Assembly and Transport of Wheat Storage Proteins

Abstract

Summary Following sequestration into the endoplasmic reticulum (ER), the storage proteins of wheat ( Triticum aestivum L.) may either be retained and packaged into protein bodies (PB) inside the organelle or be exported to the Golgi complex. To unravel the signals and mechanisms regulating the assembly and sorting of these proteins within the ER, we expressed wild type and mutant forms of a γ type gliadin in Xenopus oocytes. A considerable amount of the wild type γ-gliadin was secreted via the Golgi into the medium, while still a significant proportion was retained within the ER of the oocytes where it assembled into dense PB. A deletion mutant of the γ-gliadin, encoding only the N-terminal region, which is composed of tandem repeats of a consensus PQQPFPQ sequence, was entirely retained within the oocytes, while another deletion mutant encoding only the C-terminal unique-sequence region of this protein was entirely secreted. Retention of the γ-gliadin within the ER could not be explained by rapid precipitation or assembly into insoluble deposits inasmuch as protein could diffuse rather efficiently within the organelle for several hours. In contrast, mutants of the γ-gliadin, lacking specific conserved cysteines in the C-terminal region, were entirely retained within the oocytes and were unable to diffuse within the ER. We thus hypothesize that the assembly and sorting of wheat gliadins within the ER are determined by concerted interactions between the Nand C-terminal regions of these proteins with ER resident proteins.