Substituted hydrophobic and hydrophilic residues at methionine-68 influence the chaperone-like function of alphaB-crystallin.

Amino acid residues 57-69 in alphaB-crystallin have been implicated as a target protein binding site. Moreover, a direct correlation between the extent of alpha-crystallin hydrophobicity and chaperone-like activity has been demonstrated. The purpose of this study was to mutate a moderately hydrophobic residue Met-68 (M-68) in the above region to strongly hydrophobic and hydrophilic residues and show whether chaperoning ability is affected with or without structural changes. Mutation of M-68 to Val, Ile or Thr did not result in significant changes in molecular mass and secondary and tertiary structures. However, the Val and Ile mutants showed significant improvement and the Thr mutant showed substantial loss in chaperone activity. Differences in chaperone function in the absence of any structural changes confirmed that the hydrophobicity or hydrophilicity of the substituted amino acid in the putative target protein binding site was the only contributing factor.