POLO-LIKE KINASE CDC5 CONTRIBUTES TO MITOTIC SPINDLE ELONGATION VIA THE KINESIN-5 MOTOR PROTEIN CIN8

Proper chromosome segregation requires an orderly sequence of events, whereby spindle elongation follows the dissolution of sister chromatid linkages. Chromosome segregation starts at the onset of anaphase when the separase triggers the cleavage of cohesin, a protein complex that holds sister chromatids together. Next, chromatids are segregated into the daughter cells by the pulling force of the mitotic spindle. The mitotic spindle is a sophisticated and complex machinery built of microtubules, microtubule associated proteins and motor proteins. Despite the fundamental role of the mitotic spindle, the molecular mechanisms underlying its regulation remain elusive. Proper spindle function requires that microtubule dynamics are stabilized at anaphase. This change in microtubule dynamics is perceived as dictated by a shift in the balance of kinase and phosphatase activities in favor of the phosphatases. The finding that cells simultaneously lacking the polo-like kinase Cdc5 and the phosphatase Cdc14 cannot progress through anaphase albeit having cleaved cohesin due to defects in spindle elongation, challenges the view of mitotic exit as a time for protein dephosphorylation. The aim of my work is to understand the molecular mechanism by which the two proteins contribute to anaphase spindle elongation, with a particular focus on the role of Cdc5. We identified the kinesin 5 motor protein Cin8 as a key target of the “Cdc14-Cdc5” spindle elongation pathway. We show that besides being dephosphorylated by Cdc14, Cin8 is also phosphorylated by Cdc5 on residues S409 and S441, and that this phosphorylation is crucial for the function of the kinesin in anaphase spindle elongation, likely because of the impact it has on the interaction between Cin8 and microtubules. Since these residues, S409 and S441, are located within a highly conserved stretch of amino acids, it will be interesting to test whether this regulation is conserved in other vertebrates as well. The finding that Cin8 is simultaneously a substrate of a kinase and a phosphatase sheds light on the complexity of mitotic exit regulation and is in complete agreement with recent data showing that approximately equal numbers of phosphosites are phosphorylated and dephosphorylated during mitotic progression and exit. Since it appears that phosphorylation and dephosphorylation events are equally important to the point that kinases and phosphatases cooperate to regulate the same substrates, the view of mitotic exit as the realm of phosphatases is dismantled and the continuous need for single molecule studies in addition to global analyses investigation is put forward.