Isopeptag
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Isopeptag is a 16-amino acid peptide tag (TDKDMTITFTNKKDAE) that can be genetically linked to proteins without interfering with protein folding.[1] What makes the isopeptag different from other peptide tags is that it can bind its binding protein through a permanent and irreversible covalent bond. 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's covalent binding to its target overcomes these barriers and allows target proteins to be studied in harsher molecular environments.
The isopeptag was developed by dissecting the pilin protein (Spy0128) from Streptococcus pyogenes. Spy0128 contains two intramolecular isopeptide bonds,[2] and to generate the isopeptag one of these bonds was split by removing the last β-strand in the protein.