Sequence comparison of gamma-crystallins from the reptilian and other vertebrate species

The gradient index lens of the eye: an opto-biological synchrony

The refractive power of a lens is determined largely by its surface curvatures and the refractive index of its medium. These properties can also be used to control the sharpness of focus and hence the image quality. One of the most effective ways of doing this is with a gradient index. Eye lenses of all species, thus far, measured, are gradient index (GRIN) structures. The index gradation is one that increases from the periphery of the lens to its centre but the steepness of the gradient and the magnitudes of the refractive index vary so that the optics of the lens accords with visual demands. The structural proteins, the crystallins, which create the index gradient, also vary from species to species, in type and relative distribution across the tissue. The crystallin classes do not contribute equally to the refractive index, and this may be related to their structure and amino acid content. This article compares GRIN forms in eye lenses of varying species, the relevance of these forms to visual requirements, and the relationship between refractive index and the structural proteins. Consideration is given to the dynamics of a living lens, potential variations in the GRIN form with physiological changes and the possible link between discontinuities in the gradient and growth. Finally, the property of birefringence and the characteristic polarisation patterns seen in highly ordered crystals that have also been observed in specially prepared eye lenses are described and discussed.

Vitamin A2 bound to cellular retinol-binding protein as ultraviolet filter in the eye lens of the gecko Lygodactylus picturatus

The yellow eye lenses of the diurnal gecko Lygodactylus picturatus contain, in addition to the usual crystallins, a monomeric protein with a molecular mass of 16kDa. It comprises 6-8% of the total water-soluble lens proteins. We here identify it as a novel type of crystallin, most closely related with cellular retinol-binding protein I (CRBP I). Because of its tiny size, we designate it as iota-crystallin. The typical endogenous ligand of CRBP is all-trans-retinol. In the gecko lens, however, the ligand of iota-crystallin turns out to be 3-dehydroretinol (vitamin A2), which causes the yellow color of this lens. The iota-crystallin.3-dehydroretinol complex absorbs shortwave radiation, supposedly improving the optical quality of the dioptric apparatus and protecting the retina against ultraviolet damage. Whereas other crystallins have been recruited from stress proteins and metabolic enzymes, iota-crystallin represents a completely new class of taxon-specific lens proteins. Also, its ligand 3-dehydroretinol represents a novel type of lens pigment.

Kinetic comparison of caiman epsilon-crystallin and authentic lactate dehydrogenases of vertebrates

Kinetic comparison of epsilon-crystallins isolated from the avian and reptilian species and the authentic lactate dehydrogenases (LDHs) was undertaken in order to clarify the identities of these structural lens proteins in relation to their enzymatic activity. Caiman epsilon-crystallin similar to the previously characterized duck epsilon-crystallin appeared to possess a genuine and stable LDH activity as detected by nitro blue tetrazolium staining on polyacrylamide gels and conventional kinetic assays. Kinetic parameters for pyruvate, L-lactate, NAD+, and three structural analogues of the coenzyme in this epsilon-crystallin catalyzed reaction were also determined and compared. Despite the structural similarities between epsilon-crystallins and chicken heart LDH, differences in charge and kinetic properties have been revealed by native isozyme electrophoresis and kinetic analysis as examined by initial velocity and substrate inhibition studies. It is found that the kinetic data analyzed for caiman epsilon-crystallin were more fitted with a compulsory ordered Bi-Bi sequential mechanism similar to those for the authentic LDHs and duck epsilon-crystallin. Caiman epsilon-crystallin has for the first time been established as a heart-type LDH based on the kinetic analysis and comparison with the authentic heart- and muscle-type LDHs from pig and chicken.

Lens crystallin changes associated with amphibian metamorphosis: involvement of a beta-crystallin polypeptide

Lens crystallins isolated from the tadpole and frog lenses were compared with regard to the developmental changes of crystallin compositions. The major changes during the process of metamorphosis were (1) the total contents of alpha- and gamma-crystallins decrease from more than 70% to less than 60% and (2) one of the major beta-crystallin polypeptides increases from less than 1% to about 6% and (3) an amphibian-specific rho-crystallin also increases from about 6% to more than 10% of total soluble proteins of the lens. We have characterized the metamorphosis-dependent beta-crystallin polypeptide by peptide mapping and sequence determination of the protease-digested fragments. This polypeptide showed very high sequence homology to that of the major beta Bp-crystallin chain reported for the mammalian lenses. The changes of the relative abundance of various crystallins and the gradually-elevated levels of the expression of this beta Bp-like crystallin in the developing lens during metamorphosis may also have some bearing on the maintenance of lens stability in the adult frog lenses.

Physicochemical characterization of alpha-crystallins from bovine lenses: hydrodynamic and conformational properties

A detailed investigation of hydrodynamic and conformational behavior has been made of the HM alpha-crystallin and alpha-crystallins of bovine lens. Results from this study indicated that HM alpha (high-molecular-weight alpha-crystallin) and alpha (low-molecular-weight alpha-crystallin) possess considerable size and charge heterogeneities in their native structures and subunit polypeptides, respectively. Sedimentation velocity showed a heterogeneous polydisperse system of HM alpha with an average sedimentation coefficient of about 50S and a more homogeneous system of alpha-crystallin of 20 S. Viscosity and circular dichroism studies pointed to a compact and globular shape of dominant beta-sheet conformation for alpha-crystallin, yet a highly asymmetrical and aggregated form for HM alpha. The conformational stability of alpha-crystallin was investigated in the presence of various denaturants. The evidence presented shows that hydrogen bonding is the main force in maintaining the quaternary structure of compact native alpha-crystallin. Conformational flexibility of alpha-crystallin demonstrated in the equilibrium unfolding study indicated a multistep transition that made the extraction of thermodynamic data from the heat denaturation study difficult. Temperature perturbation on alpha-crystallin suggested the possible involvement of hydrophobic interaction in the aggregation process, leading to the formation of HM alpha from alpha-crystallin. The comparison of conformational properties between HM alpha and alpha-crystallin strongly indicated that HM alpha is a denatured form of alpha-crystallin.

Biochemical characterization of lens crystallins from three mammalian species

Lens crystallins were isolated from the homogenates of mammalian eye lenses derived from three different species by gel permeation chromatography and characterized by SDS-gel electrophoresis, isoelectric focusing, amino acid analysis and N-terminal sequence analysis. Five fractions corresponding to HM alpha-, alpha-, beta H-, beta L- and gamma-crystallins were obtained for the crystallins from these phylogenetically distant species. 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 gel pattern of gamma-crystallins from the marmot lens appeared to be more complex than those of gibbon and deer lenses. Comparison of the amino acid contents of each orthologous class of mammalian crystallins with those of evolutionarily distant species still exhibited similarity in their amino acid compositions. The charge heterogeneity of each crystallin fraction can be detected by isoelectric focusing under denaturing conditions. 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 mammalian gamma-crystallin polypeptides were found, which suggested the close relatedness of gamma-crystallins amongst different species of mammals and also established the heterogeneous nature of this multigene family.

The protein sequence homology of gamma-crystallins among major vertebrate classes and their DNA sequence homology to heat-shock protein genes

A systematic characterization of lens crystallins from five major classes of vertebrates was carried out by exclusion gel filtration, cation-exchange chromatography and N-terminal sequence determination. All crystallin fractions except that of gamma-crystallin were found to be N-terminally blocked. gamma-Crystallin is present in major classes of vertebrates except the bird, showing none, or decreased amounts, of this protein in chicken and duck lenses, respectively. N-Terminal sequence analysis of the purified gamma-crystallin polypeptides showed extensive homology between different classes of vertebrates, supporting the close relatedness of this family of crystallin even from the evolutionarily distant species. Comparison of nucleotide sequences and their predicted amino acid sequences between gamma-crystallins of carp and rat lenses and heat-shock proteins demonstrated partial sequence homology of the encoded polypeptides and striking homology at the gene level. The unexpected strong homology of complementary DNA (cDNA) lies in the regions coding for 40 N-terminal residues of carp gamma-II, rat gamma 2-1, and the middle segments of 23,000- and 70,000-Mr heat-shock proteins. The optimal alignment of DNA sequences along these two segments shows about 50% homology. The percentage of protein sequence identity for the corresponding aligned segments is only 20%. The weak sequence homology at the protein level is also found between the invertebrate squid crystallin and rat gamma-crystallin polypeptides. These results pointed to the possibility of unifying three major classes of vertebrate crystallins into one alpha/beta/gamma superfamily and corroborated the previous supposition that the existing crystallins in the animal kingdom are probably mutually interrelated, sharing a common ancestry.

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.

N-terminal sequences of gamma-crystallins from the amphibian lens and their homology with gamma-crystallins of other major classes of vertebrates

gamma-Crystallins were isolated from the homogenate of frog eye lenses (Rana catesbeiana) by exclusion gel chromatography and further purified by cation-exchange chromatography. They were the only group of crystallins possessing free amino groups amenable to sequence analysis by Edman degradation. Comparison of the amino acid contents of the purified subfractions of gamma-crystallins indicated their close relatedness in amino acid compositions and probably sequence homology as well. The amino-terminal sequence analysis of the purified gamma-crystallin subfractions showed extensive homology between these amphibian gamma-crystallin polypeptides themselves and also those from other vertebrate species, suggesting the existence of a multigene family and their close relatedness to gamma-crystallins of other vertebrates. The sequence comparison of the gamma-crystallin polypeptides from all major classes of vertebrates has provided strong support for the divergent evolution of gamma-crystallin family.