br Introduction Sister chromatid cohesion is
Introduction Sister-chromatid cohesion is established during DNA replication by a ring-shaped cohesin complex consisting of the core subunits Smc1, Smc3, Scc1, and SA1 or SA2, which recruit regulatory subunits Pds5 (Pds5A/B in vertebrates), Wapl, and Sororin to regulate the association of cohesin with chromatin . To ensure accurate chromosome segregation, cohesin between sister chromatids is released in two steps in higher eukaryotes [2, 3]. During prophase and prometaphase, the majority of cohesin on chromosome arms is phosphorylated by mitotic kinases and removed by its antagonist Wapl [4, 5, 6, 7, 8, 9], resulting in sister-chromatid resolution. However, a small population of cohesin is retained at centromeres to ensure chromosome biorientation until its proteolytic cleavage by the protease separase at Cytarabine onset [2, 10]. How the “prophase pathway” of cohesin removal is limited to chromosome arms has been inadequately addressed. Wapl-mediated cohesin release from chromatin requires its interaction with Pds5 [4, 5, 11, 12, 13], a process that is antagonized by Sororin binding to Pds5 . In early mitosis of vertebrate cells, Sororin is phosphorylated and dissociates from Pds5 [14, 15, 16], allowing Wapl to gain access to Pds5 and to remove cohesin from chromosome arms. Protection of cohesin at mitotic centromeres requires Shugoshin-1 (Sgo1) . In prometaphase, Sgo1 is recruited to the inner centromeres, where it occupies the Wapl-binding site at the Scc1-SA2 interface [18, 19, 20], thereby shielding cohesin from Wapl. Centromeric Sgo1 also collaborates with protein phosphatase 2A (PP2A) to prevent centromeric SA2 phosphorylation and disassociation [21, 22] and to counteract Sororin phosphorylation [15, 18]. It remains unclear whether additional proteins also contribute to Wapl inhibition at mitotic centromeres. The protein kinase Haspin phosphorylates histone H3 at Thr-3 (H3pT3) in mitosis, particularly at centromeres . We and others previously showed that this phospho-histone mark is directly recognized by the chromosomal passenger complex (CPC) [24, 25, 26], a key regulator of mitosis , thereby promoting chromosome alignment. RNAi-mediated depletion of Haspin caused premature chromatid separation (PCS) in human cells, indicating a requirement for Haspin in chromosome cohesion . However, it is unknown whether, and how, Haspin is directly involved in cohesion protection.
Discussion Taken together, our data suggest that at mitotic centromeres, there is a pool of cohesin complex in which Haspin binds to Pds5B through a conserved YGA/R motif in its flexible N terminus, which antagonizes the YSR-motif-dependent Wapl-Pds5B interaction. The Haspin-Wapl antagonism works together with the previously identified Sgo1/Sororin-Wapl antagonism to prevent premature removal of centromeric cohesin and maintain proper cohesion at mitotic centromeres (Figure 7I). Pds5 promotes cohesin release through binding to the YSR motif of Wapl, which is antagonized by Sororin, which also uses an YSR motif to directly bind the same site on Pds5 . The Phe-Gly-Phe (FGF) motifs of Wapl and Sororin also contribute to their association with Pds5 and are implicated in the Sororin-Wapl antagonism [13, 14, 41]. Interestingly, at least when overexpressed, Sororin mutants lacking either YSR or FGF motifs are able to support cohesion in human cells . Ouyang et al. proposed that “another protein might act redundantly with the YSR and FGF motifs of Sororin to antagonize these motifs of Wapl in human cells” . Our results indicate that the YGA/R-motif-containing Haspin is such a protein. Moreover, whereas Wapl orthologs exist in species from yeast to humans , Sororin has only been identified in metazoans so far . The high conservation of the YGA/R motif suggests that the mechanism underlying cohesion protection by Haspin may be widely conserved in eukaryotes and that Wapl activity may be antagonized by Haspin in species lacking Sororin orthologs.