A relaxed specificity in interchain disulfide bond formation characterises the assembly of a low-molecular-weight glutenin subunit in the endoplasmic reticulum.

Wheat grains contain large protein polymers constituted by two main classes of polypeptides: the high-molecular-weight and the low-molecular-weight glutenin subunits. These polymers are among the largest protein molecules known in nature and are the main determinants of the superior technological properties of wheat flours. However, little is known about the mechanisms controlling the assembly of the different subunits and the way they are arranged in the final polymer. Here we have addressed these issues by analysing the formation of interchain disulfide bonds between identical and different low-molecular-weight glutenin subunits, and by studying the assembly of mutants lacking individual intrachain disulfides. Our results indicate that individual cysteine residues that remain available for disulfide bond formation in the folded monomer can form interchain disulfide bonds with a variety of different cysteine residues present in a companion subunit. These results imply that the coordinated expression of many different low-molecular-weight glutenin subunits in wheat endosperm cells can potentially lead to the formation of a large set of distinct polymeric structures, in which subunits can be arranged in different configurations. In addition, we show that not all intrachain disulfide bonds are necessary for the generation of an assembly-competent structure, and that the retention of a low-molecular-weight glutenin subunit in the early secretory pathway is not dependent on polymer formation.