th Haspin, which phosphorylates H3 on Thr3 to recruit the CPC and activates Aurora B, to support spindle assembly. Double depletion of XSgo2 and H3pThr3 severely inhibited bipolar spindle formation and generated asters. Generation of residual microtubules in these conditions is most likely due to the presence of CPC-microtubule interaction, which also supports spindle microtubule assembly. As previously shown for human Sgo2, we found that targeting of XSgo2 to centromeres depends both on Bub1 and on Aurora B. In Xenopus, depletion of Aurora B completely abolishes centromeric accumulation of Bub1 and thus directing Bub1 to centromeres could be the main function of Aurora B in the targeting of XSgo1 and XSgo2. Sgo proteins have been recently proposed to act as CPC adaptors so that their interaction is important for co-targeting to centromeres, PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/19828810 a role apparently shared by Sgo1 and Sgo2 in human cells. In contrast, XSgo1 and XSgo2 affect distinct aspects of CPC regulation and function. Depletion of XSgo1 alters CPC distribution, whereas depletion of XSgo2 impairs CPC activation. Because the spatial regulation of the CPC is thought to be essential for the coordination of many mitotic events, one would expect that its anomalous distribution in the absence of XSgo1 would also affect CPC function. However, we have found that depletion of XSgo1 diminishes significantly the amount of total Aurora B present at centromeres, but it barely changes the amount of & 2012 European Molecular Biology Organization active Aurora B. Consistently, the lack of XSgo1 does not affect MCAK distribution or spindle assembly. Downregulation of Sgo1 in human cells causes delocalization of Aurora B from centromeres but also in this case centromeric MCAK is unaffected. Thus, an excess of the CPC apparently accumulates at the centromeric region. It is likely that different sub-populations of the complex exist that can be modulated by proteins other than Sgo2 such as Sds22/PP1 or TD60. How do XSgo1 and XSgo2 carry out their specific functions in chromosome segregation Our results showing preferential interaction of the two proteins with distinct PP2A-B56 subunits–which could dictate the substrate specificity of the enzyme–illuminate one possible answer to this question. This is consistent with the proposal that the association of Sgo proteins with PP2A serves to specify the substrate of the phosphatase by the recruitment of different PP2A complexes to centromeres. We have shown that depletion of B56 gamma removes XSgo1 from the extract and, consequently, from centromeres, but does not affect XSgo2 levels, localization or function. To produce GFP-XSgo2, this cDNA was inserted between ClaI and XhoI sites of the pAFS210 vector. A cDNA encoding X. laevis PP2A-B56e was amplified from IMAGE clone 6318521 and cloned in pcDNA3.2-V5-DEST using the Gateway Cloning System. PP2A-B56e was in-vitro translated using the TNT Quick coupled transcription/translation system according to manufacturer’s instructions and diluted five-fold in the egg extracts. Antibodies Rabbit polyclonal sera against XSgo2 were obtained by using a buy Neuromedin N synthetic peptide as immunogen and affinity purified. A second antibody raised against a C-terminal fragment of Sgo2 was also obtained and used in some experiments with indistinguishable results. Other antibodies used in this study were Haspin, INCENP and Aurora B; Dasra A; Survivin; CENP-A, Bub1 and XSgo1; MCAK and MCAK pS196 ; PP2A-B56g and PP2A-B56e; PP2A-B56a;