Combining cell biological, biochemical, and biophysical methods, our research program aims to understand cellular mechanisms governing genomic stability at the molecular and atomic levels. Genome maintenance relies on high-fidelity DNA replication, sister-chromatid cohesion, and chromosome segregation. Defects in these processes cause genomic instability and aneuploidy, which can promote tumorigenesis depending on context. Successful execution of these processes requires their interactions with underlying chromatin and proper coordination during the cell cycle, particularly in S phase and mitosis.
In S phase, DNA replication is coupled to the establishment of sister-chromatid cohesion. Cohesion establishment requires coordinated interactions among the ring-shaped Cohesin and its regulators, including Pds5, Sororin, and Wapl. Sororin and Wapl are positive and negative regulators of cohesin, respectively. Pds5 has dual roles, and binds to Sororin and Wapl mutually exclusively. Binding of the Pds5-Sororin heterodimer to Cohesin with its Smc3 subunit acetylated establishes cohesion, whereas binding of Pds5-Wapl releases Cohesin from chromatin and inhibits cohesion. Antagonistic binding of Sororin and Wapl to Pds5 thus dictates the status of sister-chromatid cohesion.
During early mitosis, the kinases Plk1 and Cdk1 phosphorylate Cohesin and Sororin, triggering their Wapl-dependent removal from chromosome arms. A complex of shugoshin (Sgo1) and PP2A protects cohesin from these kinases at centromeres. At metaphase, centromeric cohesion enables sister kinetochores to attach to microtubules emanating from opposite poles, a state termed bi-orientation. The balance of cohesion and spindle-pulling force creates tension across kinetochores. After all kinetochores are bi-oriented, the protease Separase cleaves centromeric cohesin to initiate anaphase. Through attachment to microtubules anchored at opposing poles, the separated chromatids are evenly partitioned into daughter cells. Kinetochores not bi-oriented activate the spindle checkpoint. This checkpoint inhibits the anaphase-promoting complex/cyclosome (APC/C), a multi-subunit ubiquitin ligase, to block separase activation, anaphase onset, and mitotic exit.
Our research program focuses on, and has contributed to, two major interconnected areas: (1) the spindle checkpoint and (2) sister-chromatid cohesion and chromosome segregation. Our research highlights an emerging principle in cell biology: exquisite spatiotemporal coordination of opposing activities or functionalities underlies transitions between cellular states.