Other peptide tags generally bind their targets through weak non-covalent interactions, thus limiting their use in applications where molecules experience extreme forces.
The isopeptag was developed by dissecting the pilin protein (Spy0128) from Streptococcus pyogenes.
When the isopeptag is bound to a target protein, it spontaneously binds its binding partner through an isopeptide bond, an amide bond formed autocatalytically.
The reaction is robust and occurs at various temperatures from 4-37 °C, a pH range of 5–8, and in the presence of commonly used detergents.
Also, the reaction is independent of the redox state of the environment and can occur equally well in both reducing and oxidizing conditions.