Affinity tagged recombinant proteins

Based on the vectors for the intracellular production and purification of recombinant proteins, the vector system was further expanded with vectors for the extracellular recombinant protein production. Affinity tag aided protein purification from the cell-free growth medium was made possible by the addition of the corresponding His6- and StrepII-tags [20, 35]. 

Separate the labeled protein from excess label by gel filtration or recover the protein using the appropriate affinity matrix if it is a tagged recombinant protein. 

Nickel affinity chromatography was chosen as the primary purification technique because it is a fast and reliable one-step assay and purified complexes can often be used in downstream applications without the necessity of removing the polyhistidine tag. In addition, the polyhistidine tag is smaller than many other affinity tags targeted by commercially available affinity resins and, in most cases, does not seem to interfere with the structure and function of the recombinant protein. 

The following protocol for EPL, including purification using a CBD fusion tag followed by native chemical ligation, is based on the methods of Muir et al. (1998), Chong et al. (1997, 1998), Evans et al. (1998), Severinov and Muir (1998), and the NEB instruction manual for the IMPACT-TWIN system. The recombinant protein is recovered from the affinity column as the thioester derivative ready for reaction with a N-terminal Cys peptide or another tag containing a Cys residue. 

Chong, S., Mersha, F.B., Comb, D.G., Scott, M.E., Landry, D., Vence, L.M., Perler, F.B., Benner, J., Kucera, R.B., Hirvonen, C.A., Pelletier, J.J., Paulus, H., and Xu, M.-Q. (1997) Single-column purification of free recombinant proteins using a self-cleavable affinity tag derived from a protein splicing element. Gene 192, 271-281. 

Metal chelate affinity chromatography finds most prominent application in the affinity purification of recombinant proteins to which a histidine tag has been attached (described later). As protein binding occurs via the histidine residues, this technique is no more inherently useful for the purification of metalloproteins than for the purification of non-metalloproteins (a common misconception, given its name). 

Among these amino acids histidine is the most commonly used one. Attachment of histidine tags to the recombinant proteins polypeptides is the most known development in the field of IMAC. Histidine and other metal affinity tags are widely used for protein purification [26], Adsorbents may be prepared by binding chelators onto the surface and metals to the chelators. Free coordination sites of the metal ions are needed for the analyte to bind to metal ions [25]. 

This mode of separation, as the name suggests, uses stationary phases with a special affinity for a specific analyte. The affinity ligand immobilized on the stationary phase varies dramatically from peptide, to protein, to oligonucleotide, to monoclonal antibody. In some cases the target molecule is labelled with an affinity tag to simplify the separation. This approach is common in the synthesis of recombinant proteins where the system can be engineered so that the target biomolecule expresses a tag such as polyhistidine. A stationary phase functionalized with aminodiacetic acid and nickel chelate is then used to fish out the required molecule by chelating with the polyhistidine tag. 

A key constraint in characterizing the products of these cDNAs is that often these sequences have no known function. Therefore, in expressing these cDNAs as recombinant proteins, one does not have an assay for activity to follow purification. Consequently, efforts to express these recombinant proteins rely on fusion constructs, multiple expression systems, and purification schemes that depend entirely on the presence of affinity tags. Expression and purification methods are designed to be independent of the characteristics of the protein of interest. 

Over several decades, multiple vector systems for recombinant gene expression in E. coli have been developed. Modem vectors suitable for recombinant protein production vary in the used promoter system in the presence or absence of coding sequences for affinity tags upstream or downstream of the multiple cloning site (MCS) and of sequences coding for leader peptides for the protein export. Moreover, different origins of replication (ori), antibiotic selection marker genes and MCS are used. 

Table 1 Comparison of the recombinant production and affinity chromatographic purification of GFP from B. megaterium [30]. Different affinity tag fusion forms of GFP were produced in II. megaterium WH323. Purification was performed using affinity chromatography. Amounts of purified GFP-Strep were determined using a Bradford protein assay kit (Bio-Rad Munich Germany) and BSA (Perbio Rockford USA) as standard. Amounts of purified GFP-His, His-TEV-GFP, Strep-Xa-GFP and Strep-TEV-GFP were calculated via their relative fluorescence per mg protein...

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