Bacterial actin homolog ParM: arguments for an apolar, antiparallel double helix

Structure of the ParM filament at 8.5Å resolution

The actin-like protein ParM forms the cytomotive filament of the ParMRC system, a type II plasmid segregation system encoded by Escherichia coli R1 plasmid. We report an 8.5 Å resolution reconstruction of the ParM filament, obtained using cryo-electron microscopy. Fitting of the 3D density reconstruction with monomeric crystal structures of ParM provides insights into dynamic instability of ParM filaments. The structural analysis suggests that a ParM conformation, corresponding to a metastable state, is held within the filament by intrafilament contacts. This filament conformation of ParM can be attained only from the ATP-bound state, and induces a change in conformation of the bound nucleotide. The structural analysis also provides a rationale for the observed stimulation of hydrolysis upon polymerisation into the filament.

The bacterial cytoskeleton: more than twisted filaments

Far from being simple 'bags' of enzymes, bacteria are richly endowed with ultrastructures that challenge and expand standard definitions of the cytoskeleton. Here we review rods, rings, twisted pairs, tubes, sheets, spirals, moving patches, meshes and composites, and suggest defining the term 'bacterial cytoskeleton' as all cytoplasmic protein filaments and their superstructures that move or scaffold (stabilize/position/recruit) other cellular materials. The evolution of each superstructure has been driven by specific functional requirements. As a result, while homologous proteins with different functions have evolved to form surprisingly divergent superstructures, those of unrelated proteins with similar functions have converged.

Are ParM filaments polar or bipolar?

A recent perspective [Erickson, H. (2012). Bacterial actin homolog ParM: arguments for an apolar, antiparallel double helix. J. Mol. Biol., 422, 461-463] by Harold Erickson has suggested that published reconstructions of bacterial ParM filaments from three different laboratories may have artifactually imposed polarity upon a filament that is really bipolar, with the two strands running in opposite directions. We show that Erickson's model of a bipolar filament can be easily distinguished from a polar filament by helical diffraction, since the asymmetric unit in a bipolar filament would be twice the size as that in a polar filament. Existing data from both electron cryo-microscopy and X-ray diffraction exclude a bipolar model. We adopt the suggestion put forward by Erickson to process filaments, assuming that they are bipolar, and show that the resulting filaments are polar.