Purchase this article with an account.
V. Lapko, D.L. Smith, J.B. Smith; S-Methylation and Carbamylation of Human Gamma-Crystallins . Invest. Ophthalmol. Vis. Sci. 2003;44(13):2355.
Download citation file:
© ARVO (1962-2015); The Authors (2016-present)
Purpose: Cataractogenesis is characterized by aggregation and insolubilization of lens crystallins. Disulfide-bonding is among the major contributors to formation of high molecular weight aggregates. We recently demonstrated that in vivo methylation of specific cysteine residues of human gamma S-crystallins could be a protective mechanism against disulfide bonding. The goal of this study was to determine the extent of S-methylation of other gamma crystallins. Methods: Gamma crystallins were purified from nuclear and cortical parts of clear human lenses (ages 11 days to 83 years) using size exclusion chromatography and reversed phase HPLC. The molecular weights of the intact proteins were determined by electrospray ionization mass spectrometry. Enzymatic digestions of the proteins and peptide mapping were used to determine the sites and the abundance of the major modifications. Results: Methylation of specific cysteine residues and carbamylation of the N-terminus are major in vivo post-translational modifications of gamma crystallins. Nuclear gamma D-crystallins are heavily methylated at Cys 110, ranging from 37% in a 19 year-old lens to about 60% in a 50 year-old lens. Gamma C- and gamma B-crystallins are methylated at both Cys 22 and Cys 79, but only to a maximum of about 13%. Besides S-methylation, the major post-translational modifications in gamma D-crystallins are N-terminal carbamylation and truncation of the C-terminal Ser residue. Carbamylation of the N-terminus is also major in vivo post-translational modification of gamma C- and gamma B-crystallins. Conclusions: The major modifications of human gamma crystallins include S-methylation of cysteines and carbamylation of free N-termini. The high specificity of the methylation reactions strongly supports an enzymatic mechanism for the reaction.
This PDF is available to Subscribers Only