How do transglutaminases select the reactive lysine residues in their protein substrates?

Transglutaminases catalyze the formation of N'(7-glutamyl)lysine bridges between proteins. Apart from targeting only a few proteins in complex biological systems, these enzymes also show exquisite specificities for selecting the Q and K residues for reaction (PNAS 89, 11161, 1992; JBC 269, 22907, 1994). No linear consensus sequence emerged thus far for identifying either of these sites. To investigate the possibility that the higher order structure of the protein substrate might exert a significant influence on the reactivities of the K (electron donor) sites, we studied the human erythrocyte enzyme-catalyzed incorporation of the fluorescent dansyl-t-aminocaproyl-QQIV probe into bovine pancreatic trypsin inhibitor. Knowing that this protein contains four K residues in different environments on its surface (Ear. J. Biochem. 62, 103, 1976; J. Mol. Biol. 241, 588, 1994), it was interesting to find that the acylation with the probe occurred only at K15 which, on account of being in the most positively charged environment, would be the preferred nucleophile at neutral pH. Aided by NIH grant HL-02212.