Inability of chaperones to fold mutant zeta-crystallin, an aggregation-prone eye lens protein

PURPOSE: zeta -crystallin is a quinone oxido-reductase, recruited in the ey e lens of hystricomorphic rodents and camels. A deletion mutation constitut ing the NADPH-binding domain causes congenital cataract in a strain of guin ea pigs. The presence of large quantities of alpha -crystallin, a molecular chaperone, does not provide any protection against this. In order to inves tigate whether the underlying reason for the lack of protection is the form ation of a folding-incompetent protein, we have expressed the mutant protei n in a heterologous system along with other known chaperones. METHODS: We expressed the mutant zeta -crystallin in E. coli along with oth er chaperones such as GroEL/ES and DnaK/DnaJ/GrpE and then analyzed whether these chaperones could increase the amount of protein partitioning into th e soluble fraction of E. coli cells. RESULTS: These chaperones were unable to rescue the mutant protein from par titioning into inclusion bodies, although they could increase the yield of soluble wild-type zeta -crystallin. CONCLUSIONS: The deletion of 34 amino acids, constituting the NADPH-binding domain of zeta -crystallin, makes the protein incompetent to fold correctl y and thus form insoluble aggregates. It perhaps suggests why the mutant st rain of guinea pigs have cataract at birth even though their lenses contain high amounts of alpha -crystallin. This study also shows that certain muta tions can render proteins incompetent to fold into soluble molecules despit e abundant assistance.