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Methylation and carbamylation of human -crystallins

Department of Chemistry, University of Nebraska, Lincoln, Nebraska 68588, USA

Reprint requests to: Jean B. Smith, Department of Chemistry, Hamilton Hall, University of Nebraska, Lincoln, NE 68588, USA; e-mail: jbsmith{at}unlserve.unl.edu; fax: (402) 472-9402.

Accessible sulfhydryls of cysteine residues are likely sites of reaction in long-lived proteins such as human lens crystallins. Disulfide bonding between cysteines is a major contributor to intermolecular cross-linking and aggregation of crystallins. A recently reported modification of S-crystallins, S-methylation of cysteine residues, can prevent disulfide formation. The aim of this study was to determine whether cysteines in C-, D-, and B-crystallins are also S-methylated. Our data show that all the -crystallins are S-methylated, but only at specific cysteines. In D-crystallin, methylation is exclusively at Cys 110, whereas in C- and B-crystallins, the principal methylation site is Cys 22 with minor methylation at Cys 79. D-crystallin is the most heavily methylated -crystallin. D-Crystallins from adult lenses are 37%–70% methylated, whereas C and B are ~12% methylated. The specificity of -crystallin methylation and its occurrence in young clear lenses supports the idea that inhibition of disulfide bonding by S-methylation may play a protective role against cataract. Another modification, not reported previously, is carbamylation of the N termini of B-, C-, D-crystallins. N-terminal carbamylation is likely a developmentally related modification that does not negatively impact crystallin function.

Keywords: Human lens crystallins; cataract; cysteine S-methylation; N-terminal carbamylation; N-terminal acetylation

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