Induction of protein body formation in plant leaves by elastin-like polypeptide fusions.

Andrew J Conley,Rima Menassa,Jim E Brandle,Jussi J Joensuu

doi:10.1186/1741-7007-7-48

Abstract

BackgroundElastin-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.ResultsThe 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.ConclusionAn 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.

Highlights

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
We found that green fluorescent protein (GFP)-elastin-like polypeptide (ELP) fusions targeted to the endoplasmic reticulum (ER) tended to form novel protein body (PB)-like structures in leaves, which may exclude the heterologous protein from normal physiological turnover and may be responsible for ELP's positive effect on recombinant protein accumulation
The presence of an ELP fusion tag had a negligible effect on the concentration of GFP in the cytoplasm and apoplast, whereas it decreased the accumulation of GFP in the chloroplasts

Summary

Introduction

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. Elastin-like polypeptide fusions have been shown to enhance the accumulation of a range of different recombinant proteins in plants, addressing the major limitation of plant-based expression systems, which is a low production yield. Seeds provide an attractive alternative to conventional large-scale recombinant protein expression systems since they can produce relatively high heterologous protein yields in a stable, compact environment for long periods of time, assisting in storage, handling, and transport of the transgenic product [1]. There is a strong reluctance among scientists, regulators, and the general public to use seeds of major crops (that is, maize, rice and wheat) for biopharmaceutical production, given the possibility of contaminating the food chain [9]. The low production yields of many recombinant proteins in tobacco remains a serious problem for this host system, since foreign proteins are often unstable and susceptible to proteolytic degradation in the aqueous environment of leafy crops [13,14]

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