Mutation R120G in alpha B-crystallin, which is linked to a desmin-related myopathy, results in an irregular structure and defective chaperone-like function

alpha B-crystallin, a member of the small heat shock protein family, posses ses chaperone-like function. Recently, it has been shown that a missense mu tation in alpha B-crystallin, R120G, is genetically linked to a desmin-rela ted myopathy as well as to cataracts [Vicart, P., Caron, A., Guicheney, P., Li, A., Prevost, M.-C., Faure, A., Chateau, D., Chapon, F., Tome, P., Dupr et, J.-M., et al. (1998) Not. Genet. 20, 92-95]. By using alpha-lactalbumin , alcohol dehydrogenase, and insulin as target proteins, in vitro assays in dicated that R120G alpha B-crystallin had reduced or completely lost chaper one-like function. The addition of R120G alpha B-crystallin to unfolding al pha-lactalbumin enhanced the kinetics and extent of its aggregation. R120G alpha B-crystallin became entangled with unfolding alpha-lactalbumin and wa s a major portion of the resulting insoluble pellet. Similarly, incubation of R120G alpha B-crystallin with alcohol dehydrogenase and insulin also res ulted in the presence of R120G alpha B-crystallin in the insoluble pellets. Far and near UV CD indicate that R120G alpha B-crystallin has decreased be ta-sheet secondary structure and an altered aromatic residue environment co mpared with wild-type alpha B-crystallin. The apparent molecular mass of R1 20G alpha B-crystallin, as determined by gel filtration chromatography, is 1.4 MDa, which is more than twice the molecular mass of wild-type alpha B-c rystallin (650 kDa). Images obtained from cryoelectron microscopy indicate that R120G alpha B-crystallin possesses an irregular quaternary structure w ith an absence of a clear central cavity. The results of this study show, t hrough biochemical analysis, that an altered structure and defective chaper one-like function of alpha B-crystallin are associated with a point mutatio n that leads to a desmin-related myopathy and cataracts.