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Kirsten J. Lampi, Cade Fox, Larry David; Formation of βB2 and βA3 Crystallin Heterodimer induces an Open to Closed Conformation in βB2 leading to a more Compact Structure than either Homodimer. Invest. Ophthalmol. Vis. Sci. 2012;53(14):1300.
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© ARVO (1962-2015); The Authors (2016-present)
Close packing of lens crystallin subunits during maturation is necessary for lens transparency. Later, during normal aging and cataracts, aberrant protein interactions can lead to aggregation and insolubilization. In order to identify these aberrant interactions, normal interactions were first identified. βB1-crystallin has a closed structure with a bent linker connecting the N- and C- terminal domains, while βB2-crystallin has an open structure with an extended linker. The purpose of this study was to determine the structure of βA3, whose structure is not known, and the structure of βB2 in complex with βA3.
Complexes were formed after incubating recombinant human βA3 and βB2 cystallins at 37 degrees C for 18 hours. Heterodimers were isolated by size exclusion chromatography and confirmed by blue-native gels. Solvent accessibility changes were detected by analysis with hydrogen/deuterium exchange coupled to high-resolution mass spectrometry.
The average deuterium uptake per exchangeable residue was greatest for the N-terminal extensions, with no change upon complex formation, suggesting the extensions were not involved in the hetero-oligomer formation. Upon hetero-oligomer complex formation, significant decreases in exchange occurred in residues 89-109, 140-162, and 163-215 of βA3, suggesting a decrease in accessibility at the peptide in the N-terminal interface between the subunits and an overall more compactness of the C-terminal domain. For βB2, decreases in exchange occurred in regions 70-84 and 121-163, similar to results for βA3, but were more pronounced. The regions of greatest decrease were localized at the interface known to be important in the βB2 homo-tetramer and homologous to the intramolecular interface in the βB1 homodimer. These regions would only be buried in a closed model of a βB2-βA3 complex.
Our data strongly suggests that formation of a βB2 and βA3 crystallin heterodimer induces an open to closed conformation in βB2 leading to a more compact structure than either homodimer. Disruption of this complex during aging and cataracts may disrupt the compact packing of lens crystallins and lead to loss of vision.
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