What is a His tag? How does it work?

A His tag, also known as a polyhistidine tag, is an amino acid motif in proteins consisting of 6-10 histidine (His) residues. It is usually attached to the N- or C- terminus of recombinant proteins, serving as an affinity tag for the purification of proteins.

Among the amino acids constituting proteins, histidine can strongly coordinate with metal ions. Therefore, when a protein is fused with the His tag, it can tightly bind to the carrier on which a metal ion such as nickel is immobilized. Other proteins without His tags do not bind to the carrier or bind only very weakly, which would be removed by washing the carrier with an appropriate buffer. Thereafter, a high concentration solution of imidazole can be used to elute the His-tagged protein with high purity.