Role of conserved cysteines of a wheat gliadin in its transport and assembly into protein bodies in Xenopus oocytes.

Following sequestration into the endoplasmic reticulum, wheat gliadin storage proteins may either be retained and packaged into protein bodies inside the organelle or be transported via the Golgi apparatus to vacuoles and condense into protein bodies at a post-endoplasmic reticulum location. To unravel the mechanism of this complex process of deposition, we expressed wild-type and mutant forms of two closely related gamma and aggregated gliadins in Xenopus oocytes. Although a considerable amount of the gamma-gliadin was secreted to the medium, its closely related aggregated gliadin was entirely retained within the oocytes. This differential secretion was largely due to structural variations in the C-terminal regions of the proteins. Retention of the wild-type aggregated and gamma-gliadins within the endoplasmic reticulum could not be explained by rapid assembly into insoluble deposits inasmuch as both proteins could diffuse rather efficiently within the organelle for several hours. To address more closely the role of the C-terminal region in the transport and assembly of the gamma-gliadin within the endoplasmic reticulum, 3 cysteine codons in this region were mutated, one at a time, to serine codons. The cysteine-replacement mutants improperly aggregated within the endoplasmic reticulum forming denser deposits compared with the wild-type protein.