RESUMO
BACKGROUND: Lens transparency is due to the ordered arrangement of the major structural proteins, called crystallins. ßB2 crystallin in the lens of the eye readily forms dimers with other ß-crystallin subunits, but the resulting heterodimer structures are not known and were investigated in this study. METHODS: Structures of ßA3 and ßB2 crystallin homodimers and the ßA3/ßB2 crystallin heterodimers were probed by measuring changes in solvent accessibility using hydrogen-deuterium exchange with mass spectrometry. We further mimicked deamidation in ßB2 and probed the effect on the ßA3/ßB2 heterodimer. Results were confirmed with chemical crosslinking and NMR. RESULTS: Both ßA3 and ßB2 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 ßB2 was supported by the identification of chemical crosslinks between lysines in ßB2 within the heterodimer that were inconsistent with ßB2's extended homodimeric structure. The compact structure of ßA3 was supported by an overall decrease in mobility of ßA3 in the heterodimer detected by NMR. In ßB2, 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 ßA3, 97-109 and 134-149, had 25-50% decreases in deuterium levels in the heterodimer. These peptides are probable sites of interaction between ßB2 and ßA3 and are located at the predicted interface between subunits with bent linkers. Deamidation at Q184 in ßB2 at this predicted interface led to a less compact ßB2 in the heterodimer. The more compact structure of the ßA3/ßB2 heterodimer was also more heat stable than either of the homodimers. CONCLUSIONS: The major structural proteins in the lens, the ß-crystallins, are not static, but dynamic in solution, with differences in accessibility between the homo-and hetero-dimers. This structural flexibility, particularly of ßB2, may facilitate formation of different size higher-ordered structures found in the transparent lens. GENERAL SIGNIFICANCE: Understanding complex hetero-oligomer interactions between ß-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.