THE BEAN SEED STORAGE PROTEIN BETA-PHASEOLIN IS SYNTHESIZED, PROCESSED, AND ACCUMULATED IN THE VACUOLAR TYPE-II PROTEIN BODIES OF TRANSGENIC RICE ENDOSPERM

The seed storage protein beta-phaseolin of the common bean (Phaseolus vulgaris L.) was expressed in the endosperm of transgenic rice (Oryza sativa L.) plants. The 5.1- or 1.8-kb promoter fragment of the rice se ed storage protein glutelin Ctl gene was fused transcriptionally to ei ther the genomic or cDNA coding sequence of the beta-phaseolin gene. T he highest quantity of phaseolin estimated by enzyme-linked immunosorb ent assay was 4.0% of the total endosperm protein in the transgenic ri ce seeds. The phaseolin trait was segregated as a single dominant trai t with a positive gene dosage effect and was stably inherited through three successive generations. Both phaseolin genomic and cDNA coding s equences were used to synthesize four isoforms of mature phaseolin pro tein with apparent molecular masses of 51, 48, 47, and 45 kD. Enzyme d eglycosylation experiments indicated that the 51-kD form contains high -mannose N-glycans; the 48- and 47-kD forms have further modified N-gl ycans; and the 45-kD form is a nonglycosylated protein. Immunolabeling studies using light and electron microscopy demonstrated that phaseol in accumulates primarily in the vacuolar type-it protein bodies locate d at the periphery of the endosperm near the aleurone layer. We discus s the implications of these results on nutritional improvement of rice grains.