THE CARBOXY-TERMINAL LYSINE OF ALPHA-B-CRYSTALLIN IS AN AMINE-DONOR SUBSTRATE FOR TISSUE TRANSGLUTAMINASE

A hexapeptide, corresponding to the sequence around the glutamine in beta-A3-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 beta-crystallins but, unexpectedly, also to alpha-B-crystallin. The C-terminal lysine residue of alpha-B-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, alpha-B-crystallin can be covalently crosslinked to beta-crystallins by transglutaminase. The transglutaminase-mediated crosslinking of alpha-B-crystallin may have implications for its involvement in normal and pathological processes in lens and other tissues.