An SRLLR motif downstream of the scissile bond enhances enterokinase cleavage efficiency

Epitope-directed monoclonal antibody production using a mixed antigen cocktail facilitates antibody characterization and validation

High quality, well-validated antibodies are needed to mitigate irreproducibility and clarify conflicting data in science. We describe an epitope-directed monoclonal antibody (mAb) production method that addresses issues of antibody quality, validation and utility. The workflow is illustrated by generating mAbs against multiple in silico-predicted epitopes on human ankyrin repeat domain 1 (hANKRD1) in a single hybridoma production cycle. Antigenic peptides (13-24 residues long) presented as three-copy inserts on the surface exposed loop of a thioredoxin carrier produced high affinity mAbs that are reactive to native and denatured hANKRD1. ELISA assay miniaturization afforded by novel DEXT microplates allowed rapid hybridoma screening with concomitant epitope identification. Antibodies against spatially distant sites on hANKRD1 facilitated validation schemes applicable to two-site ELISA, western blotting and immunocytochemistry. The use of short antigenic peptides of known sequence facilitated direct epitope mapping crucial for antibody characterization. This robust method motivates its ready adoption for other protein targets.

Monoclonal Antibodies against Specific p53 Hotspot Mutants as Potential Tools for Precision Medicine

The large number of mutations identified across all cancers represents an untapped reservoir of targets that can be useful for therapeutic targeting if highly selective, mutation-specific reagents are available. We report here our attempt to generate such reagents: monoclonal antibodies against the most common R175H, R248Q, and R273H hotspot mutants of the tumor suppressor p53. These antibodies recognize their intended specific alterations without any cross-reactivity against wild-type (WT) p53 or other p53 mutants, including at the same position (as exemplified by anti-R248Q antibody, which does not recognize the R248W mutation), evaluated by direct immunoblotting, immunoprecipitation, and immunofluorescence methods on transfected and endogenous proteins. Moreover, their clinical utility to diagnose the presence of specific p53 mutants in human tumor microarrays by immunohistochemistry is also shown. Together, the data demonstrate that antibodies against specific single-amino-acid alterations can be generated reproducibly and highlight their utility, which could potentially be extended to therapeutic settings.

Expression, purification, and characterization of mouse nesfatin-1 in Escherichia coli

Nesfatin-1 is a newly discovered satiety molecule expressed mainly in the hypothalamic nuclei. It suppresses both short-term and long-term appetite. Six synthetic deoxyoligonucleotides overlapped by PCR encoding nesfatin-1 were cloned into a pET28a vector after the hexa-histidine-tagged multiple cloning sites sequence with an enterokinase recognition site incorporated in-between. The recombinant plasmid was transformed into Escherichia coli strain Rosetta to express the fusion protein, which constituted 27% of the total cell proteins. After purified by Ni-sepharose affinity chromatography, the fusion protein was treated with enterokinase to release nesfatin-1. The nesfatin-1 sample was further purified with reverse-phase high performance liquid chromatography (HPLC), and its molecular weight was determined by mass spectrometry. The biological activities of recombinant nesfatin-1 were also assessed using in vivo animal models. The method described here promises to produce about 8 mg biologically active nesfatin-1 with homogeneity over 98% from 1-L shaking flask culture of E. coli, which can be considered as an easy and cost-effective way to synthesize nesfatin-1.

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.

A His6-SUMO-eXact tag for producing human prepro-urocortin 2 in Escherichia coli for raising monoclonal antibodies

This is a first report of recombinant production of human prepro-Urocortin 2 in Escherichia coli by N-terminal fusion with a triple His₆-SUMO-eXact tag and its subsequent use as an antigen for the production and screening of very high affinity monoclonal antibodies. The rationale for this combinatorial construct is that the His tag allows first step protein purification of insoluble and soluble proteins, the SUMO tag enhances protein expression level and solubility, while the eXact tag facilitates anion-triggered on-column cleavage of the triple tag to recover pure native proteins in a simple two-step protein purification procedure. Compared with an eXact fusion alone, the presence of the SUMO moiety enhanced overall expression levels by 4 to 10 fold but not the solubility of the highly basic prepro-Urocortin 2. Insoluble SUMO-eXact-preproUCN2 was purified in milligram quantities by denaturing IMAC and solubilized in native phosphate buffer by on-column refolding or step-wise dialysis. Only a small fraction of this solubilized protein was able to bind onto the eXact™ affinity column and cleaved by NaF treatment. To test whether binding and cleavage failure was due to improperly refolded SUMO-eXact-preproUCN2 or to the presence of N- and C-terminal sequences flanking the eXact moiety, we created a SUMO-eXact-thioredoxin construct which was overexpressed mainly in the soluble form. This protein bound to and was cleaved efficiently on the eXact™ column to yield native thioredoxin. Solubilized SUMO-eXact-preproUCN2 was used successfully to generate two high affinity mouse monoclonal antibodies (KD~10⁻¹⁰ and 10⁻¹¹ M) specific to the pro-region of Urocortin 2.

Fusion expression and purification of four disulfide-rich peptides reveals enterokinase secondary cleavage sites in animal toxins

Animal toxins are powerful tools for testing the pharmacological, physiological, and structural characteristics of ion channels, proteases, and other receptors. However, most animal toxins are disulfide-rich peptides that are difficult to produce functionally. Here, a glutathione S-transferase (GST) fusion expression strategy was used to produce four recombinant animal toxin peptides, ChTX, StKTx23, BmP01, and ImKTx1, with different isoelectric points from 4.7 to 9.2. GST tags were removed by enterokinase, a widely used and effective commercial protease that cleaves after lysine at the cleavage site DDDDK. Using this strategy, two disulfide-rich animal toxins ChTX and StKTx23 were obtained successfully with a yield of approximately 1-2 mg/l culture. Electrophysiological experiments further showed that these two recombinant toxins showed good bioactivities, indicating that our method was effective in producing large amounts of functional disulfide-rich animal toxins. Interestingly, by analyzing the separated fractions of BmP01, StKTx23, and ImKTx1 using matrix-assisted laser desorption ionization time-of-flight mass spectrometry, four new enterokinase secondary cleavage sites were found, consisting of the sequences "WEYR," "EDK," "QNAR," and "DNDK." To our knowledge, this is the first report of the presence of secondary cleavage sites for commercial enterokinase in animal toxins. These findings will help us use commercial enterokinase appropriately as a cleavage tool in the production of animal toxins.

Secreted production of self-assembling peptides in Pichia pastoris by fusion to an artificial highly hydrophilic protein

The undecapeptides CH(3)CO-Gln-Gln-Arg-Phe-Gln-Trp-Gln-Phe-Glu-Gln-Gln-NH(2) (P(11)-2) and CH(3)CO-Gln-Gln-Orn-Phe-Orn-Trp-Orn-Phe-Orn-Gln-Gln-NH(2) (P(11)-14) have unique self-assembly characteristics and broad application potential. Originally, these peptides were produced by chemical synthesis, which is costly and difficult to scale up to industrial levels in an economically feasible way. This article describes the efficient secreted production of these peptides (with free termini and ornithines replaced with lysines) in the methylotrophic yeast Pichia pastoris. The peptides were produced as enterokinase-cleavable fusions to the C-terminus of an artificial Solubility-Enhancing Protein (SEP). In vitro, the fused highly hydrophilic SEP proved to prevent self-assembly of the peptides. The SEP domain also facilitates product detection and allows convenient separation of the fusion protein from the broth by simple salt precipitation. After cleavage of the purified fusion protein with enterokinase, the free undecapeptides were obtained and P(11)-2 spontaneously assembled into a self-supporting gel, as intended. The properties of the SEP carrier could be advantageous for the production of other peptides.

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.