Folding and binding stability of the \textit{$\varepsilon $} and \textit{$\theta $} subunits of DNA polymerase III.

The \textit{$\varepsilon $} subunit of DNA polymerase III is responsible for the proofreading and repair functions of the holoenzyme during DNA replication. The \textit{$\theta $} subunit binds to the \textit{$\varepsilon $} subunit. This binding has been suggested to provide additional folding stability to the \textit{$\varepsilon $} subunit. We have studied the folding stability and the binding affinity of the two subunits at 15 \r{ }C. The midpoint of urea denaturation of the \textit{$\varepsilon $} subunit was low, at 2.4 M urea, but the slope of the unfolding free energy versus urea was high (at 2.9 kcal/mol/M). The sensitivity to urea echoes the low thermal stability. In contrast, the midpoint of urea denaturation of the \textit{$\theta $} subunit was high, at 3.8 M urea, but the slope of the unfolding free energy versus urea was low (at 1.0 kcal/mol/M). Both proteins thus showed marginal stability with respect to denaturation. Their complex exhibited much greater resistance to denaturation, with a midpoint of urea denaturation at 4.3 M.