Increased Accumulation of Recombinant Proteins in Soybean Seeds via the Combination Strategy of Polypeptide Fusion and Suppression of Endogenous Storage Proteins

Soybean seeds show great potential as a safe and cost-effective host for the large-scale production of biopharmaceuticals and industrially important macromolecules. However, the yields of desired recombinant proteins in soybean seeds are usually lower than the economic threshold for their potential commercialization. Our previous study demonstrated that polypeptide fusion such as maize gamma-zein or elastin-like polypeptide (ELP) could significantly increase the accumulation of foreign proteins. In the present study, a recombination strategy of polypeptide fusions (gamma-zein or ELP) and suppression of intrinsic storage proteins (glycinin or conglycinin) via RNA interference was further exploited to improve the yield of the target protein in soybean seeds. Transgenic soybean plants harboring both polypeptide-fused green fluorescent protein (GFP) and glycinin/conglycinin RNAi expression cassettes were generated and confirmed by molecular analysis. The results showed that on both the glycinin and conglycinin suppression backgrounds, the average accumulation levels of recombinant zein-GFP and GFP-ELP proteins were significantly increased as compared to that of their counterparts without such suppressions in our previous study. Moreover, zein-GFP and GFP-ELP accumulation was also remarkably higher than unfused GFP on the glycinin suppression background. However, no significant differences were detected in the glycinin or conglycinin suppression backgrounds for the same polypeptide fusion constructs, though suppression of one of the storage proteins in soybean seeds led to a significant increase in the other. Additionally, the increases in the recombinant protein yield did not affect the total protein content and the protein/oil ratio in soybean seeds. Taken together, the results indicate that both the fusion of the foreign protein with polypeptide tags together with the depletion of endogenous storage proteins contributed to a higher accumulation of the recombinant proteins without affecting the total protein content or the protein/oil ratio in soybean seeds.