Purification Systems Based on Bacterial Surface Proteins
Tove Boström * , Johan Nilvebrant * and Sophia Hober Royal Institute of Technology, Stockholm, Sweden
1. Introduction
Affinity purification is based on the selective and reversible interaction between two binding partners, of which one is bound to a chromatography matrix and the other may be either a native target protein or a recombinant protein fused with an affinity tag (Cuatrecasas et al. 1968). Recombinant DNA-technology allows straightforward construction of gene fusions to provide fusion proteins with two or more functions. The main intention is to facilitate downstream purification; however gene fusions may also improve solubility and proteolytic stability and assist in refolding (Waugh 2005). There are many fusion partners for which commercially available purification systems exist, ranging in size from a few amino acids to whole proteins (Flaschel & Friehs 1993; Terpe 2003). A commonly used purification handle is the poly-histidine (His) tag, enabling purification of the recombinant protein on a column with immobilized metal ions (Hochuli et al. 1988).
Other commonly used tags include the FLAG peptide (binding to anti-FLAG monoclonal antibodies), the strep-tag (binding to streptavidin), glutathione S-transferase (binding to glutathione) and maltose binding protein (binding to amylose) (Terpe 2003). Many affinity chromatography strategies also exist for the purification of native proteins, however these are slightly less specific and generally purify classes of proteins, as individual proteins each need a specific ligand. Today, many different ligands are available that can separate specific groups of proteins, for example phosphorylated, glycosylated or ubiquitinylated proteins (Azarkan et al. 2007).
Several bacterial surface proteins that show high affinity against different host proteins as immunoglobulins (Ig:s) and serum albumin, but also other host serum proteins, have been identified, see table 1 for examples. These proteins have different specificities regarding species and immunoglobulin classes and also bind to different parts of the immunoglobulin molecules. Therefore they have proven to be highly suitable for applications within protein purification. Many such proteins are expressed by pathogenic strains of the Staphylococci and Streptococci genera, and one biological function of these surface proteins is to help the bacteria evade the immune system of the host by covering the bacterium with host proteins (Achari et al. 1992; Sauer-Eriksson et al. 1995; Starovasnik et al. 1996). A significant property of serum albumin is the capability to bind other molecules and act as a transporter in the
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