mDia proteins are mammalian homologues of Drosophila diaphanous and belong to 
the formin family proteins that catalyze actin nucleation and polymerization. 
Although formin family proteins of nonmammalian species such as Drosophila 
diaphanous are essential in cytokinesis, whether and how mDia proteins function 
in cytokinesis remain unknown. Here we depleted each of the three mDia isoforms 
in NIH 3T3 cells by RNA interference and examined this issue. Depletion of mDia2 
selectively increased the number of binucleate cells, which was corrected by 
coexpression of RNAi-resistant full-length mDia2. mDia2 accumulates in the 
cleavage furrow during anaphase to telophase, and concentrates in the midbody at 
the end of cytokinesis. Depletion of mDia2 induced contraction at aberrant sites 
of dividing cells, where contractile ring components such as RhoA, myosin, 
anillin, and phosphorylated ERM accumulated. Treatment with blebbistatin 
suppressed abnormal contraction, corrected localization of the above components, 
and revealed that the amount of F-actin at the equatorial region during 
anaphase/telophase was significantly decreased with mDia2 RNAi. These results 
demonstrate that mDia2 is essential in mammalian cell cytokinesis and that 
mDia2-induced F-actin forms a scaffold for the contractile ring and maintains 
its position in the middle of a dividing cell.