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F. Shang, X. Zhang, Y. Liu, A. Taylor, B. Liu, J. J. Liang; Different Susceptibility of R120G and D140N Mutant B-Crystallins to the Ubiquitin-Proteasome Pathway May Account for the Different Myopathy Phenotypes. Invest. Ophthalmol. Vis. Sci. 2007;48(13):1527.
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© ARVO (1962-2015); The Authors (2016-present)
The R120G mutation in αB-crystallin results in cataract and myopathy, whereas the D140N mutation in αB-crystallin causes cataract without any other systematic abnormality. The objective of this work is to search for the mechanisms that underlie the different phenotypes of these two cataract-causing mutations in αB-crystallin.
Recombiant wild type, R120G and D140N mutant αB-crystallins were expressed and purified to homogeneity. The susceptibility of these αB-crystallins to ubiquitin-mediated degradation was assessed in lysates of lens epithelial or skeletal muscles. Susceptibility to degradation by trypsin was also determined. The structures of wt and mutant αB-crystallins were characterized by far UV circular dichroism, intrinsic tryptophan fluorescence, surface hydrophobicity and the thermal stability.
Wt αB-crystallin was degraded by the UPP in lysates of lens epithelial cells and skeletal muscles. R120G mutant αB-crystallin was resistant to ubiquitin-dependent degradation in lens and muscle cell lysates. In contrast, D140N mutant αB-crystallin was rapidly degraded. Both mutations had no detectable effects on trypsin-mediated degradation, indicating substrate selectivity and specificity of the UPP. Far UV circular dichroism spectra indicate that the mutations did not alter the secondary structure significantly. However, intrinsic tryptophan fluorescence spectra and Bis-ANS fluorescence spectra indicate that both mutations result in alterations in tertiary and/or quaternary structures and surface hydrophobicity of αB-crystallin. The order of thermal ostability was R120G mutant αB-crystallin > wt αB-crystallin > D140N mutant αB-crystallin.
Although R120G and D140N mutant αB-crystallins share many common characteristics, they show different susceptibility to the UPP mediated degradation. The rapid degradation of D140N mutant αB-crystallin by the UPP in muscles may prevent the myopathy phenotype as seen in R120G mutation in αB-crystallin.
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