Expression in Escherichia coli, purification, and spectroscopic characterization of two mutant Bet v 1 proteins

Bet v 1 is the major birch pollen allergen. A highly efficient expression and purification scheme for mutant forms of this protein was developed on the basis of the pET expression system in order to provide the high quantities of protein needed for spectroscopic and structural work. Bet v 1 (M139L) protein could be purified at high yield (approx. 30 mg from 1 liter of LB medium) in a two-step procedure by the use of metal-affinity chromatography. Matrix assisted laser desorption ionisation mass spectroscopy, and size exclusion chromatography demonstrate the homogeneity and purity of the prepared protein. Spectroscopic methods were used to show that Bet v 1 (M139L) is structurally similar to wild type Bet v 1. Furthermore, we investigated the influence of the nature of amino acid 139 on the thermodynamic behaviour of the protein by replacing the leucine residue by alanine. While there appears to be no global structural effect of this mutation, the thermostability of Bet v 1 is greatly decreased.