The cataract-associated V41M mutant of human γS-crystallin shows specific structural changes that directly enhance local surface hydrophobicity

The major crystallins expressed in the human lens are gamma S-, gamma C- and gamma D-crystallins. Several mutations in gamma S-crystallin are associated with hereditary cataracts, one of which involves the substitution of a highly conserved Valine at position 41 to Methionine. According to a recent report, the mutant protein, V41M, shows lower stability and increased surface hydrophobicity compared to the wild-type, and a propensity for self-aggregation. Here we address the structural differences between the two proteins, with residue-level specificity using NMR spectroscopy. Based on the structural model of the mutant protein, our results clearly show that the mutation creates a major local perturbation almost at the junction of the first and second Greek-key motifs in the N-terminal domain. A larger section of the second motif (residues 4486) appears to be mainly affected. Based on the sizeable chemical shift of the imino proton of the indole side-chain of Trp46 in V41M, we suggest that the sulphur atom of Met41 is involved in an S-pi interaction with Trp46. This interaction would bring the last beta-strand of the first Greek-key motif closer to the first beta-strand of the second motif. This appears to lead to a domino effect, towards both the N- and C-terminal ends, even as it decays off substantially beyond the domain interface. During this process discreet hydrophobic surface patches are created, as revealed by ANS-binding. Such changes would not affect the secondary structure or cause a major change in the tertiary structure, but can lead to self-aggregation or aberrant binding interactions of the mutant protein in vivo, and lead to lens opacity or cataract. (C) 2013 Elsevier Inc. All rights reserved.