EXPRESSION AND PROCESSING OF AN ARABIDOPSIS 2S ALBUMIN IN TRANSGENIC TOBACCO

2S albumin seed storage proteins undergo a complex series of posttranslational proteolytic cleavages. In order to determine if this process is correctly carried out in transgenic plants, the gene AT2S1 encoding an Arabidopsis thaliana 25 albumin isoform has been expressed in transgenic tobacco. Initial experiments using a reporter gene demonstrated that the AT2S1 promoter directs seed specific expression in both transgenic tobacco and Brassica napus plants. The entire AT2S1 gene was then transferred into tobacco plants, where it showed a tissue specific and developmentally regulated expression. Arabidopsis 25 albumin accumulates up to 0.1% of the total high-salt extractable seed protein. Protein sequencing demonstrated that the amino termini of the two Arabidopsis 25 albumin subunits were correctly processed, suggesting that the protease(s) necessary for posttranslational processing of 2S albumin precursors may display common specificities among different dicot plant species, Immunocytochemical studies showed that the Arabidopsis 25 albumin is localized in the protein body matrix of tobacco endosperm and embryo. Correct processing and targeting of the 2S albumin in transgenic plants suggests that modified versions could be expressed, allowing the study of 2S albumin processing and in particular the possible roles of the processed fragments in protein stability and/or targeting.