Regulation of chromosome condensation in Saccharomyces cerevisiae during mitosis
Chromosomal condensation is a vital process required for sister chromatid separation during cell division and for maintaining the structural integrity of the chromatin. The central effector of this process is a highly conserved pentameric protein complex known as condensin. The condensin complex is found in all eukaryotes and is essential for viability.  However, it is not clearly known how condensin initiates chromosome condensation at the onset of mitosis. Condensin is a target for various cell cycle kinases during mitosis. So we hypothesized that post-translational modifications might play an important role in activating condensation in early mitosis. Consistent with our hypothesis, we showed that Cdk1 phosphorylates Smc4 in the condensin complex during early mitosis and initiates chromosomal condensation process. Importantly, the Smc4 phospho-mutant experienced defects in condensation. We have also uncovered an intermediate stage during this process, the intertwist. We further demonstrated that the CDK-dependent modification of condensin regulates the dynamic binding of the complex to chromatin, thereby promoting effective chromatin compaction during cell division. We showed that the dynamic binding of condensin is regulated by VCP/p97/Cdc48 (AAA-ATPase), a molecular chaperon known for extracting chromatin-bound proteins. Supporting this idea, we have observed chromosome condensation defects in three conditional-lethal mutants of CDC48 (cdc48-3, cdc48-6 and cdc48-9). We determined that the role of Cdc48 in chromosome condensation requires the activity of Cdc48 co-factors Ufd1-Npl4 and also this process depends on protein ubiquitylation. Finally, we show that the kinetics of condensin mobility on chromatin is dependent on Cdc48. These observations suggest that Cdc48 is acting as a molecular chaperone to facilitate the removal of condensin from chromatin thereby promoting chromosome condensation. Taken together, our work highlights novel regulatory mechanisms responsible for effective chromosome morphogenesis during cell division.
Smc4 Cdk1 Cdc48 Ufd1-Npl4 Phosphorylation Ubiquitination Chaperon Condensin Chaperone Cell cycle