Dynamic control of sister chromatid cohesion: How is DNA association of cohesin regulated by Wapl and acetylation

DNA is a macromolecule present in each cell, which acts as the genetic information carrier in all organisms (from bacteria to humans). This genetic information is passed from parents to their offspring and decides their biological characteristics. During cell proliferation, all of the DNA is precisely duplicated to form identical 'sister DNAs'. Subsequently, the sister DNAs must be accurately separated and equally distributed into the two newborn daughter cells. If anything goes wrong during this process, cells will encounter catastrophic consequences, such as cell death or malfunction. This will lead to cancer and other diseases, for example, developmental disorders. To prevent premature separation and ensure the faithful passing of genetic information from parent cells to daughter cells, the replicated sister DNAs must be held together through a mechanism called sister chromatid cohesion. Only when the cells are fully prepared and the sister DNAs are ready to separate is the sister chromatid cohesion removed, to permit the movement of the sister DNAs to opposite poles of the cell.

Sister chromatid cohesion is achieved through a protein complex called cohesin, which consists of three protein subunits. Besides sister chromatid cohesion, cohesin also plays important roles in compacting DNA, regulating gene expression, and repairing damaged DNA. Though it is responsible for multiple functions on DNA, cohesin actually has a very simple mode to interact with DNA. The three subunits of cohesin form a ring structure and DNA is entrapped within this ring. To enable cohesin to carry out its functions, the ring can open and close, allowing the DNA fiber to get in or out of the ring. Precise regulation of the opening and closing of the ring is fundamental for cohesin's versatile functions. Defects in this regulation compromise cohesin's function, leading in humans to cancer and inherited developmental disorders (such as Cornelia de Lange syndrome and Roberts syndrome).

A key protein for regulating the opening of the cohesin ring is called Wapl. Wapl is crucial for the removal of sister chromatid cohesion when sister DNAs are ready to separate. Compromising Wapl function causes delayed intellectual development during childhood. Although Wapl was discovered about twenty years ago, our knowledge of the molecular mechanism of Wapl-dependent cohesin ring opening has progressed very little. The key challenge for studying this process is caused by the transient interaction of Wapl with cohesin, which poses a barrier to understanding how Wapl physically interacts with cohesin. In this study, we will used advanced in vivo crosslink techniques to reveal which part of cohesin is bound by Wapl, revealing how Wapl opens the cohesin ring. This approach combined with biochemical analysis will unveil the molecular mechanism through which Wapl triggers disassociation of cohesin from DNA. Insight into this fundamental process will improve our understanding of how DNA segregation occurs in cell division, and how it can go wrong to cause cancer or failures in the developmental programme leading to cohesin-related diseases.

Cohesin is a key player in determining sister chromatid cohesion, interphase chromosome structure, and DNA damage repair. The three yeast cohesin subunits Smc1, Smc3, and Scc1 form a tripartite ring that embraces DNA. Wapl, a regulatory subunit of cohesin, disengages the interface between Scc1 and Smc3 to release cohesin from DNA. This process triggers the removal of sister chromatid cohesion at the early stage of mitosis. Moreover, during G1 phase regulated cohesin release by Wapl prevents the over-compaction of DNA, facilitating DNA replication and entry to S phase. During DNA replication, Smc3 is acetylated, which antagonises Wapl to prevent precocious disjunction of sister chromosomes prior to mitosis. Defects in these processes cause profound abnormalities. Failure of Smc3 acetylation causes Roberts syndrome, a cohesion-related genetic disorder. Moreover, several Wapl mutations were recently reported to be associated with delayed mental/intellectual development. Although the roles of Wapl and Smc3 acetylation were discovered a decade ago, the molecular mechanisms of their action remain enigmatic.

Our studies of yeast cohesin revealed a direct interaction of Wapl with the Scc1 N-terminus, that is enhanced by Smc3 mutations that mimic acetylation. Based on our results, we propose a model of how Wapl releases cohesin from DNA and how Smc3 acetylation inhibits this release process. This study will test our model, investigating the biochemical properties of the Wapl::N-Scc1 interaction to understand how it promotes cohesin release. We will also elucidate how Smc3 acetylation blocks this process. We will further examine whether the molecular mechanism of yeast cohesin release also operates in human cells to regulate chromosome segregation. This work will offer fundamental knowledge to understand the pathology of cohesin-related disorders caused by defects in Wapl and Smc3 acetylation, essential for their prevention and treatment.