title: Two-subunit DNA escort mechanism and inactive subunit bypass in an ultra-fast ring ATPase.
abstract: SpoIIIE is a homo-hexameric dsDNA translocase responsible for completing chromosome segregation in Bacillus subtilis. Here, we use a single-molecule approach to monitor SpoIIIE translocation when challenged with neutral-backbone DNA and non-hydrolyzable ATP analogs. We show that SpoIIIE makes multiple essential contacts with phosphates on the 5'→3' strand in the direction of translocation. Using DNA constructs with two neutral-backbone segments separated by a single charged base pair, we deduce that SpoIIIE's step size is 2 bp. Finally, experiments with non-hydrolyzable ATP analogs suggest that SpoIIIE can operate with non-consecutive inactive subunits. We propose a two-subunit escort translocation mechanism that is strict enough to enable SpoIIIE to track one DNA strand, yet sufficiently compliant to permit the motor to bypass inactive subunits without arrest. We speculate that such a flexible mechanism arose for motors that, like SpoIIIE, constitute functional bottlenecks where the inactivation of even a single motor can be lethal for the cell.
authors.affiliation: Jason L. Choy Laboratory of Single Molecule Biophysics, University of California, Berkeley, United States.|Department of Molecular and Cell Biology, University of California, Berkeley, United States.
authors.forename: Ninning
authors.initials: N
authors.lastname: Liu
authors.affiliation: Jason L. Choy Laboratory of Single Molecule Biophysics, University of California, Berkeley, United States.|Department of Physics, University of California, Berkeley, United States.
authors.forename: Gheorghe
authors.initials: G
authors.lastname: Chistol
authors.affiliation: Jason L. Choy Laboratory of Single Molecule Biophysics, University of California, Berkeley, United States.|Department of Molecular and Cell Biology, University of California, Berkeley, United States.|Department of Physics, University of California, Berkeley, United States.|California Institute for Quantitative Biosciences, Berkeley, United States.|Department of Chemistry, Howard Hughes Medical Institute, University of California, Berkeley, United States.|Physical Biosciences Division, Lawrence Berkeley National Laboratory, Berkeley, United States.|Kavli Energy NanoSciences Institute at the University of California, Berkeley and the Lawrence Berkeley National Laboratory, Berkeley, United States.
authors.forename: Carlos
authors.initials: C
authors.lastname: Bustamante
mesh_terms: D000251:Adenosine Triphosphatases / Q000378:metabolism*; D001412:Bacillus subtilis / Q000201:enzymology*; D001426:Bacterial Proteins / Q000378:metabolism*; D004269:DNA, Bacterial / Q000378:metabolism*; D008954:Models, Biological; D011485:Protein Binding
chemical_list: D001426:Bacterial Proteins; D004269:DNA, Bacterial; C030069:spore-specific proteins, Bacillus; D000251:Adenosine Triphosphatases
keywords: B. subtilis; Ring Atpase; SpoIIIE/FtsK; biophysics; intersubunit coordination; neutral DNA; single-molecule; step size; structural biology