- Identification and characterization of Sgo2 interactions - Insights into dynamic chromosomal localization, mechanism of cohesin protection and putative checkpoint function (2010)
- Sister chromatids are embraced and held together by a ring-shaped multiprotein complex called cohesin, from the time of their generation in S-phase until their separation in M-phase. Shugoshins protect centromeric cohesin from premature dissociation and, hence, are important regulators of genome stability. This work demonstrates that hSgo2, one of the two shugoshins in humans, first binds to kinetochores in prophase, then localizes to centromeres in prometaphase before finally traveling back to kinetochores in metaphase. It is further shown that (1) chromatin binding requires the C-terminus of hSgo2, (2) the catalytic activity of the aurora B kinase is essential to focus hSgo2 at centromeres, and (3) attachment of microtubules to kinetochores is not only necessary for the metaphase-specific re-localization of hSgo2 but also sufficient; pulling forces are not required. These newly discovered regulations allow to formulate a mechanistic model that explains shugoshin’s dynamic subcellular localization. An existing conflict in the literature concerns the relative importance of mammalian Sgo1 and Sgo2 in mitosis. Cell biological analyses now demonstrate unambiguously that, contrary to earlier claims, hSgo2 is dispensable for several aspects of mitotic cell divisions. Thus, shugoshin functions are strictly separated, being fulfilled by hSgo1 in mitosis and by hSgo2 in meiosis. Initially, it was unclear how shugoshins exert their cohesin protective function at the molecular level. Using a biochemical approach, protein phosphatase 2A (PP2A) was identified as a prominent interactor of shugoshins in this thesis. Shortly thereafter, it was reported that shugoshins protect cohesin by mediating PP2A-dependent dephosphorylation. Nevertheless, it is shown here for the first time that hSgo2 binds PP2A via its N-terminus. A Sgo2 point mutant deficient in PP2A binding is created and characterized by cell biological experiments. Importantly, biochemical assays demonstrate that hSgo2 greatly stimulates PP2A’s enzymatic activity. Shugoshin function therefore extends beyond simple provision of a linkage between cohesin and PP2A. Mitosis and meiosis are chiefly controlled by the spindle assembly checkpoint (SAC), which allows anaphase to take place only after all chromosomes have become properly attached to the mitotic spindle. Central to SAC signaling, kinetochore bound Mad1-Mad2 complex catalyzes a conformational switch of soluble Mad2, thereby allowing its inhibitory binding to the downstream effector protein Cdc20. SAC-dependent inhibition of anaphase correlates well in timing with shugoshin-dependent protection of cohesin. Here, Mad2 is identified as another novel interactor of hSgo2. Precise mapping reveal a conserved Mad2 interaction motif (MIM) in hSgo2, which is shared by the known Mad2 interactors Mad1 and Cdc20. In fact, several lines of evidence show that the Sgo2-Mad2 complex is structurally very similar to the Mad1-Mad2 and the Cdc20-Mad2 complexes. These biochemical studies challenge the current “one source (kinetochore bound Mad1) – one target (Cdc20)” dogma in the field of SAC research. Mad2 binding is conserved in the only Xenopus laevis shugoshin, xSgo1, indicating an important function of this interaction during vertebrate meiosis.