Large-scale expression and purification of high-molecular-weight glutenin subunits

The high-molecular-weight glutenin subunits (HMW-GSs) are considered to be one of the most important components of wheat gluten, contributing to the unique viscoelastic properties of wheat dough. The HMW-GSs are highly homologous in sequence and structure and a mixture of subunits is usually present in wheat flours. Consequently, it is difficult to purify these proteins separately in appreciable amounts. Expression in heterologous systems provides a clear opportunity to produce large amounts of single HMW-GS proteins, amounts (up to 100 mg) which are required for in vitro analysis of these proteins. However, since the first expression studies of HMW-GSs, over 10 years ago, this technology has not been widely utilized. Previous studies have been analytical or small scale (5-100 ml) and in most cases only partial purity was obtained. In the present paper, we describe in detail the expression of the HMW-GSs Glu1-Dx2, Dx5, Dy10, and Dy12 for the first time on a large scale, producing up to 100 mg of target protein from a 2-liter bacterial culture, using a Biostat fermenter. Our results include optimization of expression conditions to increase yield and stability of proteins. Results also include localization, differences between x- and y-type expression and small-scale versus large-scale expression. We also developed a large-scale purification procedure. The bacterially expressed proteins have the same molecular weight on SDS-PAGE and the same retention times on RP-HPLC as their native counterparts extracted from flour. Functionality tests, on the bacterially produced proteins, have shown a clear correlation with the equivalent native proteins from flour. These results provide a clear opportunity to produce protein in amounts necessary for more detailed studies of the structure and function of the HMW-GSs and glutenin polymers on dough development and quality.