Acceptor-donor relationships in the transglutaminase-mediated cross-linking of lens beta-crystallin subunits

Following the isolation of the N epsilon-(gamma-glutamyl)lysine-containing polymers from human cataracts, our efforts were directed to induce such cross-links experimentally in rabbit lens, and evidence was obtained for the selective reactivities of certain beta-crystallin subunits in this transglutaminase-catalyzed event. In the present work, we examined the enzymatic cross-linking of purified crystallins individually (alpha, beta H, beta L, and gamma) and in combinations, with particular emphasis on forming the approximately 55K dimer. This species was the primary product in the cross-linking of beta H-crystallins; beta L also reacted with transglutaminase. Neither alpha- nor gamma-crystallins formed appreciable amounts of cross-linked structures with transglutaminase. Dansylcadaverine, known to compete against the reactive lysines of proteins in forming N epsilon-(gamma-glutamyl)lysine cross-bridges, was shown to inhibit the generation of dimeric and higher ordered oligomers from beta H and beta L. The fluorescent amine specifically labeled only two subunits in beta H (approximately 29-30K and approximately 26K) and one in beta L (approximately 26K), identifying these substrates as possessing transglutaminase-reactive endo-gamma-glutaminyl residues. An antiserum to bovine beta Bp recognized the approximately 23K subunit of rabbit beta-crystallins and also the approximately 55K dimer, suggesting that the approximately 23K protein participates as a lysine donor in generating the cross-linked dimer with transglutaminase. Inasmuch as the same antiserum reacts with an approximately 50K material reported to appear in increasing amounts with age in human lens, the results lend added support to the physiological significance of transglutaminase in the aging of lens.