Mammalian transglutaminase (TGase) catalyzes covalent crosslinking of peptide-bound lysine residues or incorporation of primary amines to limited glutamine residues in substrate proteins. Using an unbiased M13 phage display random peptide library, we developed a screening system to elucidate primary structures surrounding reactive glutamine residue(s) that are preferred by TGase. Screening was performed by selecting phage clones expressing peptides that incorporated biotin-labeled primary amine by the catalytic reactions of TGase 2 and activated Factor XIII (Factor XIIIa). We identified several amino acid sequences that were preferred as glutamine donor substrates, most of which have a marked tendency for individual TGases: TGase 2, QxPD(P), QxP, and QxxDP; Factor XIIIa, QxxxWP (where x and represent a non-conserved and a hydrophobic amino acid, respectively). We further confirmed that the sequences were favored for transamidation using modified glutathione S-transferase (GST) for recombinant peptide-GST fusion proteins. Most of the fusion proteins exhibited a considerable increase in incorporation of primary amines over that of modified GST alone. Furthermore, we identified the amino acid sequences that demonstrated higher specificity and inhibitory activity in the cross-linking reactions by TGase 2 and Factor XIIIa.Transglutaminase (TGase, 2 2.3.2.13) is an enzyme that catalyzes the formation of isopeptide cross-links between glutamine and lysine residues in a variety of proteins and also attaches other primary amines to peptide-bound glutamines (1-5). To date, eight human TGase isozymes (Factor XIII, TGases 1-7) have been identified, comprising a large protein family with unique tissue distributions and physiological roles. Among the isozymes, TGase 2 and Factor XIII are two major members, although their locations and regulation differ. TGase 2 is expressed ubiquitously and has been implicated in many biological processes, including apoptosis, stabilization of the extracellular matrix, and regulation of growth factors (6, 7). Plasma Factor XIII is synthesized as a zymogen that comprises two A and two B subunits (8). The A subunit contains the catalytic domain and is converted by thrombin-dependent proteolysis into its active form during clot formation. Activated Factor XIII (Factor XIIIa) is involved in fibrin stabilization and wound healing (9).Generally, TGase reactions involve a Ca 2ϩ -dependent acyl transfer via a double displacement mechanism. In the first step, a glutamine ␥-carboxyamide group in the substrate binds to a cysteine at the active site, resulting in formation of a ␥-glutamylthioester bond and release of ammonia. Formation of the covalent acyl enzyme intermediate is the rate-limiting step. The -amino group of a peptide-bound lysine, as a nucleophilic substrate, binds to the acyl enzyme intermediate and then attacks the thioester bond, thereby generating an intermolecular isopeptide -(␥-glutamyl)lysine cross-link. Primary amines can replace lysine in transamidation reactions and become incorp...

Transglutaminase 1 (TGase 1) is an essential enzyme for cornified envelope formation in stratified squamous epithelia. This enzyme catalyzes the cross‐linking of glutamine and lysine residues in structural proteins in differentiating keratinocytes. To gain insight into the preferred substrate structure of TGase 1, we used a phage‐displayed random peptide library to screen primary amino acid sequences that are preferentially selected by human TGase 1. The peptides selected as glutamine donor substrate exhibited a marked tendency in primary structure, conforming to the sequence: QxK/RψxxxWP (where x and ψ represent non‐conserved and hydrophobic amino acids, respectively). Using glutathione S‐transferase (GST) fusion proteins of the selected peptides, we identified several sequences as preferred substrates and confirmed that they were isozyme‐specific. We generated GST‐fused alanine mutants of the most reactive sequence (K5) to determine the residues that were critical for reactivity. Even in peptide form, K5 appeared to have high and specific reactivity as substrate. In situ analysis of mouse skin sections using fluorescence‐conjugated K5 peptide resulted in detection of TGase 1 activity with high sensitivity, but no signal was detected in a TGase 1‐null mouse. In conclusion, we were successful in generating a novel substrate peptide for sensitive detection of endogenous TGase 1 activity in the skin.

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