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Vatsala Sagar, Graeme Wistow; A crystallographic model for mechanisms of aggregation in γ-crystallins. Invest. Ophthalmol. Vis. Sci. 2017;58(8):5593.
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Cataracts, like other disease involving protein deposition, are associated with age-related changes in protein structure. The x-ray crystallographic structure of a γS-crystallin under an oxidative environment was investigated to explore mechanisms for lens protein aggregation in cataract.
Recombinant proteins for native and variant γS-crystallins of chicken and mouse were purified. Proteins were crystallized under various conditions with limiting amounts of DTT at room temperature. Crystals were sent to the synchrotron for diffraction. Protein structures were solved by molecular replacement.
Using x-ray crystallography, we have determined a remarkable structure for what appears to be an aggregation intermediate for a normally monomeric gS-crystallin. The structure for mouse γS-L16 reveals an octameric complex with unusual domain-swapping, loss of organized secondary structure and intra- and intermolecular disulfide bond formation that causes local distortions of the protein backbone. The N-domains are arranged as two layers of non-covalently interacting tetrameric rings. The C- domains extend outwards from the ring. Each N-domain of one tetrameric ring forms an intermolecular disulfide bond to another the N-domain in the tetrameric assembly directly below it. The C-domain of each monomer domain-swaps to interact with an N-domain of a monomer in other layer of the octamer, forming four domain-swapped dimers arranged in a ring. The combination of strained domain-swapping and intermolecular disulfide bonds apparently causes stress to the protein resulting in loss of secondary structure, particularly in the outer ring of C-domains, potentially making them available for other interactions, including further disulfide formation.
This is the first high resolution structure to illustrate important features of the postulated intermediates of a crystallin aggregation cascade. It also shows how intermolecular disulfides, long implicated in formation of age-related cataract, can form in γ-crystallins.
This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.
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