Intramolecular disulfide bonds between conserved cysteines in wheat gliadins control their deposition into protein bodies

Abstract

Following synthesis, wheat gliadin storage proteins are deposited into protein bodies inside the endomembrane system in a way that enables not only their efficient accumulation and dehydration during seed maturation, but also their rapid rehydration and degradation during germination. In the present report, we studied the mechanism of gliadin deposition and whether it was controlled by the conformation of these proteins. Although gliadins are generally known to be insoluble in aqueous solutions, sucrose gradient analysis showed that a considerable amount of these proteins appeared as relatively soluble monomers in developing grains. In vitro reduction of the intramolecular disulfide bonds that are present in natural monomeric gliadins caused their precipitation into insoluble aggregates. In addition, pulse- chase experiments in the absence or presence of reducing agents showed that formation of intramolecular disulfide bonds also played a major role in folding and deposition of the gliadins in vivo. Our results imply that following sequestration into the endoplasmic reticulum, the gliadins fold into relatively soluble monomers, which are incompetent for rapid aggregation and gradually assemble into protein bodies. This pattern of deposition apparently depends on the conformation of the gliadins, which is stabilized by intramolecular disulfide bonds formed between the conserved cysteines. The contribution of this study to the understanding of the evolution and function of gliadins is discussed.