ASSEMBLY AND TRANSPORT OF WHEAT STORAGE PROTEINS

Following sequestration into the endoplasmic reticulum (ER), the stora ge 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 regulatin g the assembly and sorting of these proteins within the ER, we express ed wild type and mutant forms of a gamma type gliadin in Xenopus oocyt es. A considerable amount of the wild type gamma-gliadin was secreted via the Golgi into the medium, while still a significant proportion wa s retained within the ER of the oocytes where it assembled into dense PB. A deletion mutant of the gamma-gliadin, encoding only the N-termin al region, which is composed of tandem repeats of a consensus PQQPFPQ sequence, was entirely retained within the oocytes, while another dele tion mutant encoding only the C-terminal unique-sequence region of thi s protein was entirely secreted. Retention of the gamma-gliadin within the ER could not be explained by rapid precipitation or assembly into insoluble deposits inasmuch as protein could diffuse rather efficient ly within the organelle for several hours. In contrast, mutants of the gamma-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 sorti ng of wheat gliadins within the ER are determined by concerted interac tions between the N and C-terminal regions of these proteins with ER r esident proteins.