Towards understanding the mechanism of cohesin loading

Thesis

When a cell divides into two, it is imperative that each resultant daughter receives a full complement of chromosomes; DNA is ultimately responsible for all cellular processes. Cohesion between sister chromatids from the moment of their generation in S phase is central to ensuring the fidelity of chromosome segregation. Smc1 and Smc3 proteins interact with each other via their hinges and with a bridging kleisin subunit via their heads to form the cohesin ring. It is cohesin, through entrapment of sister chromatid within its ring, that confers sister chromatid cohesion. The process of cohesin’s loading onto DNA is poorly understood. While it is thought to depend on ATP hydrolysis, opening of the ring at one of its three interfaces, and the as yet undefined action of the kollerin complex, comprising Scc2 and Scc4 proteins, the sequence of events as they occur are yet to be defined.

A recent screen for suppressors of a thermosensitive scc4 allele in budding yeast revealed a mutation within Smc1’s hinge that could bypass the kollerin subunit. Here, the Smc1 suppressor mutation is investigated. Through targeted mutagenesis, the Smc1D588Y mutant identified in the screen and two additional point mutants, Smc1D588F and Smc1D588W, are herein proven able to bypass Scc4 function completely. Thus we provide the strongest evidence to date to suggest that cohesin’s hinge is a critical factor in its loading. Biochemical evidence shows that isolated Smc1 hinge mutants are defective in their binding to Smc3 hinges. This, together with the genetic link made between the hinge and loading complex, suggests that hinge opening might be a requisite for loading.

Through mutagenesis of Scc2 and Scc4 we show that the N-terminus of each protein is responsible for their dimerisation. Furthermore, the N- terminus of Scc2 confers no function other than in its binding to Scc4. Finally, we show that Scc4 is required for the enrichment of both Scc2 and cohesin at centromeres, but not at arm loci. Our results are therefore indicative of there being two different pathways of cohesin loading.

Authors: Dixon, SE

• Publication date: 2013
• DOI: 10.5287/ora-nzx17vmgy
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: biology; cell division; biochemistry; cohesin; chromosome
• Subjects: Cell Biology (see also Plant sciences); Biology; Biochemistry