R120G αB-crystallin promotes the unfolding of reduced α-lactalbumin and is inherently unstable

Abstract

α-Crystallin is the principal lens protein which, in addition to its structural role, also acts as a molecular chaperone, to prevent aggregation and precipitation of other lens proteins. One of its two subunits, αB-crystallin, is also expressed in many nonlenticular tissues, and a natural missense mutation, R120G, has been associated with cataract and desmin-related myopathy, a disorder of skeletal muscles [Vicart P, Caron A, Guicheney P, Li Z, Prevost MC, Faure A, Chateau D, Chapon F, Tome F, Dupret JM, Paulin D & Fardeau M (1998) Nat Genet 20, 92-95]. In the present study, real-time 1H-NMR spectroscopy showed that the ability of R 120G αB-crystallin to stabilize the partially folded, molten globule state of α-lactalbumin was significantly reduced in comparison with wild-type αB-crystallin. The mutant showed enhanced interaction with, and promoted unfolding of, reduced α-lactalbumin, but showed limited chaperone activity for other target proteins. Using NMR spectroscopy, gel electrophoresis, and MS, we observed that, unlike the wild-type protein, R120G αB-crystallin is intrinsically unstable in solution, with unfolding of the protein over time leading to aggregation and progressive truncation from the C-terminus. Light scattering, MS, and size-exclusion chromatography data indicated that R120G αB-crystallin exists as a larger oligomer than wild-type αB-crystallin, and its size increases with time. It is likely that removal of the positive charge from R120 of αB-crystallin causes partial unfolding, increased exposure of hydrophobic regions, and enhances its susceptibility to proteolysis, thus reducing its solubility and promoting its aggregation and complexation with other proteins. These characteristics may explain the involvement of R120G αB-crystallin with human disease states.