Figure 1. Schematic diagram showing the principle of affinity chromatography. A protein with a high affinity for a ligand can selectively bind to an affinity matrix on which the ligand is immobilized. After washing away the unbound proteins, the protein can be eluted and purified under conditions that disrupt the interaction between the protein and its ligand.

Figure 2. Chemical structures of the substrates and ligands of affinity tag proteins.

Figure 3. The scheme of using an affinity tag to purify a recombinant target protein. The gene of a target protein is fused to the gene of an affinity tag. Expression of this fusion gene in cells produces a recombinant fusion protein that can selectively bind to an affinity matrix owing to the presence of the affinity tag in the fusion protein. This allows the fusion protein to be purified by affinity chromatography. After elution of the fusion protein, the target protein can be separated from the affinity tag by a cleavage reaction catalysed by a site-specific protease.