Tagged protein purification uses affinity chromatography to purify recombinant proteins that have been engineered to include a specific peptide or protein sequence (tag). Adding a tag to a target protein allows you to simplify the purification protocol greatly, sometimes to the extent that you can use a standard protocol.
Common choices for affinity tags are polyhistidine (histidine-tag), Glutathione S-transferase (GST), maltose-binding protein (MBP), Strep-tag II and FLAG™.
The target protein with affinity tag is specifically and reversibly bound by a complementary binding substance (ligand) that recognizes the tag. The sample is applied under conditions that favor binding to the ligand. Unbound material is washed out of the column.
The bound tagged protein is recovered (eluted), commonly using a competitive ligand. The eluted protein is usually at a high concentration. If tag removal is needed prior to use of the protein, cleavage can be performed using a site-specific protease.