The accumulation of recombinant miraculin is independent of fruit size in tomato

Improvement of recombinant miraculin production in transgenic tomato by crossbreeding-based genetic background modification

An important optimization step in plant-based recombinant protein production systems is the selection of an appropriate cultivar after a potential host has been determined. Previously, we have shown that transgenic tomatoes of the variety 'Micro-Tom' accumulate incredibly high levels of miraculin (MIR) due to the introduction of MIR gene controlled by a CaMV35S promoter and a heat-shock protein terminator. However, 'Micro-Tom' is unsuitable for commercial production of MIR as it is a dwarf cultivar characterized by small-sized fruit and poor yield. Here, we used the crossbreeding approach to transfer the high MIR accumulation trait of transgenic 'Micro-Tom' tomatoes to 'Natsunokoma' and 'Aichi First', two commercial cultivars producing medium and large fruit sizes, respectively. Fruits of the resultant crossbred lines were larger (~ 95 times), but their miraculin accumulation levels (~ 1,062 μg/g fresh mass) were comparable to the donor cultivar, indicating that the high miraculin accumulation trait was preserved regardless of fruit size or cultivar. Further, the transferred trait resulted in a 3-4 fold increase in overall miraculin production than that of the previously reported line 5B. These findings demonstrate the effectiveness of crossbreeding in improving MIR production in tomatoes and could pave the way for a more efficient production of recombinant proteins in other plants.

Effect of fruit maturation on N-glycosylation of plant-derived native and recombinant miraculin

Miracle fruit, Synsepalum dulcificum, produces a unique taste-modifying protein, miraculin (MIR), which has an attractive potential for commercial application as a novel low-calorie sweetener. To establish a stable supply system for MIR, a previous study established a platform for recombinant MIR (rMIR) production in tomato plants and demonstrated that native miraculin from miracle fruit (nMIR) and rMIR were almost identical in their protein modifications with N-glycan. However, neither N-glycosylation nor the influence of fruit maturation on the structural changes of N-glycan have been fully characterized in detail. Here, with a focus on N-glycosylation and the contribution of fruit maturation to N-glycan, we reanalyzed the N-glycosylation of the natural maturation stages of nMIR and rMIR, and then compared the N-glycan structures on MIRs prepared from the fruit at two different maturation stages. The detailed peptide mapping and N-glycosylation analysis of MIRs provided evidence that MIRs have variants, which were derived mainly from the differences in the N-glycan structure in nMIR and the N-glycosylation in rMIR and not from the cleavage of the peptide backbone. N-Glycan analysis of MIRs from the maturation stage of fruits demonstrated that N-glycan structures were similar among nMIRs and rMIRs at every maturation stage. These results indicated that the heterogeneously expressed rMIRs had the same characteristics in post-translational modifications, especially N-glycosylation and N-glycan structures, throughout the maturation stages. This study demonstrated the potential of recombinant protein expressed in tomato plants and paves the way for the commercial use of rMIR.