Monday, April 25, 2016

The Martin lab and collaborators from UCSB have recently learned that the extraordinary aggregation resistance of eye lens proteins is partly due to their unique interactions with water.  Crystallins are highly soluble proteins in the eye lens that form a densely-packed transparent hydrogel with a high refractive index.  This work, published in the Journal of the American Chemical Society, explores the effects of water on protein crowding in human γS-crystallin (γS-WT) and the cataract-related variant γS-G18V, which is found in individuals with childhood-onset cataract. Using a combination of Overhauser dynamic nuclear polarization (ODNP) relaxometry and continuous wave paramagnetic resonance (CWEPR) lineshape analysis, the dynamics of the surface hydration water and of the interstitial water were explored as a function of increasing protein concentration for both γS-WT and γS-G18V. These experiments revealed the wild-type (“healthy”) protein to have a very stable and robust hydration shell, while the hydration shell of the G18V variant was be more fragile, as indicated by a dramatic slowing of the hydration water dynamics along with irreversible aggregation. These bulk data were correlated with microscopic differences in the interaction between specific protein residues and the solvent, using high-resolution solution-state NMR. The amide proton temperature coefficient (Δδ/ΔT) was measured for each backbone residue to distinguish intramolecular hydrogen bonds from those to the solvent . Overall, the results showed that more residues in γS-G18V form hydrogen bonds to water, meaning that the cataract variant has more solvent-exposed residues than wild-type, many of them hydrophobic. This increase in solvent interaction leads to decreased solubility, consistent with the current understanding that hydrophobic protein surfaces are more easily dewet than hydrophilic ones. These results suggest that the core function of γS-crystallin in the eye lens may be its capacity to resist aggregation by preserving its highly fluid hydration shell.


Their  recent manuscript has been accepted into JACS.