The Bacterial Cytoskeleton

ParM filament condensation and unidirectional elongation leads to bipolar plasmid segregation. Low copy-number plasmids utilise partitioning systems for the segregation of DNA to the two daughter cells. R1 plasmid contains the ParMRC system, comprising of a centromeric DNA region parC, an actin-like protein ParM and the adaptor protein ParR that binds to the ParM filaments and parC. ParMRC forms a bipolar spindle by growing a filament between the sister plasmids, pushing them to the poles. Here we show that ParR binding locks ParM into a filament-like conformation, functioning similar to eukaryotic actin polymerising proteins. ParR binding is restricted to the barbed-end of the ParM filament, leading to unidirectional elongation, only. ParM filaments condense and slide against each other to and this property is likely to stabilise two or more antiparallel ParM filaments, stabilised at the barbed-end by the ParRC complex. This then leads to the formation of a bipolar spindle, comprised of a bundle of growing and sliding filaments, pushing plasmids in opposite direction.