Structure-function Relationships in the Evolution of Elastin

Abstract

The evolution of the structure of the rubber-like protein elastin, found in connective tissues which are subjected to periodic physiological stress, was studied with respect to its phylogenetic distribution, fiber morphology and arrangement, response to deformation, and amino acid composition. Aortae and other tissues from several vertebrates and invertebrates were examined for the presence of elastin, which was defined on the basis of a characteristic amino acid composition, the presence of the unique crosslinks desmosine and isodesmosine, and by histologic criteria. The protein was present in all vertebrates except the primitive jawless fishes and was absent from all invertebrates which were examined. In addition, the morphology of aortic elastin fibers differed markedly among the vertebrate families. Biochemical analysis revealed increases in both the degree of crosslinking and hydrophobicity in elastins from higher vertebrates (mammals, birds) as compared to those from bony fish. Mammalian elastin displayed an increased tendency toward coacervation (polymerization into aggregated structures) at 37 degrees C and behaved differently from a conventional elastomer when stretched in a microcalorimeter. Selection for an increasingly hydrophobic elastin appears to have paralleled the development of a highly-pressurized, closed circulatory system in homeothermic animals. The data do not support a common genetic origin for elastin and other connective tissue proteins. Significant variations in amino acid composition among aortic elastins from different species, however, indicate that genetically distinct elastin types could have arisen by divergence from a common ancestral gene.