Conformational properties of substrate proteins bound to a molecular chaperone alpha-crystallin.

alpha-Crystallin, the major protein of the ocular lens, acts as a molecular chaperone by suppressing the nonspecific aggregation of damaged proteins. To investigate the mechanism of the interaction between alpha-crystallin and substrate proteins, we prepared a tryptophan-free mutant of human alpha A-crystallin and assessed the conformation of thermally destabilized proteins captured by this chaperone using fluorescence spectroscopy. The fluorescence emission characteristics of bound substrates (rhodanese and gamma-crystallin) and the results of fluorescence quenching experiments indicate that the proteins captured by alpha-crystallin are characterized by a very low degree of unfolding. In particular, the structure of rhodanese bound to alpha A-crystallin appears to be considerably more native-like compared to that of the enzyme bound to the chaperonin GroEL. We postulate that alpha-crystallin (and likely other small heat shock proteins) recognize preferentially the aggregation-prone conformers that occur very early on the denaturation pathway. With its ability to capture and stabilize these early non-native structures, alpha-crystallin appears to be uniquely well suited to chaperone the transparency properties of the ocular lens.