Structural perturbation and enhancement of the chaperone-like activity of α-crystallin by arginine hydrochloride

Structural perturbation of a-crystallin is shown to enhance its molecular chaperone-like activity in preventing aggregation of target proteins. We demonstrate that arginine, a biologically compatible molecule that is known to bind to the peptide backbone and negatively charged side-chains, increases the chaperone-like activity of calf eye lens alpha-crystallin as well as recombinant human alphaA- and alphaB-crystallins. Arginine-induced increase in the chaperone activity is more pronounced for alphaB-crystallin than for alphaA-crystallin. Other guanidinium compounds such as aminoguanidine hydrochloride and guanidine hydrochloride also show a similar effect, but to different extents. A point mutation, R120G, in alphaB-crystallin that is associated with desmin-related myopathy, results in a significant loss of chaperone-like activity. Arginine restores the activity of mutant protein to a considerable extent. We have investigated the effect of arginine on the structural changes of alpha-crystallin by circular dichroism, fluorescence, and glycerol gradient sedimentation. Far-UV CD spectra show no significant changes in secondary structure, whereas near-UV CD spectra show subtle changes in the presence of arginine. Glycerol gradient sedimentation shows a significant decrease in the size of alpha-crystallin oligomer in the presence of arginine. Increased exposure of hydrophobic surfaces of alpha-crystallin, as monitored by pyrene-solubilization and ANS-fluorescence, is observed in the presence of arginine. These results show that arginine brings about subtle changes in the tertiary structure and significant changes in the quaternary structure of alpha-crystallin and enhances its chaperone-like activity significantly. This study should prove useful in designing strategies to improve chaperone function for therapeutic applications.