Protein fusions for high level expression and purification of pharmaceutical and industrial proteins in plants

The production of recombinant proteins in plants is an active area of research and many different high-value proteins have now been produced in plants. We have worked on several pharmaceutical and industrial proteins such as interleukin-10 for the treatment of inflammatory bowel disease, interleukin-24 for the treatment of cancer, erythropoietin for tissue protection, spider silk proteins for the production of high tensile strength fibers and enzymes for biofuels production including cellulases and amylases. However, two major challenges for economical production of recombinant proteins include inadequate accumulation levels and the lack of efficient purification methods. We have recently described two fusion partners that allow the accumulation of recombinant proteins to very high levels while allowing simple non-chromatographic purification. Elastin-like polypeptides (ELPs) are synthetic biopolymers composed of a repeating pentapeptide 'VPGXG' sequence that enhance the accumulation of a range of different recombinant proteins in plants, thus addressing a major limitation of plant-based expression systems. Hydrophobins are small proteins derived from filamentous fungi that are capable of altering the hydrophobicity of their respective fusion partner to enable purification by surfactant-based aqueous two-phase separation (ATPS). A fusion of GFP with HFBI has produced a very high level of recombinant protein in plants, up to 51% total soluble protein, and allowed us to recover 91% of the fusion in one ATPS step. Both fusion systems dramatically increase accumulation levels of fusion partners, and concurrently the presence of discrete protein bodies has been observed. It is possible that the packing of fusions into these protein bodies may exclude the recombinant protein from normal physiological turnover, as well as protect plant tissue from the toxicity of high levels of foreign proteins. We are currently further characterizing these protein bodies and their biogenesis as well as testing their occurrence with a number of fusion partners.