Expression of S331E Aurora B rescues chromosome alignment or segregation after Chk2 inhibition. (A) Western blot analysis of total Mps1 and actin in cells transfected with negative siRNA (control) or Chk2 siRNA (siChk2) followed by shake-off. Values at control were taken as 1. MI, mitotic index. (B) Tetracycline-induced CHO S331E Aurora B cells expressing Mps1-GFP were untreated or treated with Chk2 inhibitor II (inh II) in the presence of 2 µM AZ3146 for 3 h. Boxed values show mean Mps1-GFP/CENP-B fluorescence intensity ± SDs. Values in square brackets show kinetochore pairs and number of cells analyzed. Insets show 1.7× magnification of kinetochores. (C and D) Tetracycline-induced WT or S331E Aurora B cells were treated with Chk2 inhibitor II for 2 h (C) or with Chk2 inhibitor and MG132 for 1 h (D). (C, left) Anaphase with lagging chromatin (indicated by an arrow). (C, right) Normal anaphase. (D, top) Metaphase with misaligned chromosomes. (D, bottom) Normal metaphase. The frequency of cells exhibiting the respective phenotype is shown in parentheses. (E) Model for the role of Chk2 in mitosis. p indicates phosphorylation. Dotted lines indicate prolonged checkpoint activation. Bars, 5 µm.