Application of electrospray ionization mass spectrometry to study the hydrophobic interaction between the ε and θ subunits of DNA polymerase III

The interactions between the N-terminal domain of the ε (ε186) and θ subunits of DNA polymerase III of Escherichia coli were investigated using electrospray ionization mass spectrometry. The ε186-θ complex was stable in 9 M ammonium actetate (pH 8), suggesting that hydrophobic interactions have a predominant contribution to the stability of the complex. Addition of primary alkanols to ε186-θ in 0.1 M ammonium acetate (pH 8), led to dissociation of the complex, as observed in the mass spectrometer. The concentrations of methanol, ethanol, and 1-propanol required to dissociate 50% of the complex were 8.9 M, 4.8 M, and 1.7 M, respectively. Closer scrutiny of the effect of alkanols on ε186, θ, and ε186-θ showed that ε186 formed soluble aggregates prior to precipitation, and that the association of ε186 with θ stabilized ε186. In-source collision-induced dissociation experiments and other results suggested that the ε186-θ complex dissociated in the mass spectrometer, and that the stability (with respect to dissociation) of the complex in vacuo was dependent on the solution from which it was sampled.