ParA2, a Vibrio cholerae chromosome partitioning protein, forms left-handed helical filaments on DNA

摘要

Most bacterial chromosomes contain homologs of plasmid partitioning (par) loci. These loci encode ATPases called Par A that are thought to contribute to the mechanical force required for chromosome and plasmid segregation. In Vibrio cholerae. the chromosome II (chrll) par locus is essential for chrll segregation. Here, we found that purified ParA2 had ATPase activities comparable to other Par A homologs, but, unlike many other Par A homologs, did not form high molecular weight complexes in the presence of ATP alone. Instead, formation of high molecular weight ParA2 polymers required DNA. Electron microscopy and three-dimensional reconstruction revealed that ParA2 formed bipolar helical filaments on double-stranded DNA in a sequence-independent manner. These filaments had a distinct change in pitch when ParA2 was polymerized in the presence of ATP versus in the absence of a nucleotide cofactor. Fitting a crystal structure of a Par A protein into our filament reconstruction showed how a dimer of ParA2 binds the DNA. The filaments formed with ATP are left-handed, but surprisingly these filaments exert no topological changes on the right-handed B-DNA to which they are bound. The stoichiometry of binding is one dimer for every eight base pairs, and this determines the geometry of the ParA2 filaments with 4.4 dimers per 120 A pitch left-handed turn. Our findings will be critical for understanding how Par A proteins function in plasmid and chromosome segregation.