Insufficient accumulation levels of recombinant proteins in plants and the lack of efficient purification methods for recovering these valuable proteins have hindered the development of plant biotechnology applications. Hydrophobins are small and surface-active proteins derived from filamentous fungi that can be easily purified by a surfactant-based aqueous two-phase system. In this study, the hydrophobin HFBI sequence from Trichoderma reesei was fused to green fluorescent protein (GFP) and transiently expressed in Nicotiana benthamiana plants by Agrobacterium tumefaciens infiltration. The HFBI fusion significantly enhanced the accumulation of GFP, with the concentration of the fusion protein reaching 51% of total soluble protein, while also delaying necrosis of the infiltrated leaves. Furthermore, the endoplasmic reticulum-targeted GFP-HFBI fusion induced the formation of large novel protein bodies. A simple and scalable surfactant-based aqueous two-phase system was optimized to recover the HFBI fusion proteins from leaf extracts. The single-step phase separation was able to selectively recover up to 91% of the GFP-HFBI up to concentrations of 10 mg mL⁻¹. HFBI fusions increased the expression levels of plant-made recombinant proteins while also providing a simple means for their subsequent purification. This hydrophobin fusion technology, when combined with the speed and posttranslational modification capabilities of plants, enhances the value of transient plant-based expression systems.

Background: Elastin-like polypeptides are synthetic biopolymers composed of a repeating pentapeptide 'VPGXG' sequence that are valuable for the simple non-chromatographic purification of recombinant proteins. In addition, elastin-like polypeptide fusions have been shown to enhance the accumulation of a range of different recombinant proteins in plants, thus addressing the major limitation of plant-based expression systems, which is a low production yield. This study's main objectives were to determine the general utility of elastin-like polypeptide protein fusions in various intracellular compartments and to elucidate elastin-like polypeptide's mechanism of action for increasing recombinant protein accumulation in the endoplasmic reticulum of plants.Results: The effect of elastin-like polypeptide fusions on the accumulation of green fluorescent protein targeted to the cytoplasm, chloroplasts, apoplast, and endoplasmic reticulum was evaluated. The endoplasmic reticulum was the only intracellular compartment in which an elastin-like polypeptide tag was shown to significantly enhance recombinant protein accumulation. Interestingly, endoplasmic reticulum-targeted elastin-like polypeptide fusions induced the formation of a novel type of protein body, which may be responsible for elastin-like polypeptide's positive effect on recombinant protein accumulation by excluding the heterologous protein from normal physiological turnover. Although expressed in the leaves of plants, these novel protein bodies appeared similar in size and morphology to the prolamin-based protein bodies naturally found in plant seeds. The elastin-like polypeptide-induced protein bodies were highly mobile organelles, exhibiting various dynamic patterns of movement throughout the cells, which were dependent on intact actin microfilaments and a functional actomyosin motility system.Conclusion: An endoplasmic reticulum-targeted elastin-like polypeptide fusion approach provides an effective strategy for depositing large amounts of concentrated heterologous protein within the limited space of the cell via storage in stable protein bodies. Furthermore, encapsulation of recombinant proteins into physiologically inert organelles can function to insulate the protein from normal cellular mechanisms, thus limiting unnecessary stress to the host cell. Since elastin-like polypeptide is a mammalian-derived protein, this study demonstrates that plant seed-specific factors are not required for the formation of protein bodies in vegetative plant tissues, suggesting that the endoplasmic reticulum possesses an intrinsic ability to form protein body-like accretions in eukaryotic cells when overexpressing particular proteins.