Affinity Purification & Expression Tags

After successful expression of a recombinant protein, it is often desirable to purify the protein to permit its characterization or its use in various applications. There are many strategies that can be applied to purify proteins; however, one type of purification that is commonly used is affinity chromatography. This method is often preferred because it can rapidly yield small but useful quantities of pure recombinant protein in a single chromatography step. In its most typical application, a protein or peptide “tag” that has affinity for a specific immobilized substrate is genetically encoded and expressed as a fusion to the target protein. The tagged protein can then be recovered from a complex mixture (such as a cell lysate) by contacting the mixture with a chromatography resin that displays a substrate to which the tag selectively binds. Common tags include polyhistidine sequences (“His-tags”) that specifically bind to resins displaying nickel or cobalt ions, peptide epitopes that interact with specific immobilized ligands or antibodies, or protein domains (e.g. maltose binding protein or chitin binding domain) that bind to chromatography resins that display a specific sugar. Chromatography resins may be used in several formats including packed columns or spin columns. Additionally, substrates or proteins that specifically bind to tags may be immobilized to magnetic beads to permit isolation of tagged proteins upon their exposure to a magnetic field.