Beta B1 crystallin is an amine-donor substrate for tissue transglutaminase

The effects of tissue transglutaminase on the water-soluble proteins in bovine lens homogenates are described. Addition of liver transglutaminase and Ca2+ to calf lens homogenates resulted not only in the appearance of 50- and 57-kDa dimers, but also in a decrease in the amount of beta B1 crystallin and the almost complete disappearance of beta B3 and beta A3. This is not the result of Ca2+-induced proteolysis, since histamine completely inhibits this phenomenon. It may be concluded that these polypeptides are involved in beta-crystallin crosslinking by transglutaminase. This notion was confirmed by using beta B1- and beta Bp-specific antisera. Both sera reacted with the 57-kDa dimer; the beta Bp-specific antiserum also reacted with the 50-kDa dimer. No reaction in the region 50-57 kDa was detectable when EDTA was used instead of Ca2+. Using reconstituted mixtures of beta B1- and beta Bp-crystallin chains, and N-terminally truncated derivatives thereof, it was shown that in the beta B1/beta Bp dimer, glutamine residue -9 of beta Bp crosslinks to one of the lysine residues in the N-terminal extension of beta B1.