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Here we report the discovery of a bacterial DNA-segregating actinlike protein (BtParM) from Bacillus thuringiensis, which forms novel antiparallel, two-stranded, supercoiled, nonpolar helical filaments, as determined by electron microscopy. The BtParM filament features of supercoiling and forming antiparallel double-strands are unique within the actin fold superfamily, and entirely different to the straight, double-stranded, polar helical filaments of all other known ParMs and of eukaryotic F-actin. The BtParM polymers show dynamic assembly and subsequent disassembly in the presence of ATP. BtParR, the DNA-BtParM linking protein, stimulated ATP hydrolysis/phosphate release by BtParM and paired two supercoiled BtParM filaments to form a cylinder, comprised of four strands with inner and outer diameters of 57 Å and 145 Å, respectively. Thus, in this prokaryote, the actin fold has evolved to produce a filament system with comparable features to the eukaryotic chromosomesegregating microtubule.D uring bacterial cell division, equal distribution of replicated plasmids to the two daughter cells ensures their stable inheritance. Type II plasmid segregation systems consist of an actin-like protein (ParM) capable of nucleotide-dependent filament formation and a centrosome-like DNA region (parC), which are linked by an adaptor protein ParR. The model ParCMR system is that of the Escherichia coli R1 plasmid (1). ParM-R1 forms actin-like double-helical straight polar filaments (2), which are paired into randomly oriented bundles. The antiparallel pairing of at least two filaments is required to push apart two R1-ParR/parC complexes (3). All other ParMs, which have been experimentally verified to segregate DNA, including AlfA from Bacillus subtilis (4) and ParM-pSK41 from Staphylococcus aureus (5), have also been shown by electron microscopy to form polar, double-stranded straight filaments with diameters between 80 and 90 Å, similar to eukaryotic F-actin (6).Actins and microtubules have gained dedicated functions during evolution that vary between eukaryotes and prokaryotes. During cell division, the contractile ring in prokaryotes depends on the microtubule-like protein FtsZ, whereas this task relies on actin in eukaryotes. In contrast, DNA segregation in eukaryotes is orchestrated by microtubules, whereas in prokaryotes plasmid DNA segregation depends largely on the actin-like proteins ParMs, although Walker-type ATPase ParA (type I) systems (7) and microtubule-like TubZ (type III) systems have also been found (8). Therefore, a long-standing question has been whether a functional equivalent of the microtubule-like DNA segregating architecture, a hollow cylinder, can be found in bacteria.Using X-ray crystallography, electron microscopy and biochemical assays, we have identified and characterized a novel DNA partitioning ParCMR system from Bacillus thuringiensis (Bt) encoded on the plasmid pBMB67 (9). The filament-forming motor protein, BtParM, proved to be entirely different from all previously studied ParMs; in contrast t...

Microfilaments (actin) and microtubules represent the extremes in eukaryotic cytoskeleton cross-sectional dimensions, raising the question of whether filament architectures are limited by protein fold. Here, we report the cryoelectron microscopy structure of a complex filament formed from 15 protofilaments of an actin-like protein. This actin-like ParM is encoded on the large pCBH Clostridium botulinum plasmid. In cross-section, the ~26 nm diameter filament comprises a central helical protofilament surrounded by intermediate and outer layers of six and eight twisted protofilaments, respectively. Alternating polarity of the layers allows for similar lateral contacts between each layer. This filament design is stiffer than the actin filament, and has likely been selected for during evolution to move large cargos. The comparable sizes of microtubule and pCBH ParM filaments indicate that larger filament architectures are not limited by the protomer fold. Instead, function appears to have been the evolutionary driving force to produce broad, complex filaments.