The histidine affinity tag consists of 6 histidine amino acid
residues located at the N- or C- terminus of recombinant protein.
Purification of the recombination protein involves the use of
immobilized metal affinity chromatography, which takes advantage of
the high affinity of the hexahistidine residues for transition
metal ions (e.g Ni2+ (most common), Zn2+, Cu2+ or Co2+).
Other proteins present in the lysate or solution are unlikely to
bind to the immobilized metal ions, thus allowing for the isolation
of the recombinant protein. To release the recombinant
protein from the resin containing the immobilized metal ions, the
pH is lowered or a strong chelating agent (e.g. imidazole or EDTA)
is used. Compared to other purification tags, histidine tags are
small, have low toxicity and low levels of immunogenicity.