beta-Crystallin: endogenous substrate of lens transglutaminase. Characterization of the acyl-donor site in the beta Bp chain.

Incubation of calf lens cortex homogenate with [14C]putrescine or dansylcadaverine, followed by two-dimensional gel electrophoresis and fluorography, enabled the identification of three different beta-crystallin chains as the endogenous substrates of Ca2+-dependent lens transglutaminase (R-glutaminyl-peptide:amine-gamma-glutamylyltransferase, EC 2.3.2.13). One of these is beta Bp, the predominant subunit of beta-crystallin, of which the amino acid sequence is known. The site of amine-labeling in beta Bp could be located, by limited proteolysis, in the N-terminal domain of this chain. Tryptic digestion of the N-terminal domain and subdigestion with elastase of the N-terminal tryptic peptide identified glutamine-7 as the single residue to which the amines are bound. This is the first example of an endogenous substrate of intracellular transglutaminase in which the site of the acyl-donor glutamine residue has been established. Tryptic digestion of the putrescine-labeled beta-crystallin aggregate, followed by high-voltage paper electrophoresis, provided a preliminary characterization of the labeled peptides originating from the other two labeled beta subunits.