Crystal structure of the chi:psi sub-assembly of the Escherichia coli DNA polymerase clamp-loader complex.

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TitleCrystal structure of the chi:psi sub-assembly of the Escherichia coli DNA polymerase clamp-loader complex.
Publication TypeJournal Article
Year of Publication2004
AuthorsGulbis, JM, Kazmirski, SL, Finkelstein, J, Kelman, Z, O'Donnell, M, Kuriyan, J
JournalEur J Biochem
Date Published2004 Jan
KeywordsAdenosine Triphosphate, Amino Acid Sequence, Crystallization, Crystallography, X-Ray, Dimerization, DNA Polymerase III, DNA Replication, DNA, Single-Stranded, DNA-Binding Proteins, Escherichia coli, Models, Molecular, Molecular Sequence Data, Protein Binding, Protein Conformation, Protein Folding, Protein Subunits, Sequence Homology, Amino Acid

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.

Alternate JournalEur. J. Biochem.
PubMed ID14717711
Grant ListGM 38839 / GM / NIGMS NIH HHS / United States
GM 45547 / GM / NIGMS NIH HHS / United States