Lys-17 is the amine-donor substrate site for transglutaminase in beta A3-crystallin

The bovine lens protein beta A3-crystallin has recently been shown to be an amine-donor (Lys) substrate for tissue-type transglutaminase, using a newly developed amine-acceptor hexapeptide as a probe (Groenen, P.J.T.A., Seccia, M., Smulders, R.H.P.H., Gravela, E., Cheeseman, K.H., Bloemendal, H., and de Jong, W.W. (1993) Biochem. J. 295, 399-404). In the present study, the reactive amine-donor site has been identified by site-directed mutagenesis of the putative substrate lysine. The mutation Lys-17-->Arg abolishes the substrate capacity. This residue, located in the N-terminal extension of the polypeptide, thus acts as the sole amine-donor substrate in beta A3-crystallin. Our finding reinforces the notion that, in the crystallins, all amine-donor as well as amine-acceptor substrate sites reside in the N- or C-terminal arms. Transglutaminase-mediated cross-linking of beta A3-crystallin also gives rise to a beta A3 dimer, presumably due to the fact that Lys-17 can be cross-linked to the previously established Gln-7 or Gln-8 amine-acceptor site.