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

Abstract

Incubation of calf lens cortex homogenate with [14C]putrescine or dansylcadaverine, followed by 2 dimensional gel electrophoresis and fluorography, enabled the identification of 3 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 1st 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 2 labeled .beta. subunits.