strol washout into Haspin inhibitors, allowing substantial error correction before Haspin-dependent Aurora B targets became dephosphorylated. We therefore conducted assays in which Aurora B was initially inhibited but activation was allowed upon monastrol and Hesperadin washout. In this format, all three Haspin inhibitors strongly hindered chromosome alignment at all tested doses. In these assays, we were unable to determine if retargeting Aurora B to centromeres could rescue the defect because expression of CENP-BINCENP itself disrupts error correction, presumably because the increased local concentration of Aurora B near kinetochores decreases microtubule binding. Nevertheless, the results indicate that the CPC population targeted by the HaspinH3T3ph pathway is required for efficient error correction. Phosphorylation of several KMN network proteins including KNL1, Dsn1, and Hec1/Ndc80 at kinetochores contributes to the regulation of microtubule attachment. PBTZ 169 Consistent with a role of the Haspin-dependent CPC population in error correction, Haspin inhibitors strongly reduced the phosphorylation of Dsn1 at the Aurora B target residue S109 in Aurora B reactivation assays, and Dsn1 phosphorylation could be largely restored by retargeting Aurora B to centromeres using CENP-BINCENP. Live imaging of cells treated with Haspin inhibitors To directly observe the effects of Haspin inhibitors on mitosis, we performed time-lapse microscopy of U2OS cells expressing histone H2B-mRFP and -tubulinGFP. All three inhibitors caused a moderate increase Haspin inhibitors and centromeric Aurora B Wang et al. 257 258 JCB VOLUME 199 NUMBER 2 2012 in the length of mitosis, defined as the period between nuclear envelope breakdown and anaphase onset. This was reminiscent of a similar extension of mitosis reported for cells treated with Aurora B inhibitors. We also PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/19834673 noted a dose-dependent decline in the number of cells entering mitosis. This effect was not apparent in prior RNAi studies, and whether it reflects a role for Haspin outside mitosis or off-target effects of the compounds requires further investigation. All three compounds caused a dose-dependent increase in the proportion of defective mitoses. Lagging chromosomes at anaphase were often observed, even at relatively low inhibitor concentrations. At higher concentrations, cells that entered anaphase with chromosomes that had not congressed, or that entered anaphase with ill-defined or “loose” metaphase plates, became increasingly apparent. At 10 M of the most potent inhibitor, 5-iodotubercidin, cytokinesis often occurred without obvious chromosome disjunction. In these cases, the cytokinetic furrow impinged upon the chromosome mass, resulting in a “cut-like” phenotype resembling that seen upon microinjection of antibodies against H3T3ph. Similar mitotic figures, in which central spindle formation was evident in the absence of obvious anaphase chromosome movements, were seen in fixed cells previously treated with 10 M 5-iodotubercidin or 100 M LDN-211898. These results support the conclusion that Haspin inhibition causes defects in error correction, but that it does not affect the central spindle functions of Aurora B or prevent cytokinesis. Haspin inhibitors compromise maintenance of the spindle checkpoint The finding that inhibitor-treated cells could exit mitosis before chromosomes were fully aligned suggested either that the spindle checkpoint was satisfied on such spindles, or that a defect in the spind