Interaction of detergents with bovine lens alpha-crystallin: evidence for an oligomeric structure based on...

We have studied the quaternary structure of alpha-crystallin in the presence of increasing concentrations of amphiphilic and neutral detergents using gel filtration, light-scattering, boundary and equilibrium sedimentation. We observed a continuous reduction of the molar mass of the polymeric alpha-crystallin on increasing the concentration of sodium dodecyl sulphate from 0.1 mM to 5 mM, ending up with the monomeric peptides. Dodecyltrimethylammonium bromide also disrupts the oligomeric structure of alpha-crystallin but the interaction appears to be cooperative: in the sharp transition region (for a 1 mg/ml protein solution) from 3 to 8 mM of the detergent, only the native protein and a mixture of monomeric and dimeric peptide-DTAB complexes can be observed. Concomitant studies of the circular dichroism in the far UV revealed a substantial decrease of the beta-sheet and increase of the alpha-helix secondary structure. The latter can be related to the presence of amphiphilic polypeptide sequences in the constituent alpha A and alpha B peptides. These studies reveal for the first time a direct relation between changes in the secondary structure of the alpha A and alpha B peptides and the formation of the oligomeric alpha-crystallin structure: the binding of the amphiphilic detergent reduces the beta-sheet content, induces the formation of alpha-helix secondary structure and reduces the tendency of the peptide to form large aggregates. The different mechanisms for reducing the oligomeric size by anionic and cationic detergents with identical apolar parts stresses the importance of charge interactions. Our findings support some aspects of the micelle model of alpha-crystallin and can be related to its chaperone activity.