title: Are ParM filaments polar or bipolar?
abstract: 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.
authors.affiliation: Department of Biochemistry and Molecular Genetics, University of Virginia, Box 800733, Charlottesville, VA 22908-0733, USA.
authors.forename: Vitold E
authors.initials: VE
authors.lastname: Galkin
authors.forename: Albina
authors.initials: A
authors.lastname: Orlova
authors.forename: Edward H
authors.initials: EH
authors.lastname: Egelman
mesh_terms: D008841:Actin Cytoskeleton / Q000737:chemistry*; D000199:Actins / Q000737:chemistry*; D029968:Escherichia coli Proteins / Q000737:chemistry*
chemical_list: D000199:Actins; D029968:Escherichia coli Proteins