partly stabilizes spindles in ase1Δ cells. In addition, SUMOylation of Cin8 and Slx5 are required for timely actomyosin-ring contraction and Cin8 clearance from the mitotic spindle. We propose that SUMOylation regulates Cin8 activity and that SUMO-targeted ubiquitylation facilitates removal Cin8 from the spindle, promoting spindle disassembly at the end of mitosis.