Overexpression and purification of isotopically labeled Escherichia coli MutH for NMR studies

Abstract

MutH is one of the enzymes involved in the methyl directed -GATC-based DNA repair system. We report a significantly optimized protocol to prepare isotopically (¹⁵N and/or ¹³C) labeled MutH in minimal medium with high yields for NMR studies. Under the various conditions that we have standardized for the affinity purification of His(6) MutH, the yield of the purified MutH has been estimated to be 35-40 mg of protein from 1liter of M9 minimal media. The yield, thus, obtained by this method is significantly higher than those of previously reported methods. Matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectroscopy analysis revealed that the protein was pure and existed essentially in a monomeric form. Uniformly ¹⁵N-labeled protein, thus, produced has been characterized by recording a sensitivity enhanced 2D [¹⁵N]-[¹H] HSQC spectrum. The dispersion seen in ¹⁵N-1H cross-peaks indicates the presence of a well-ordered structure for MutH and proper folding of the purified protein. The spectrum confirms further the existence of MutH as a monomer.