Expression and affinity purification of recombinant proteins from plants

Development and Applications of a Calmodulin-Based Fusion Protein System for the Expression and Purification of WW and Zinc Finger Modules

Calmodulin from Homo sapiens is an α-helical calcium-binding protein that expresses to high levels in E. coli. When the N-terminus of a calmodulin variant is bound to Ca²⁺, it undergoes a conformational change, exposing hydrophobic pockets. This property can be utilized for purification purposes, as these pockets bind to phenyl sepharose resin with high affinity. Washing with EDTA chelates the Ca²⁺ ions from the protein, inducing a conformational change back to the more folded state and eluting the protein from the column. We describe herein the use of a protein expression and purification technique using the calmodulin variant and a short linker for proteolytic cleavage by the mutant NIa-Pro tobacco etch virus protease. We have shown this approach to be useful in obtaining purified quantities of various small proteins that could not be expressed using other methods, including high enough concentrations of a designed WW domain protein for NMR structural analysis. We have also obtained promising results on the usefulness of this procedure to express and purify zinc finger proteins without the addition of zinc ions or other cofactors.

Optimization of Ammonium Sulfate Concentration for Purification of Colorectal Cancer Vaccine Candidate Recombinant Protein GA733-FcK Isolated from Plants

A protein purification procedure is required to obtain high-value recombinant injectable vaccine proteins produced in plants as a bioreactor. However, existing purification procedures for plant-derived recombinant proteins are often not optimized and are inefficient, with low recovery rates. In our previous study, we used 25-30% ammonium sulfate to precipitate total soluble proteins (TSPs) in purification process for recombinant proteins from plant leaf biomass which has not been optimized. Thus, the objective in this study is to optimize the conditions for plant-derived protein purification procedures. Various ammonium sulfate concentrations (15-80%) were compared to determine their effects on TSPs yield. With 50% ammonium sulfate, the yield of precipitated TSP was the highest, and that of the plant-derived colorectal cancer-specific surface glycoprotein GA733 fused to the Fc fragment of human IgG tagged with endoplasmic reticulum retention signal KDEL (GA733(P)-FcK) protein significantly increased 1.8-fold. SDS-PAGE analysis showed that the purity of GA733(P)-FcK protein band appeared to be similar to that of an equal dose of mammalian-derived GA733-Fc (GA733(M)-Fc). The binding activity of purified GA733(P)-FcK to anti-GA733 mAb was as efficient as the native GA733(M)-Fc. Thus, the purification process was effectively optimized for obtaining a high yield of plant-derived antigenic protein with good quality. In conclusion, the purification recovery rate of large quantities of recombinant protein from plant expression systems can be enhanced via optimization of ammonium sulfate concentration during downstream processes, thereby offering a promising solution for production of recombinant GA733-Fc protein in plants.

Review article: commercialization of whole-plant systems for biomanufacturing of protein products: evolution and prospects

Technology for enabling plants to biomanufacture nonnative proteins in commercially significant quantities has been available for just over 20 years. During that time, the agricultural world has witnessed rapid commercialization and widespread adoption of transgenic crops enhanced for agronomic performance (herbicide-tolerance, insect-resistance), while plant-made pharmaceuticals (PMPs) and plant-made industrial products (PMIPs) have been limited to experimental and small-scale commercial production. This difference in the rate of commercial implementation likely reflects the very different business-development challenges associated with 'product' technologies compared with 'enabling' ('platform') technologies. However, considerable progress has been made in advancing and refining plant-based production of proteins, both technologically and in regard to identifying optimal business prospects. This review summarizes these developments, contrasting today's technologies and prospective applications with those of the industry's formative years, and suggesting how the PM(I)P industry's evolution has generated a very positive outlook for the 'plant-made' paradigm.

Production of recombinant single chain antibodies (scFv) in vegetatively reproductive Kalanchoe pinnata by in planta transformation

We developed an asexual reproductive plant, Kalanchoe pinnata, as a new bioreactor for plant-based molecular farming using a newly developed transformation method. Leaf crenate margins were pin-pricked to infect the plant with the Agrobacterium strain LBA4404 and vacuum infiltration was also applied to introduce the target gene into the plants. Subsequently, the young mother leaf produced new clones at the leaf crenate margins without the need for time- and labor-consuming tissue culture procedures. The average transformation rates were approximately 77 and 84% for pin-prickling and vacuum-infiltration methods, respectively. To functionally characterize an introduced target protein, a nucleic acid hydrolyzing recombinant 3D8 scFv was selected and the plant based 3D8 scFv proteins were purified and analyzed. Based on abzyme analysis, the purified protein expressed with this system had catalytic activity and exhibited all of properties of the protein produced in an E. coli system. This result suggested that vegetatively reproductive K. pinnata can be a novel and potent bioreactor for bio-pharmaceutical proteins.

Considerations for extraction of monoclonal antibodies targeted to different subcellular compartments in transgenic tobacco plants

SUMMARY

Monoclonal antibody production from transgenic tobacco plants offers many advantages over other heterologous production systems, creating the prospect of production at a scale that will allow new prophylactic and therapeutic applications in global human and animal health. However, information on the major processing factors to consider for large-scale purification of antibodies from transgenic plants is currently limited, and is in urgent need of attention. The purpose of this project was to investigate methods for the initial extraction of recombinant immunoglobulin G (IgG) antibodies from transgenic tobacco leaf tissue. Three different transgenic plant lines were studied in order to establish the parameters for optimal extraction of monoclonal antibodies that accumulate in the apoplasm, at the plasma membrane or within the endoplasmic reticulum. For each transgenic line, seven techniques for physical extraction were compared. The factors that determine the optimal extraction of antibodies from plants have a direct influence on the initial choice of expression strategy, and so must be considered at an early stage. The use of small-scale techniques that are applicable to large-scale purification was a particularly important consideration. The optimal extraction technique varied with the target location of IgG in the plant cell, and the dependence of antibody yield on the physical extraction methodology employed, the pH of the extraction buffer and the extraction temperature was demonstrated in each case. The addition of detergent to the extraction buffer may improve the yield, but this was found to be dependent on the site of accumulation of IgG within the plant cell.

Purification of an acidic recombinant protein from transgenic tobacco

Tobacco has proven to be a promising alternative for the production of recombinant therapeutic proteins and offers numerous advantages over other plants as a host system. However, the recovery and purification steps needed to obtain a protein at high recovery and purity have not been well investigated. In this study, a process was developed to purify a model acidic protein, recombinant beta-glucuronidase (rGUS) from transgenic tobacco leaf tissue, in three main steps after extraction: polyelectrolyte precipitation, hydrophobic interaction chromatography (HIC), and hydroxyapatite chromatography (HAC). Using this three-step process, up to 40% of the initial rGUS activity could be recovered to near homogeneity as judged by SDS-PAGE. This work demonstrates that acidic recombinant proteins expressed in tobacco may be purified to high yield with high purity in a minimal amount of steps that are suitable for scale-up. Furthermore, the general steps used in this process may suggest that a wide variety of acidic recombinant proteins may be purified in a similar manner from transgenic tobacco or other leafy crops.

Calmodulin-mediated reversible immobilization of enzymes

This work demonstrates the use of the protein calmodulin, CaM, as an affinity tag for the reversible immobilization of enzymes on surfaces. Our strategy takes advantage of the of the reversible, calcium-mediated binding of CaM to its ligand phenothiazine and of the ability to produce fusion proteins between CaM and a variety of enzymes to reversibly immobilize enzymes in an oriented fashion to different surfaces. Specifically, we employed two different enzymes, organophosphorus hydrolase (OPH) and beta-lactamase and two different solid supports, a silica surface and cellulose membrane modified by covalently attaching a phenothiazine ligand, to demonstrate the versatility of our immobilization method. Fusion proteins between CaM-OPH and CaM-beta-lactamase were prepared by using genetic engineering strategies to introduce the calmodulin tail at the N-terminus of each of the two enzymes. In the presence of Ca(2+), CaM adopts a conformation that favors interaction between hydrophobic pockets in CaM and phenothiazine, while in the presence of a Ca(2+)-chelating agent such as EGTA, the interaction between CaM and phenothiazine is disrupted, thus allowing for removal of the CaM-fusion protein from the surface under mild conditions. CaM also acts as a spacer molecule, orienting the enzyme away from the surface and toward the solution, which minimizes enzyme interactions with the immobilization surface. Since the method is based on the highly selective binding of CaM to its phenothiazine ligand, and this is covalently immobilized on the surface, the method does not suffer from ligand leaching nor from interference from other proteins present in the cell extract. An additional advantage lies in that the support can be regenerated by passing through EGTA, and then reused for the immobilization of the same or, if desired, a different enzyme. Using a fusion protein approach for immobilization purposes avoids the use of harsh conditions in the immobilization and/or regeneration steps, which could cause inactivation of the immobilized enzyme. Moreover, we have demonstrated that the CaM affinity tag allows immobilization of enzymes on a variety of surfaces without compromising their enzymatic activity substantially; for example, the immobilized OPH retained more than 80% of the activity of the free enzyme. Our results with beta-lactamase showed the feasibility of using a phenothiazine surface in several consecutive loading and regeneration cycles. This can be advantageous when expensive and/or difficult to obtain immobilization surfaces have to be employed; the immobilization surface could be reused to immobilize the same or a different enzyme using the CaM affinity tail. We also determined that the phenothiazine-modified silica particles are stable for long periods of time, i.e., up to 2 years when stored at 4 degrees C. It is envisioned that this type of reversible immobilization may find applications in the development of reversible, reusable biosensors and bioreactors endowed with the additional advantage that the biological element at the surface of the sensor or bioreactor could be replaced under mild conditions when needed to sense or process a different target molecule.

Expression of single-chain antibody-barstar fusion in plants

We successfully cloned and expressed a single-chain antibody (425scFv), that is directed to human epidermal growth factor receptor HER1 (EGFR) in transgenic tobacco plants as a fusion with bacterial barstar gene (425scFv-barstar). Plant-produced recombinant 425scFv-barstar was recovered using barstar-barnase system. Based on barstar-barnase affinity, during purification of the plant-produced 425scFv-barstar, we generated bispecific scFv-antibody heterodimers from individual single-chain fragments initially produced in different host systems with binding activity to both HER1 and HER2/neu tumor antigens. We demonstrated by flow cytometry and indirect immunofluorescent microscopy that both the components of heterodimer retain its specific cell-binding activity.

Real-time GUS analysis using Q-PCR instrumentation

The development of new technology within biological sciences has resulted in a number of real-time PCR instruments that have become essential tools within molecular biology. This equipment has facilitated high throughput analysis of samples and optimal information gathering of completed PCR reactions for example estimating the copy number of a gene of interest that is inserted into particular genomes. Real-time PCR instruments frequently come with optional filter sets, e.g. the ALEXA filter set which has parameters in common with excitation and emission wavelengths of sodium methyl umbelliferone (NaMU) widely used in beta-glucuronidase reporter gene assays. Using these filter sets it has been possible to quantify and measure gus A activity of Ulmus procera SR4 in real-time removing the necessity for aliquots of reactions to be stopped by pipetting into carbonate buffer for each time point. The introduction of real-time GUS analysis leads to faster, more accurate and reproducible assays with reduced potential for pipetting errors, requires fewer manipulations and encourages high throughput analysis of inter-individual gene expression variation.

Downstream processing of recombinant proteins from transgenic feedstock

The search for inexpensive production systems capable of producing large quantities of recombinant protein has resulted in the development of new technology platforms based on transgenic plants and animals. Over the past decade, these transgenic systems have been used to produce several products and potential therapeutic proteins. Improvements continue to be made, not only in how the proteins are expressed but also in how the end products are obtained. As improvements in expression are realized, cost-saving measures will increasingly focus on downstream processing.