Protein body-inducing fusions for high-level production and purification of recombinant proteins in plants

For the past two decades, therapeutic and industrially important proteins have been expressed in plants with varying levels of success. The two major challenges hindering the economical production of plant‐made recombinant proteins include inadequate accumulation levels and the lack of efficient purification methods. To address these limitations, several fusion protein strategies have been recently developed to significantly enhance the production yield of plant‐made recombinant proteins, while simultaneously assisting in their subsequent purification. Elastin‐like polypeptides are thermally responsive biopolymers composed of a repeating pentapeptide ‘VPGXG’ sequence that are valuable for the purification of recombinant proteins. Hydrophobins are small fungal proteins capable of altering the hydrophobicity of their respective fusion partner, thus enabling efficient purification by surfactant‐based aqueous two‐phase systems. Zera, a domain of the maize seed storage protein γ‐zein, can induce the formation of protein storage bodies, thus facilitating the recovery of fused proteins using density‐based separation methods. These three novel protein fusion systems have also been shown to enhance the accumulation of a range of different recombinant proteins, while concurrently inducing the formation of protein bodies. The packing of these fusion proteins into protein bodies may exclude the recombinant protein from normal physiological turnover. Furthermore, these systems allow for quick, simple and inexpensive nonchromatographic purification of the recombinant protein, which can be scaled up to industrial levels of protein production. This review will focus on the similarities and differences of these artificial storage organelles, their biogenesis and their implication for the production of recombinant proteins in plants and their subsequent purification.