Tissue transglutaminase catalyzes the deamidation of glutamines in lens βB₂- and βB₃-crystallins

Tissue transglutaminase (tTG) is a Ca²⁺-dependent enzyme catalyzing the formation of covalent crosslinks between peptide-bound glutamine and lysine residues. Lens crystallins, including αB-crystallin and several β-crystallins, are in vitro substrates for tTG. In both human and bovine fetal lens extracts treated with commercially available guinea pig liver tTG we detected the formation of high molecular weight (HMW) aggregates containing crosslinked βB₂- and βA₃-crystallin. More interestingly, 2D-gel electrophoresis combined with mass spectrometry analysis revealed that glutamines present in the N-terminal arms of βB₂- and βB₃-crystallins deamidate readily in the presence of tTG. We found that both tTG-catalyzed crosslinking and deamidation disrupt the β-crystallin complex, suggesting that these tTG-catalyzed modifications can influence the macromolecular assembly of lens crystallins. These data together suggest that tTG can contribute to the age-related deamidation of glutamine residues of lens crystallins.