Mimicking phosphorylation of αB-crystallin affects its chaperone activity

alpha B-crystallin is a member of the sHsp (small heat-shock protein) family that prevents misfolded target proteins from aggregating and precipitating. Phosphorylation at three serine residues (Ser(19), Ser(45) and Ser(59)) is a major post-translational modification that occurs to alpha B-crystallin. In the present study, we produced recombinant proteins designed to mimic phosphorylation of alpha B-crystallin by incorporating a negative charge at these sites. We employed these mimics to undertake a mechanistic and structural investigation of the effect of phosphorylation on the chaperone activity of alpha B-crystallin to protect against two types of protein misfolding, i.e. amorphous aggregation and amyloid fibril assembly. We show that mimicking phosphorylation of alpha B-crystallin results in more efficient chaperone activity against both heat-induced and reduction-induced amorphous aggregation of target proteins. Mimicking phosphorylation increased the chaperone activity of alpha B-crystallin against one amyloid-forming target protein (kappa-casein), but decreased it against another (cc beta-Trp peptide). We observed that both target protein identity and solution (buffer) conditions are critical factors in determining the relative chaperone ability of wild-type and phosphorylated alpha B-crystallins. The present study provides evidence for the regulation of the chaperone activity of alpha B-crystallin by phosphorylation and indicates that this may play an important role in alleviating the pathogenic effects associated with protein conformational diseases.