THE MUTATION ASP69-]SER AFFECTS THE CHAPERONE-LIKE ACTIVITY OF ALPHA-A-CRYSTALLIN

alpha-Crystallins are members of the family of small heat-shock protei ns. The conformation and mode of action of these 'junior chaperones' a re unknown. To investigate the structure and chaperone-like activity, four mutants of bovine alpha A-crystallin were generated by site-direc ted mutagenesis. In comparison with wild-type alpha A-crystallin, the D69S mutant, in which a highly conserved charged residue has been repl aced, forms larger multimers and displays a threefold reduced heat-pro tection capacity. The conformation and thermal stability of this mutan t are not noticeably affected. Three other mutations, replacing hydrop hobic by uncharged hydrophilic residues, were aimed at disturbing hydr ophobic intersubunit interactions. None of these mutations resulted in major structural perturbations and only minor differences in heat-pro tective capacity were observed. Although it is assumed that small heat -shock proteins interact with denaturing proteins via their hydrophobi c surfaces, this study clearly shows that charged residues in alpha-cr ystallin can also influence the efficiency of substrate binding.