gamma-Crystallins are associated with cataract in both human and animal models. They may also have stress related roles in other eye tissues, notably retina. We have shown that they can play a role in stabilization of cytoskeleton in lens. g-Crystallins have highly unusual solution properties that fit them for high protein concentration environments. They have stable, tightly folded structures but can unfold to form amyloid like fibrils. We have studied unfolding/refolding in members of the gS-crystallin family from birds and mammals. We have crystallized and solved the three-dimensional structure of chicken gS-crystallin, the first gS to be crystallized. The crystal structure reveals an unexpected mode for gS dimerization. In particular this monomeric protein forms a crystal lattice contact (QR)identical in orientation to the dimerization interface in beta-crystallins. We show that proteins adopting the QR interface exhibit additive molecule dipoles, which suggests an important mechanism for association of crystallins. Although initial experiments suggested that this dimerization does not occur in solution, new AUC data with our collaborators show that gS-crystallins do dimerize at high concentrations similar to those found in the lens. We are also investigating how oxidation and other stresses contribute to crystallin unfolding and aggregation, We now have a crystal structure for a variant of mouse gS under oxidizing conditions. This remarkable structure appears to be an aggregation intermediate involving intra- and inter-molecular S-S bonds, strained domain swapping and progressive loss of secondary structure. This adds important insight into the processes that lead to cataract. Further experiments show that the octameric structure in the crystal forms by dimerization of a disulfide linked tetramer but that geometry does not allow full disulfide formation.
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