The benefits of being β-crystallin heteromers: βB1-crystallin protects βA3-crystallin against aggregation during co-refolding.
β-Crystallins are the major structural proteins in mammalian lens, and their stability is critical in maintaining the transparency and refraction index of the lens. Among the seven β-crystallins, βA3-crystallin and βB1-crystallin, an acidic and a basic β-crystallin, respectively, can form heteromers in vivo. However, the physiological roles of the heteromer have not been fully elucidated. In this research, we studied whether the basic β-crystallin facilitates the folding of acidic β-crystallin. Equilibrium folding studies revealed that the βA3-crystallin and βB1-crystallin homomers and the βA3/βB1-crystallin heteromer all undergo similar five-state folding pathways which include one dimeric and two monomeric intermediates. βA3-Crystallin was found to be the most unstable among the three proteins, and the transition curve of βA3/βB1-crystallin was close to that of βB1-crystallin. The dimeric intermediate may be a critical determinant in the aggregation process and thus is crucial to the lifelong stability of the β-crystallins. A comparison of the Gibbs free energy of the equilibrium folding suggested that the formation of heteromer contributed to the stabilization of the dimer interface. On the other hand, βA3-crystallin, the only protein whose refolding is challenged by serious aggregation, can be protected by βB1-crystallin in a dose-dependent manner during the kinetic co-refolding. However, the protection is not observed in the presence of the pre-existed well-folded βB1-crystallin. These findings suggested that the formation of β-crystallin heteromers not only stabilizes the unstable acidic β-crystallin but also protects them against aggregation during refolding from the stress-denatured states.
State Key Laboratory of Biomembrane and Membrane Biotechnology, School of Life Sciences, Tsinghua University, Beijing 100084, China.
Sha
S
Wang
Xiao-Yao
XY
Leng
Yong-Bin
YB
Yan
D019281:Dimerization; D006801:Humans; D055503:Protein Multimerization; D058849:Protein Refolding; D038205:beta-Crystallin A Chain / Q000737:chemistry* / Q000378:metabolism*; D038206:beta-Crystallin B Chain / Q000737:chemistry* / Q000502:physiology*
C489108:CRYBA1 protein, human; C105601:CRYBB1 protein, human; D038205:beta-Crystallin A Chain; D038206:beta-Crystallin B Chain