Figure 5.

Effects of calponin 2 on the rate and fate of cytokinesis. Green fluorescent protein (GFP)-fusion calponin 2 was expressed in SM3 cells via transient transfection for 24 h. Individual cells were monitored with fluorescence microscopy using a stage culture chamber. GFP fluorescent images were recorded throughout the course of mitosis. The representative recordings show the correlation between calponin 2 level and the velocity and fate of cytokinesis. A: Group I cells expressed appropriate levels of calponin 2 with an effective decrease after metaphase and showed normal completion of cytokinesis within 1 h. B: Group II cells expressed higher levels of calponin 2 with an effective decrease after metaphase could also complete cytokinesis although with a significantly longer time. C: in contrast, Group III cells expressed high levels of calponin 2 without notable decrease after metaphase failed to complete cytokinesis and ended up with cell death after a few hours. The lower level of GFP-fusion calponin 2 seen in the cleavage furrow of successfully dividing cell is indicated by arrows. D: quantification of GFP-calponin 2 fluorescence intensity shows the level and change of calponin 2 in metaphase and cytokinesis. The pattern illustrates that cells expressing normal or high level of calponin 2 can complete cytokinesis by having an effective degradation after metaphase. The longer duration of cytokinesis for cells that need to remove high level of calponin 2 suggest that calponin 2 degradation is a determinant of the rate of cytokinesis, by which the failure of post metaphase degradation of calponin 2 results in failure of cytokinesis followed by cell death. Values are presented as means ± SD.