The carboxy‐terminal lysine of αB‐crystallin is an amine‐donor substrate for tissue transglutaminase

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Abstract
A hexapeptide, corresponding to the sequence around the glutamine in βA3‐crystallin that functions as amine‐acceptor for transglutaminase, was synthesized. This peptide was biotinylated and used as a probe to identify amine‐donor substrates for transglutaminase among lens proteins. It was found that Ca2+‐activated transglutaminase linked this peptide not only to several β‐crystallins but, unexpectedly, also to αB‐crystallin. The C‐terminal lysine residue of αB‐crystalline could be identified as the site of linkage. This strengthens the notion that, at least in crystallins, all transglutaminase substrate residues are located in terminal extensions of the polypeptides. It was shown that in lens homogenate, αB‐crystallin can be covalently crosslinked to β‐crystallins by transglutaminase. The transglutaminase‐mediated crosslinking of αB‐crystallin may have implications for its involvement in normal and pathological processes in lens and other tissues.