Differences in solution dynamics between lens ?-crystallin homodimers and heterodimers probed by hydrogen-deuterium exchange and deamidation.

ABSTRACT: Lens transparency is due to the ordered arrangement of the major structural proteins, called crystallins. ?B2 crystallin in the lens of the eye readily forms dimers with other ?-crystallin subunits, but the resulting heterodimer structures are not known and were investigated in this study.Structures of ?A3 and ?B2 crystallin homodimers and the ?A3/?B2 crystallin heterodimers were probed by measuring changes in solvent accessibility using hydrogen-deuterium exchange with mass spectrometry. We further mimicked deamidation in ?B2 and probed the effect on the ?A3/?B2 heterodimer. Results were confirmed with chemical crosslinking and NMR.Both ?A3 and ?B2 had significantly decreased deuterium levels in the heterodimer compared to their respective homodimers, suggesting that they had less solvent accessibility and were more compact in the heterodimer. The compact structure of ?B2 was supported by the identification of chemical crosslinks between lysines in ?B2 within the heterodimer that were inconsistent with ?B2's extended homodimeric structure. The compact structure of ?A3 was supported by an overall decrease in mobility of ?A3 in the heterodimer detected by NMR. In ?B2, peptides 70-84 and 121-134 were exposed in the homodimer, but buried in the heterodimer with ?50% decreases in deuterium levels. Homologous peptides in ?A3, 97-109 and 134-149, had 25-50% decreases in deuterium levels in the heterodimer. These peptides are probable sites of interaction between ?B2 and ?A3 and are located at the predicted interface between subunits with bent linkers. Deamidation at Q184 in ?B2 at this predicted interface led to a less compact ?B2 in the heterodimer. The more compact structure of the ?A3/?B2 heterodimer was also more heat stable than either of the homodimers.The major structural proteins in the lens, the ?-crystallins, are not static, but dynamic in solution, with differences in accessibility between the homo-and hetero-dimers. This structural flexibility, particularly of ?B2, may facilitate formation of different size higher-ordered structures found in the transparent lens.Understanding complex hetero-oligomer interactions between ?-crystallins in normal lens and how these interactions change during aging is fundamental to understanding the cause of cataracts. This article is part of a Special Issue entitled Crystallin Biochemistry in Health and Disease.

SUBMITTER: Lampi KJ 

PROVIDER: S-EPMC4673030 | biostudies-literature | 2016 Jan

REPOSITORIES: biostudies-literature