Figure 1.

The Cdk inhibitor Flavopiridol induces reversible mitotic exit and cytokinesis if proteasome activity is inhibited. a, Treatment of mitotic cells with Flavopiridol induces premature mitotic exit and cytokinesis without chromatid separation. A Xenopus S3 cell expressing GFP-α-tubulin was treated with 5 μM Flavopiridol at time 0. The black arrow indicates a chromosome not yet at metaphase when Flavopiridol was added. The white arrowhead indicates the reassembled nuclear envelope. The complete video sequence is available as Supplementary Video 1. Time is indicated as hours:minutes:seconds (a.- c.). b, Flavopiridol induces mitotic exit in cells arrested at metaphase with MG132. An MG132-arrested cell was treated with 10 μM Flavopiridol at time 0. The complete video sequence is found as Supplementary Video 2. c, Flavopiridol-induced mitotic exit and cytokinesis are reversible if the proteasome is inhibited. An MG132-arrested cell was treated with 5 μM Flavopiridol at time 0. The Flavopiridol was removed at 25 min. The complete video sequence is available as Supplementary Video 3. d, Flavopiridol induces normal mitotic exit changes in the distributions of Aurora B kinase and MKLP1 that are reversible. Xenopus S3 cells were incubated in medium containing MG132 and were fixed before and after treatment with 5 μM Flavopiridol. Samples were labeled for Aurora B kinase (upper panels) or MKLP1 (lower panels). Initial and final images (0 min and 60 min) were scaled identically for brightness and contrast. Others were scaled individually. Bars = 10 μm.