title: Loading a ring: structure of the Bacillus subtilis DnaB protein, a co-loader of the replicative helicase.
abstract: Loading of the ring-shaped replicative helicase is a critical step in the initiation of DNA replication. Bacillus subtilis has adopted a two-protein strategy to load its hexameric replicative helicase: DnaB and DnaI interact with the helicase and mediate its delivery onto DNA. We present here the 3D electron microscopy structure of the DnaB protein, along with a detailed analysis of both its oligomeric state and its domain organization. DnaB is organized as an asymmetric tetramer that is comprised of two stacked components, one arranged as a closed collar and the other as an open sigma shape. Intriguingly, the 3D map of DnaB exhibits an overall architecture similar to the structure of the Escherichia coli gamma-complex, the loader of the ring-shaped processivity factor. We propose a model whereby each DnaB monomer participates in both stacked components of the tetramer and displays a different overall shape. This asymmetric quaternary organization could be a general feature of ring loaders.
authors.affiliation: Departmento de Estructura de Macromoléculas, Centro Nacional de Biotecnología, CSIC, Campus Universidad Autónoma de Madrid, 28049 Madrid, Spain.
authors.forename: Rafael
authors.initials: R
authors.lastname: Núñez-Ramírez
authors.forename: Marion
authors.initials: M
authors.lastname: Velten
authors.forename: Germán
authors.initials: G
authors.lastname: Rivas
authors.forename: Patrice
authors.initials: P
authors.lastname: Polard
authors.forename: José María
authors.initials: JM
authors.lastname: Carazo
authors.forename: Luis Enrique
authors.initials: LE
authors.lastname: Donate
mesh_terms: D001412:Bacillus subtilis / Q000201:enzymology*; D053488:DnaB Helicases / Q000737:chemistry* / Q000648:ultrastructure*; D008854:Microscopy, Electron; D008958:Models, Molecular; D011487:Protein Conformation; D020836:Protein Structure, Quaternary; D017434:Protein Structure, Tertiary; D011994:Recombinant Proteins / Q000737:chemistry / Q000648:ultrastructure
chemical_list: D011994:Recombinant Proteins; D053488:DnaB Helicases