title: Crystal structure of the chi:psi sub-assembly of the Escherichia coli DNA polymerase clamp-loader complex.
abstract: The chi (chi) and psi (psi) subunits of Escherichia coli DNA polymerase III form a heterodimer that is associated with the ATP-dependent clamp-loader machinery. In E. coli, the chi:psi heterodimer serves as a bridge between the clamp-loader complex and the single-stranded DNA-binding protein. We determined the crystal structure of the chi:psi heterodimer at 2.1 A resolution. Although neither chi (147 residues) nor psi (137 residues) bind to nucleotides, the fold of each protein is similar to the folds of mononucleotide-(chi) or dinucleotide-(psi) binding proteins, without marked similarity to the structures of the clamp-loader subunits. Genes encoding chi and psi proteins are found to be readily identifiable in several bacterial genomes and sequence alignments showed that residues at the chi:psi interface are highly conserved in both proteins, suggesting that the heterodimeric interaction is of functional significance. The conservation of surface-exposed residues is restricted to the interfacial region and to just two other regions in the chi:psi complex. One of the conserved regions was found to be located on chi, distal to the psi interaction region, and we identified this as the binding site for a C-terminal segment of the single-stranded DNA-binding protein. The other region of sequence conservation is localized to an N-terminal segment of psi (26 residues) that is disordered in the crystal structure. We speculate that psi is linked to the clamp-loader complex by this flexible, but conserved, N-terminal segment, and that the chi:psi unit is linked to the single-stranded DNA-binding protein via the distal surface of chi. The base of the clamp-loader complex has an open C-shaped structure, and the shape of the chi:psi complex is suggestive of a loose docking within the crevice formed by the open faces of the delta and delta' subunits of the clamp-loader.
authors.affiliation: Laboratory of Molecular Biophysics, Howard Hughes Medical Institute, The Rockefeller University, New York, NY, USA.
authors.forename: Jacqueline M
authors.initials: JM
authors.lastname: Gulbis
authors.forename: Steven L
authors.initials: SL
authors.lastname: Kazmirski
authors.forename: Jeff
authors.initials: J
authors.lastname: Finkelstein
authors.forename: Zvi
authors.initials: Z
authors.lastname: Kelman
authors.forename: Mike
authors.initials: M
authors.lastname: O'Donnell
authors.forename: John
authors.initials: J
authors.lastname: Kuriyan
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism*; D000595:Amino Acid Sequence; D003460:Crystallization; D018360:Crystallography, X-Ray; D004258:DNA Polymerase III / Q000737:chemistry* / Q000378:metabolism; D004261:DNA Replication; D004277:DNA, Single-Stranded; D004268:DNA-Binding Proteins / Q000378:metabolism; D019281:Dimerization; D004926:Escherichia coli / Q000201:enzymology*; D008958:Models, Molecular; D008969:Molecular Sequence Data; D011485:Protein Binding; D011487:Protein Conformation; D017510:Protein Folding; D021122:Protein Subunits; D017386:Sequence Homology, Amino Acid
chemical_list: D004277:DNA, Single-Stranded; D004268:DNA-Binding Proteins; D021122:Protein Subunits; D000255:Adenosine Triphosphate; D004258:DNA Polymerase III