Structural basis for PP2A-mediated regulation of accurate chromosome segregation

The dynamic crosstalk between kinases and phosphatases is essential to ensure faithful mitosis. Protein phosphatases PP2A(B55) and PP2A(B56), and the Chromosomal Passenger Complex -CPC, containing Aurora B kinase-, establish a signalling network that controls sister chromatid cohesion, kinetochore-microtubule attachments, and timely chromosome segregation. PP2A(B56), together with Shugoshin1 (Sgo1), opposes Aurora B during early mitosis to protect cohesin from being disassembled at the inner centromere. PP2A(B56) and Aurora B also act antagonistically in controlling the processes of error correction and activation of the Spindle Assembly Checkpoint, both of which are essential to achieve chromosome biorientation. PP2A(B55) is the master regulator of mitotic exit, as it triggers late mitotic events by counteracting Cdk1 activity. Combining biochemistry, structural biology and biophysics, my work sheds light into the mechanistic basis of how PP2A regulates these key mitotic events through its interaction with Shugoshin1, the Chromosomal Passenger Complex and the Ska complex. Recent studies have shown that individual PP2A(B56) isoforms might have differential roles during mitosis. Nevertheless, the structural and functional basis of this isoform specificity and how it affects Sgo1 function remain unclear. In my thesis, I have explored the intricacies of PP2A(B56)-Sgo1 interactions and have discovered that Sgo1 binds PP2A(B56) in a B56- isoform-dependent manner. Our ongoing work hints that this specificity is mediated by dynamic interactions between Sgo1 and the B56 subunits, and aims to decipher the functional contribution of these interactions in achieving faithful chromosome segregation My work also unveils a novel interaction between PP2A(B56) and the CPC. Structural analysis of this complex shows that Borealin – one of the components of the CPC – contains a short linear motif that binds PP2A(B56) via the LxxIxE-binding pocket of B56. This highly conserved pocket is key for the recognition of PP2A(B56) interactors, and it is heavily involved in the regulation of PP2A activity and localisation during mitosis. The importance of this interaction in PP2A(B56) and/or CPC localisation and function in vivo is an outstanding question that our ongoing research aims to resolve. Recent studies suggested that PP2A(B55) might interact with the Spindle and Kinetochoreassociated (Ska) complex. The Ska complex is a key protein assembly -consisting of Ska1, Ska2 and Ska3- essential for stabilising load-bearing end-on kinetochore-microtubule attachments, and it is also found in the central spindle and midbody at the end of mitosis. Our research shows that the interaction between PP2A(B55) and the Ska complex is mediated by a short unstructured region of Ska3. This region binds the B55 subunit of PP2A and, according to our predicted models, it might involve its substrate-binding groove. Ongoing efforts in human cell lines aim to explain how this interaction controls the localisation and function of the Ska complex during late mitotic events