Structural studies on the individual domains of human γS-crystallin and its G57W mutant unfolds mechanistic insights into childhood cataracts

Abstract

Inter-domain interactions tune the exceptional stability of human γS-crystallin (γS-WT) in the eye lens, which lasts a lifetime with no protein turnover. Our recent NMR studies revealed the key role of G57W mutation in γS-WT, as the familial determinate of childhood cataracts. As the unusually exposed W57 is near the inter-domain interface, a recurring theme of this study is the upsetting of inter-domain contacts exposing hydrophobic patches, which may initiate aggregation at crystallin concentrations not so surprising in the eye lens. In this endeavour, to untangle the mechanistic pathways triggering aggregation in the cataract variant γS-G57W, we undertook high-resolution structural characterization of isolated domains vis-a-vis full length γS-crystallin. Here we report for the first time, thermodynamic and kinetic determinants of structural stability with their eccentric shifts under pathological stress employing sophisticated spectroscopy techniques. We propose that domain interface acts as an intrinsic stabilizer for the otherwise floppy N-terminal domain in γS-G57W than in γS-WT where it serves an extrinsic role. Our results present a residue resolved quantitative analysis for differential domain stabilities from non-linear temperature coefficients of ¹H^N chemical shifts using solution NMR spectroscopy. Consistent with the Ca²⁺-binding episode that lasted poorly for human lens crystallins, our results show that disease mutants attenuate it further and completely silence it in extreme cases. Overall, our study provides a compelling evidence for the diverse structural evolution of lens crystallins elucidating molecular details to apprehend lens opacification and suggests the scope of therapeutics in reducing the global trauma of cataracts.

Keywords: Ca(2+)-binding motifs; Inter-domain interactions; NMR spectroscopy; Refolding kinetics.