Protein purification is a fundamental step for analyzing individual proteins and protein complexes. Escherichia coli remains the first choice of many researchers for producing recombinant proteins due to ease of use, rapid cell growth and low cost of culture. Proteins expressed in E. coli can be purified in relatively high quantities, but these proteins, especially eukaryotic proteins, may not exhibit proper protein activity or folding. Cultured mammalian cells offer an alternative option for producing properly folded and functional mammalian proteins with appropriate post-translational modifications.
To simplify purification, affinity purification tags can be fused to a recombinant protein of interest. Common fusion tags are polypeptides, small proteins or enzymes added to the N- or C-terminus of a recombinant protein. The first step in purification of a recombinant protein is the preparation of the cell lysate or supernatant. For secreted proteins, minimal supernatant preparation is required, followed by selective binding, washing and elution of the purified protein. After purification the protein may be cleaved with a protease to remove the affinity tag.