Solution structure of Domains IVa and V of the τ subunit of Escherichia coli DNA polymerase III and interaction with the α subunit

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Abstract
The solution structure of the C-terminal Domain V of the τ subunit of E. coli DNA polymerase III was determined by nuclear magnetic resonance (NMR) spectroscopy. The fold is unique to τ subunits. Amino acid sequence conservation is pronounced for hydrophobic residues that form the structural core of the protein, indicating that the fold is representative for τ subunits from a wide range of different bacteria. The interaction between the polymerase subunits τ and α was studied by NMR experiments where α was incubated with full-length C-terminal domain ( τC 16), and domains shortened at the C-terminus by 11 and 18 residues, respectively. The only interacting residues were found in the C-terminal 30-residue segment of τ , most of which is structurally disordered in free τC 16. Since the N- and C-termini of the structured core of τC 16 are located close to each other, this limits the possible distance between α and the pentameric δτ2γδ ′ clamp–loader complex and, hence, between the two α subunits involved in leading- and lagging-strand DNA synthesis. Analysis of an N-terminally extended construct ( τC 22) showed that τC 14 presents the only part of Domains IVa and V of τ which comprises a globular fold in the absence of other interaction partners.