Molecular evolution and subunit structure of cattle lens alpha crystallin

The present studies were based on the premise that any common determinants in homologous proteins must have originated with the common ancestor of all of the taxonomic groups in which that determinant occurs. Cross-reacting antigenic determinants of lens alpha crystallin various classes of modern vertebrates were used to trace their evolutionary relationships. For quantitation of evolutionarily distinct determinants, equimolar amounts of alpha crystallin or its subunits, in either monomeric or reaggregated form, were bound to a matrix, then saturated with 125I-labeled Fab fragments of anti-cattle alpha crystallin antibodies having phylogenetically restricted specificities. This quantitative procedure has the important advantage of independence from variation in antibody responses to different determinants of the same antigenic molecule. The procedure is not impaired by steric hindrance. Both the SH-containing and SH-free subunits of cattle lens alpha crystallin were found to contain common antigenic determinants with the cyclostomata alpha crystallin. Such determinants originatd in evolution with the first vertebrates, the primitive agnatha. Antigenic determinants transferred from ancestral aquatic and land vertebrates to the mammals were found to constitute 93% of all determinants reactive in the monomeric SH-free subunits of cattle alpha crystallin. These determinants constitute only 76.5% of all determinants which are reactive in the SH-containing subunits. The antigenic determinants on both types of subunits were all found to be different. These findings indicate that evolutionary changes must have occurred more slowly in SH-free subunits than in SH-containing subunits. Significant decreases or increases were found in the content of various evolutionarily distinct determinants reactive in the reaggregated subunits as compared to the ones reactive in monomeric subunits. These differences can result from the formation of new conformational antigenic determinants during aggregation as well as from the burial or exposure of other determinants after aggregation. Different amounts of evolutionarily distinct antigenic determinants were found to be reactive in the molecules dissociated into subunits than in the intact molecules one of the reasons being that the intact molecules contain phylogenetically distinct determinants which depend on the quaternary structure of the protein molecule. The data obtained indicate that the quaternary structure of cattle alpha crystallin has, to a large degree, remained unchanged since the origin of vertebrates.