Influence of chemical modification of cysteine and histidine side chains upon subunit reassembly of alpha crystallin

Alpha crystallin, the important multimeric structural protein of mammalian eye lens, is an assembly composed of 30 alpha-A and 10 alpha-B subunits. The influence of either partial or complete chemical modification of two important amino acid side chains, cysteine and histidine, upon the integrity of native alpha crystallin assembly and also upon the mode of subunit reassembly has been investigated. It has been found that chemical modification of surface-exposed cysteine and histidine side chains does not affect the subunit-subunit interactions stabilizing the native aggregate. Cysteine modifications, either partial or complete, unlike histidine modifications, do not seem to affect the backbone conformation of the subunits refolded after denaturation. Both cysteine and histidine modifications, however, affect the packing of the refolded structural elements forming the tertiary structure of the subunits and also the mode of oligomeric reorganization. The most striking effect of histidine modification is the considerable increase in size of the aggregates upon reassociation of the modified subunits. The chaperone activity, however, has been found to remain almost unaffected in spite of these chemical modifications.