Purification and characterization of a recombinant rat prohaptoglobin expressed in baculovirus-infected Sf9 insect cells

Several affinity tags commonly used in chromatographic purification

Affinity tags have become powerful tools from basic biological research to structural and functional proteomics. They were widely used to facilitate the purification and detection of proteins of interest, as well as the separation of protein complexes. Here, we mainly discuss the benefits and drawbacks of several affinity or epitope tags frequently used, including hexahistidine tag, FLAG tag, Strep II tag, streptavidin-binding peptide (SBP) tag, calmodulin-binding peptide (CBP), glutathione S-transferase (GST), maltose-binding protein (MBP), S-tag, HA tag, and c-Myc tag. In some cases, a large-size affinity tag, such as GST or MBP, can significantly impact on the structure and biological activity of the fusion partner protein. So it is usually necessary to excise the tag by protease. The most commonly used endopeptidases are enterokinase, factor Xa, thrombin, tobacco etch virus, and human rhinovirus 3C protease. The proteolysis features of these proteases are described in order to provide a general guidance on the proteolytic removal of the affinity tags.

New ligation-independent cloning vectors compatible with a high-throughput platform for parallel construct expression evaluation using baculovirus-infected insect cells

Biomedical research has undergone a major shift in emphasis over the past decade from characterizing the genomes of organisms to characterizing their proteomes. The high-throughput approaches that were successfully applied to sequencing of genomes, such as miniaturization and automation, have been adapted for high-throughput cloning and protein production. High-throughput platforms allow for a multi-construct, multi-parallel approach to expression optimization and construct evaluation. We describe here a series of baculovirus transfer and expression vectors that contain ligation-independent cloning regions originally designed for use in high-throughput Escherichia coli expression evaluation. These new vectors allow for parallel cloning of the same gene construct into a variety of baculovirus or E. coli expression vectors. A high-throughput platform for construct expression evaluation in baculovirus-infected insect cells was developed to utilize these vectors. Data from baculovirus infection expression trials for multiple constructs of two target protein systems relevant to the study of human diseases are presented. The target proteins exhibit a wide variation in behavior and illustrate the benefit of investigating multiple cell types, fusion partners and secretion signals in optimization of constructs and conditions for eukaryotic protein production.

Strategy for improvement of enteropeptidase efficiency in tag removal processes

Enteropeptidase (synonym: enterokinase, EC 3.4.21.9) is a heterodimeric serine protease of the intestinal brush border that activates trypsinogen by highly specific cleavage of the trypsinogen activation peptide following the sequence (Asp)(4)-Lys. It has also great biotechnological interest because of the unique substrate specificity of the serine protease domain. The high degree of specificity exhibited by enteropeptidase makes it a suitable reagent for cleaving recombinant proteins to remove affinity or other tags. However often unwanted cleavages elsewhere in the protein occurred during cleavage of fusions when high amount of enzyme is required. In this study we have improved the efficiency of fusion proteins cleavage by enteropeptidase by substitution of the Lys residue by Arg in specific cleavage sequence (Asp)(4)-Lys. We have demonstrated that 3-6-fold lower amounts of the catalytic subunit of human and bovine enteropeptidase is required for 95% cleavage of Trx/TRAIL and Trx/FGF-2 fusions with (Asp)(4)-Arg cleavage sequence in comparison to native sequence (Asp)(4)-Lys. As a result, reduced amount of non-specifically cleaved peptide fragments were observed during cleavage of (Asp)(4)-Lys/Arg mutated fusions. These findings overcome limitations of enteropeptidase in tag removal processes during recombinant proteins purification and extend its commercial benefit in the biopharmaceutical industry.

Enhancing the specificity of the enterokinase cleavage reaction to promote efficient cleavage of a fusion tag

In our work with designed minimalist proteins based on the bZIP motif, we have found our His-tagged proteins to be prone to inclusion body formation and aggregation; we suspect this problem is largely due to the His tag, known to promote aggregation. Using AhR6-C/EBP, a hybrid of the AhR basic region and C/EBP leucine zipper, as representative of our bZIP-like protein family, we attempted removal of the His tag with enterokinase (EK) but obtained the desired cleavage product in very small yield. EK is known for proteolysis at noncanonical sites, and most cleavage occurred at unintended sites. We manipulated experimental conditions to improve specificity of proteolysis and analyzed the cleavage products; no effect was observed after changing pH, temperature, or the amount of EK. We then suspected the accessibility of the EK site was impeded due to protein aggregation. We found that the easily implemented strategy of addition of urea (1-4 M) greatly improved EK cleavage specificity at the canonical site and reduced adventitious cleavage. We believe that this enhancement in specificity is due to a more "open" protein structure, in which the now accessible canonical target can compete effectively with adventitious cleavage sites of related sequence.