ParM is a bacterial structural homologue of actin that segregates plasmids prior to cell division. Reporting in Science, Garner et al. show that ParM assembles into a dynamic polymerization engine.

ParM is encoded in the par operon, a partitioning locus harboured on the R1 drug-resistance plasmid. Together with ParC (centromere) and ParR (DNA-binding protein), ParM (ATPase) functions to position plasmid pairs at opposite ends of rod-shaped bacteria, ensuring plasmid distribution to both daughter cells following division.

ParM was known to polymerize to form filaments that extended along the length of the cell, with plasmids located at the ends of the filaments. But until now, how the ParM machine functions has not been clear. ParM was labelled and filament assembly monitored by dual-colour fluorescence microscopy. Unlike actin filaments, which assemble unidirectionally, ParM filaments assembled bidirectionally with monomers added to both ends of the filaments.

Assembly kinetics were analysed using fluorescence resonance energy transfer (FRET), revealing that ParM assembles into filaments 300 times faster than actin. A filament long enough to touch both ends of a rod-shaped bacterium was assembled in 10 μs. After growing the length of a rod-shaped bacterium, ParM filaments abruptly began to disassemble. A parM mutant unable to hydrolyse ATP formed a stable filament, indicating that switching between elongation and disassembly is a nucleotide-dependent dynamic instability, which has previously only been observed for eukaryotic microtubules.

Furthermore, Garner et al. confirmed that ParM polymerization stimulates ParM ATPase activity, and that ParM–ADP rapidly dissociates from the polymer. Assembly and disassembly of ParM filaments does not require any accessory factors, probably because it serves only to segregate plasmids.

Replicated plasmids are joined at their centromeres and bound up with ParR. The authors propose that ParM assembles into filaments that scout around the bacterial cell for these plasmid complexes and that bound plasmid complexes stabilize the filaments, preventing disassembly and maintaining plasmid segregation.

Whether disassembly begins at a specific filament terminus and whether MreB, another structural actin homologue, functions to segregate chromosomes using a similar mechanism are questions that should now be addressed. Finding a polymer with dynamic instability in bacteria that has no similarity to tubulin indicates convergent evolution in these lineages and reveals that bacteria are far more sophisticated than was once appreciated.