Biochemical studies of the cohesin complex

Thesis

The accurate inheritance of genetic material depends upon the establishment and maintenance of sister chromatid cohesion. Replicated chromosomes are topologically encircled by the large, tripartite protein complex cohesin, allowing bi-orientation in mitosis. To entrap and reversibly dissociate from DNA, the annular complex structure must be disrupted at either the hinge domain between Smc1 and Smc3, or the interfaces created by the kleisin subunit Scc1 bridging the two ABC-like ATPase domains. The aim of this work was to characterise the cohesin complex loading and releasing mechanisms by examining the biochemical requirements for these processes. Although the identity of a chromosomal cofactor could not be assigned, the loading reaction was found to necessitate engagement of ATPase domains in an ATP-dependent manner. Notions of allosteric modulation of ATP binding and NBD engagement by acetylation were discredited. Likewise, a direct and stable physical association of hinge domains with NBDs was shown to be an unlikely conformational intermediate in a reaction thought to promote hinge opening for loading of cohesin onto DNA. The Smc3-Scc1 kleisin interface might be exploited during the opposing process of release of cohesin from DNA. Therefore, a novel protein- protein cross-linking system was adapted for use in S. cerevisiae, with a view to (1) confirming the well-founded role of hinge dissociation in topological entrapment, and (2) validating the Smc3- Scc1 interface as the recently conjectured exit gate. Despite promising preliminary kinetics in vitro, the SpyTag-SpyCatcher system was considerably less efficient in vivo. It was thus deemed unsuitable in its current format for investigating the process of interface dissociation in live cells. Finally, the large, cohesin complex-associated HEAT repeat protein Pds5 has been either speculated or shown to participate in all of the aforementioned processes, potentially modulating the hinge and Smc-kleisin interfaces. Acting as a regulatory node in cohesin function, it was pursued as an informative target for structural studies. Although no diffractive material could be obtained, Pds5 was confirmed to bind a short, N-terminal sequence of Scc1 and was in turn bound at its N- terminus by Wapl. Together, these findings contribute to defining the structures and states of the cohesin complex.

Authors: Upcher, WR

• Publication date: 2012
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: biology chromosome cell biochemistry
• Subjects: Biochemistry; Biology