The N-terminal extension of ßB1-crystallin chaperones ß-crystallin folding and cooperates with αA-crystallin.

ß/γ-Crystallins are the major structural proteins in mammalian lens. The N-terminal truncation of ßB1-crystallin has been associated with the regulation of ß-crystallin size distributions in human lens. Herein we studied the roles of ßB1 N-terminal extension in protein structure and folding by constructing five N-terminal truncated forms. The truncations did not affect the secondary and tertiary structures of the main body as well as stability against denaturation. Truncations with more than 28 residues off the N-terminus promoted the dissociation of the dimeric ßB1 into monomers in diluted solutions. Interestingly, the N-terminal extension facilitated ßB1 to adopt the correct folding pathway, while truncated proteins were prone to undergo the misfolding/aggregation pathway during kinetic refolding. The N-terminal extension of ßB1 acted as an intramolecular chaperone (IMC) to regulate the kinetic partitioning between folding and misfolding. The IMC function of the N-terminal extension was also critical to the correct refolding of ß-crystallin heteromer and the action of the lens-specific molecular chaperone αA-crystallin. The cooperation between IMC and molecular chaperones produced a much stronger chaperoning effect than if they acted separately. To our knowledge, this is the first report showing the cooperation between IMC and molecular chaperones.

A novel mutation in CRYBB1 associated with congenital cataract-microcornea syndrome: the p.Ser129Arg mutation destabilizes the ßB1/ßA3-crystallin heteromer but not the ßB1-crystallin homomer.

Congenital cataract-microcornea syndrome (CCMC) is a clinically and genetically heterogeneous condition characterized by lens opacities and microcornea. It appears as a distinct phenotype of heritable congenital cataract. Here we report a large Chinese family with autosomal dominant congenital cataract and microcornea. Evidence for linkage was detected at marker D22S1167 (LOD score [Z]=4.49, recombination fraction [θ]=0.0), which closely flanks the â-crystallin gene cluster locus. Direct sequencing of the candidate âB1-crystallin gene (CRYBB1) revealed a c.387C>A transversion in exon 4, which cosegregated with the disease in the family and resulted in the substitution of serine by arginine at codon 129 (p.Ser129Arg). A comparison of the biophysical properties of the recombinant ß-crystallins revealed that the mutation impaired the structures of both ßB1-crystallin homomer and ßB1/ßA3-crystallin heteromer. More importantly, the mutation significantly decreased the thermal stability of ßB1/ßA3-crystallin but not ßB1-crystallin. These findings highlight the importance of protein-protein interactions among ß-crystallins in maintaining lens transparency, and provide a novel insight into the molecular mechanism underlying the pathogenesis of human CCMC.