What are the common affinity tags for protein purification?

Affinity tags are unique proteins/peptides that help streamline protein purification and make it easier to track proteins during protein expression and purification. These are 5 of the most common affinity tags for protein purification:

Glutathione-S-transferase (GST) Tag
Glutathione-S-transferase (GST), a protein consisting of 211 amino acids, is a widely-used tag that works on the principle of GST’s affinity for immobilized glutathione. Glutathione-S-transferase increases solubility of the target protein and gives good yields. The mild elution procedure helps to preserve the protein’s immunogenicity and inherent properties. However, the relatively large size of the tag can impact the functionality of the fusion protein.

Polyhistidine Tag
Poly-Histidine tag (His tag) is a polypeptide consisting of 6-8 histidine residues located on either the N or C-end of a recombinant protein. The relatively small size of the tag allows it to easily integrate into expression vectors. Their small size and stable binding make His tags popular for use in protein purification. In addition, they do not form secondary structures, neither do they affect the structure and function of the purified protein.

Calmodulin Binding Peptide (CBP)
Calmodulin Binding Peptide is a 25 amino acid fragment with a molecular weight of only 4 kDa. This tag has a strong affinity for the protein calmodulin. It purifies target proteins from bacteria using a C-terminal fragment from muscle myosin light-chain kinase. The relatively small size of Calmodulin Binding Protein makes it an ideal tag for purifying delicate proteins under mild conditions.

Maltose-binding Tag
One of the most popular fusion tags, Maltose-Binding Protein (MBP), is a relatively large periplasmic tag with a molecular weight of 43 kDa. This tag is used to increase solubility of the fusion protein and is the preferred tag when purifying membrane or lipophilic proteins. Location of the Maltose-Binding Protein on the N-end of the target protein is more efficient than on the C-end. The target protein fused with MBP can be expressed either in the cytoplasm or periplasmic space.

Streptavidin/Biotin-Based Tag
Biotin, a small molecule with a relatively small molecular weight of 244 Da forms a very strong bond with both avidin and streptavidin molecules. Affinity purification is based on this strong affinity between biotin and avidin or streptavidin. Because of its small molecular weight, biotin is unlikely to affect protein functionality.

Streptavidin/Biotin-Based Tag is widely used for protein expression, protein purification, and surface plasmon resonance.