Meta-Biol

• Pathways

Prior to anaphase onset, sister-chromatids are held together by cohesin complexes distributed along chromosomal arms and at centromeres. In prometaphase, PLK1, likely recruited to cohesin complexes by binding to CDK1-phosphorylated CDCA5 (Sororin) (Zhang et al. 2011), phosphorylates cohesin subunits STAG2 (SA2) and RAD21 (Hauf et al. 2005). PLK1-mediated phosphorylation of cohesin subunits at centromeres is counteracted by the phosphatase activity of PP2A complex (containing the regulatory subunit B56 i.e. PPP2R5), which is recruited to the kinetochore by shugoshin proteins, SGOL1 and SGOL2 (Kitajima et al. 2006). Therefore, while cohesin complexes dissociate from chromosomal arms in prometaphase (Hauf et al. 2001), they remain bound to centromeres until anaphase onset (Hauf et al. 2001, Hauf et al. 2005, Kitajima et al. 2006). When separase is activated after its inhibitor securin is degraded by APC/C at the onset of anaphase, RAD21 is cleaved by separase. Phosphorylation of RAD21 by PLK1 facilitates subsequent cleavage of RAD21 by separase (Hauf et al. 2005). There are several potential PLK1 phosphorylation sites in STAG2 and RAD21, but the exact positions of in vivo phosphorylation of STAG2 and RAD21 by PLK1 have not been explicitly established (Hauf et al. 2005).