title: The bacterial DnaC helicase loader is a DnaB ring breaker.
abstract: Dedicated AAA+ ATPases deposit hexameric ring-shaped helicases onto DNA to promote replication in cellular organisms. To understand how loading occurs, we used electron microscopy and small angle X-ray scattering (SAXS) to determine the ATP-bound structure of the intact E. coli DnaB⋅DnaC helicase/loader complex. The 480 kDa dodecamer forms a three-tiered assembly, in which DnaC adopts a spiral configuration that remodels N-terminal scaffolding and C-terminal motor regions of DnaB to produce a clear break in the helicase ring. Surprisingly, DnaC's AAA+ fold is dispensable for ring remodeling because the DnaC isolated helicase-binding domain can both load DnaB onto DNA and increase the efficiency by which the helicase acts on substrates in vitro. Our data demonstrate that DnaC opens DnaB by a mechanism akin to that of polymerase clamp loaders and indicate that bacterial replicative helicases, like their eukaryotic counterparts, possess autoregulatory elements that influence how hexameric motor domains are loaded onto and unwind DNA.
authors.affiliation: Department of Molecular and Cell Biology, California Institute for Quantitative Biosciences, University of California, Berkeley, Berkeley, CA 94720, USA.
authors.forename: Ernesto
authors.initials: E
authors.lastname: Arias-Palomo
authors.forename: Valerie L
authors.initials: VL
authors.lastname: O'Shea
authors.forename: Iris V
authors.initials: IV
authors.lastname: Hood
authors.forename: James M
authors.initials: JM
authors.lastname: Berger
mesh_terms: D004261:DNA Replication; D053488:DnaB Helicases / Q000737:chemistry / Q000378:metabolism*; D004926:Escherichia coli / Q000737:chemistry / Q000378:metabolism*; D029968:Escherichia coli Proteins / Q000737:chemistry / Q000378:metabolism*; D001411:Geobacillus stearothermophilus / Q000737:chemistry; D008854:Microscopy, Electron; D008958:Models, Molecular; D017434:Protein Structure, Tertiary; D053838:Scattering, Small Angle
chemical_list: C071702:DnaC protein, E coli; D029968:Escherichia coli Proteins; C506730:dnaB protein, E coli; D053488:DnaB Helicases
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