Biochemical comparison of lens crystallins from three reptilian species

Lens crystallins were isolated from the homogenates of reptilian eye lenses derived from three different species by gel-permeation chromatography and characterized by gel electrophoresis, amino-acid analysis, N-terminal sequence analysis and circular dichroism. Four fractions corresponding to alpha-, delta/epsilon/beta-, beta- and gamma-crystallins were obtained for the crystallins from caiman lenses, whereas delta- and gamma-crystallin fraction were present in lesser amounts or missing in the turtle and snake lenses, respectively. The native molecular masses for these purified fractions and their polypeptide compositions were determined by gel filtration and SDS-gel electrophoresis, respectively, revealing the typical subunit compositions for each classified crystallin. The spectra of circular dichroism indicate a predominant beta-sheet structure in alpha-, beta- and gamma-crystallins, and a major contribution of alpha-helical structure in delta/epsilon-crystallin fraction, which bears a resemblance to the secondary structure of delta-crystallin from the chicken lenses. Comparison of the amino-acid contents of each orthologous class of reptilian crystallins with those of evolutionary distant species still exhibited similarity in their amino-acid compositions. N-terminal sequence analysis of the crystallin fractions revealed that all fractions except that of gamma-crystallin are N-terminally blocked. Extensive sequence similarity between the reptilian gamma-crystallin polypeptides and those from other vertebrate species were found, which establish the close relatedness of gamma-crystallins amongst the major classes of vertebrates.