A bipolar spindle of antiparallel ParM filaments drives bacterial plasmid segregation.

To ensure their stable inheritance by daughter cells during cell division, bacterial low-copy-number plasmids make simple DNA segregating machines that use an elongating protein filament between sister plasmids. In the ParMRC system of the Escherichia coli R1 plasmid, ParM, an actinlike protein, forms the spindle between ParRC complexes on sister plasmids. By using a combination of structural work and total internal reflection fluorescence microscopy, we show that ParRC bound and could accelerate growth at only one end of polar ParM filaments, mechanistically resembling eukaryotic formins. The architecture of ParM filaments enabled two ParRC-bound filaments to associate in an antiparallel orientation, forming a bipolar spindle. The spindle elongated as a bundle of at least two antiparallel filaments, thereby pushing two plasmid clusters toward the poles.