Structure of the bovine eye lens gamma s-crystallin gene (formerly beta s)

γS-crystallin proteins from the Antarctic nototheniid toothfish: a model system for investigating differential resistance to chemical and thermal denaturation

The γS1- and γS2-crystallins, structural eye lens proteins from the Antarctic toothfish (Dissostichus mawsoni), are homologues of the human lens protein γS-crystallin. Although γS1 has the higher thermal stability of the two, it is more susceptible to chemical denaturation by urea. The lower thermodynamic stability of both toothfish crystallins relative to human γS-crystallin is consistent with the current picture of how proteins from organisms endemic to perennially cold environments have achieved low-temperature functionality via greater structural flexibility. In some respects, the sequences of γS1- and γS2-crystallin are typical of psychrophilic proteins; however, their amino acid compositions also reflect their selection for a high refractive index increment. Like their counterparts in the human lens and those of mesophilic fish, both toothfish crystallins are relatively enriched in aromatic residues and methionine and exiguous in aliphatic residues. The sometimes contradictory requirements of selection for cold tolerance and high refractive index make the toothfish crystallins an excellent model system for further investigation of the biophysical properties of structural proteins.

Preferential and specific binding of human αB-crystallin to a cataract-related variant of γS-crystallin

Transparency in the eye lens is maintained via specific, functional interactions among the structural βγ- and chaperone α-crystallins. Here, we report the structure and α-crystallin binding interface of the G18V variant of human γS-crystallin (γS-G18V), which is linked to hereditary childhood-onset cortical cataract. Comparison of the solution nuclear magnetic resonance structures of wild-type and G18V γS-crystallin, both presented here, reveal that the increased aggregation propensity of γS-G18V results from neither global misfolding nor the solvent exposure of a hydrophobic residue but instead involves backbone rearrangement within the N-terminal domain. αB-crystallin binds more strongly to the variant, via a well-defined interaction surface observed via chemical shift differences. In the context of the αB-crystallin structure and the finding that it forms heterogeneous multimers, our structural studies suggest a potential mechanism for cataract formation via the depletion of the finite αB-crystallin population of the lens.

Clinical and experimental advances in congenital and paediatric cataracts

Cataracts (opacities of the lens) are frequent in the elderly, but rare in paediatric practice. Congenital cataracts (in industrialized countries) are mainly caused by mutations affecting lens development. Much of our knowledge about the underlying mechanisms of cataractogenesis has come from the genetic analysis of affected families: there are contributions from genes coding for transcription factors (such as FoxE3, Maf, Pitx3) and structural proteins such as crystallins or connexins. In addition, there are contributions from enzymes affecting sugar pathways (particularly the galactose pathway) and from a quite unexpected area: axon guidance molecules like ephrins and their receptors. Cataractous mouse lenses can be identified easily by visual inspection, and a remarkable number of mutant lines have now been characterized. Generally, most of the mouse mutants show a similar phenotype to their human counterparts; however, there are some remarkable differences. It should be noted that many mutations affect genes that are expressed not only in the lens, but also in tissues and organs outside the eye. There is increasing evidence for pleiotropic effects of these genes, and increasing consideration that cataracts may act as early and readily detectable biomarkers for a number of systemic syndromes.

Identification of the primary targets of carbamylation in bovine lens proteins by mass spectrometry

PURPOSE

Carbamylation, an important post-translational modification of proteins, inevitably causes conformational changes of lens proteins. It may increase aggregation between crystallin molecules and disrupt the close packing required for transparency thus leading to cataract. The aim of this study was to isolate the primary targets of carbamylation in the lens and identify them by mass spectrometry.

MATERIALS AND METHODS

Fresh intact bovine lenses were incubated with 100 mM potassium cyanate for 7 days. The proteins in the water-soluble fractions from the normal control and the cyanate-modified lens proteins were separated by two-dimensional (2-D) gel electrophoresis with identification after silver staining. Protein spots that differed between the normal and carbamylated groups were selected for further analysis using matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS).

RESULTS

The 2-D gel results showed that the major lens proteins were in the section of pI 5-8, with relative molecular masses of 20-35 kDa, and changes in the carbamylated fraction like strings of beads indicating modification. The mass spectrometry analysis and a database search identified carbamylated proteins originating from alphaA-crystallin, betaB2- and gammaS-(betaS)-crystallins.

CONCLUSIONS

These crystallins may be vulnerable proteins targeted by carbamylation. The accumulated aggregation and loss of chaperone activity may contribute to cataract formation.

Eye lens proteomics

The eye lens is a fascinating organ as it is in essence living transparent matter. Lenticular transparency is achieved through the peculiarities of lens morphology, a semi-apoptotic process where cells elongate and loose their organelles and the precise molecular arrangement of the bulk of soluble lenticular proteins, the crystallins. The 16 crystallins ubiquitous in mammals and their modifications have been extensively characterized by 2-DE, liquid chromatography, mass spectrometry and other protein analysis techniques. The various solubility dependant fractions as well as subproteomes of lenticular morphological sections have also been explored in detail. Extensive post translational modification of the crystallins is encountered throughout the lens as a result of ageing and disease resulting in a vast number of protein species. Proteomics methodology is therefore ideal to further comprehensive understanding of this organ and the factors involved in cataractogenesis.

γN-crystallin and the evolution of the βγ-crystallin superfamily in vertebrates

The beta and gamma crystallins are evolutionarily related families of proteins that make up a large part of the refractive structure of the vertebrate eye lens. Each family has a distinctive gene structure that reflects a history of successive gene duplications. A survey of gamma-crystallins expressed in mammal, reptile, bird and fish species (particularly in the zebrafish, Danio rerio) has led to the discovery of gammaN-crystallin, an evolutionary bridge between the beta and gamma families. In all species examined, gammaN-crystallins have a hybrid gene structure, half beta and half gamma, and thus appear to be the 'missing link' between the beta and gamma crystallin lineages. Overall, there are four major classes of gamma-crystallin: the terrestrial group (including mammalian gammaA-F); the aquatic group (the fish gammaM-crystallins); the gammaS group; and the novel gammaN group. Like the evolutionarily ancient beta-crystallins (but unlike the terrestrial gammaA-F and aquatic gammaM groups), both the gammaS and gammaN crystallins form distinct clades with members in fish, reptiles, birds and mammals. In rodents, gammaN is expressed in nuclear fibers of the lens and, perhaps hinting at an ancestral role for the gamma-crystallins, also in the retina. Although well conserved throughout vertebrate evolution, gammaN in primates has apparently undergone major changes and possible loss of functional expression.

Crystallins, genes and cataract

Far from being a physical entity, assembled of inanimate structural proteins, the ocular lens epitomizes the biological ingenuity that sustains an essential and near-perfect physical system of immaculate optics. Crystallins (alpha, beta, and gamma) provide transparency by dint of their high concentration, but it is debatable whether proteins that provide transparency are any different, biologically or structurally, from those that are present in non-transparent structures or tissues. It is becoming increasingly clear that crystallins may have a plethora of metabolic and regulatory functions, both within the lens as well as outside of it. Alpha-crystallins are members of a small heat shock family of proteins and beta/gamma-crystallins belong to the family of epidermis-specific differentiation proteins. Crystallin gene expression has been studied from the perspective of the lens specificity of their promoters. Mutations in alpha-, beta-, and gamma-crystallins are linked with the phenotype of the loss of transparency. Understanding catalytic, non-structural properties of crystallins may be critical for understanding the malfunction in molecular cascades that lead to cataractogenesis and its eventual therapeutic amelioration.

The gamma S-crystallin gene is mutated in autosomal recessive cataract in mouse

We established a recessive cataract model from a spontaneous mutation in the KUNMING outbred mice. Lens opacity appears 11 days after birth. Slit lamp examination reveals that the opacity mainly localizes to the nuclear region of the lens. Histological analysis shows a severe degeneration of the epithelial cells underneath the anterior lens capsule, whereas those cells in the equatorial region display an excessive proliferation and migration. Within the cortical area underneath the posterior lens capsule, both vacuoles and morgagnian-like bodies are seen. Blue-stained spherical bodies are observed in the embryonic nucleus, forming a Y-like pattern. We mapped the disease locus and found a homozygous G to A nucleotide conversion at position 489 of Crygs in mutant mice, leading to a truncated gene product (Trp163Stop). This finding suggests that CRYGS is not only a lens structural protein, but is also likely to be involved in epithelial cell proliferation, apoptosis, and migration.

A temperature-sensitive mutation of Crygs in the murine Opj cataract

In Opj, an inherited cataract in mice, opacity is associated with a mutation in Crygs, the gene for gammaS-crystallin, the first mutation to be associated with this gene. A single base change causes replacement of Phe-9, a key hydrophobic residue in the core of the N-terminal domain, by serine. Despite this highly non-conservative change, mutant protein folds normally at low temperature. However, it exhibits a marked, concentration-dependent decrease in solubility, associated with loss of secondary structure, at close to physiological temperatures. This is reminiscent of processes thought to occur in human senile cataracts in which normal proteins become altered and aggregate. The Opj cataract is progressive and more severe in Opj/Opj than in Opj/+. Lens histology shows that whereas fiber cell morphology in Opj/+ mice is essentially normal, in Opj/Opj, cortical fiber cell morphology and the loss of maturing fiber cell nuclei are both severely disrupted from early stages. This may indicate a loss of function of gammaS-crystallin which would be consistent with ideas that members of the betagamma-crystallin superfamily may have roles associated with maintenance of cytoarchitecture.

Characterization of gammaS-crystallin isoforms from a catfish: evolutionary comparison of various gamma-, gammaS-, and beta-crystallins

gammaS-Crystallin from catfish eye lenses, formerly designated betas-crystallin in mammalian lenses, is structurally characterized in this study by cDNA cloning and sequencing. To facilitate sequence characterization of gammaS-crystallin with structural properties lying between beta- and gamma-crystallins, a cDNA mixture was constructed from the poly(A)+ mRNA isolated from catfish eye lenses, and amplification by polymerase chain reaction (PCR) was carried out to obtain nucleotide segments encoding multiple gammaS-crystallin isoforms. Sequencing several positive clones revealed that at least two distinct isoforms exist in the gammaS-crystallin class of this teleostean fish, similar to the authentic gamma-crystallin family characterized previously in species of the piscine class. Comparison of protein sequences encoded by two representative catfish gammaS1 and gammaS2 cDNAs with the published sequences of beta-, gamma-, and gammaS-crystallins from shark, carp, bullfrog, bovine, and human lenses indicates that there is about 20-50% sequence homology between catfish gammaS-crystallins and various members of the related beta/gamma-crystallin superfamily from different evolutionary classes, with a higher sequence similarity being found between catfish gammaS- and mammalian gamma-crystallins than between catfish gammaS- and bovine or carp gammaS-crystallins. Phylogenetic trees constructed on the basis of the nucleotide and protein sequence divergence among various beta-, gamma-, and gammaS-crystallins corroborate the closer relatedness of catfish gammaS- to authentic gamma-crystallin than to bovine and carp gammaS-crystallins. The results suggest that evolution of catfish gammaS-crystallins follows a different path from that of bovine and carp gammaS-crystallins and may represent a more ancient offshoot from the ancestral gamma/gammaS coding gene than carp and bovine gammaS-crystallins.

Characterization of gamma S-crystallin isoforms from lip shark (Chiloscyllium colax): evolutionary comparison between gamma S and beta/gamma crystallins

gamma S-Crystallin from shark eye lenses, formerly termed beta s crystallin in mammalian lenses, is structurally characterized in this study by cDNA cloning and sequencing. To facilitate sequence characterization of gamma S-crystallin possessing intermediate structural properties between beta- and gamma-crystallins, cDNA mixture was constructed from the poly(A)+ mRNA isolated from shark eye lenses, and amplification by polymerase chain reaction (PCR) was carried out to obtain nucleotide segments encoding multiple shark gamma S-crystallins. Sequencing several positive clones revealed that a multiplicity of isoforms exists in the gamma S-crystallin class of this cartilaginous fish, similar to authentic gamma-crystallin family characterized from the same shark species. Comparison of protein sequences encoded by two representative shark gamma S1 and gamma S2 cDNAs with those published sequences of beta-, gamma-, and gamma S crystallins from bovine, human, bullfrog and carp lenses indicated that there is about 35-64% sequence homology between shark gamma S crystallins and structurally related crystallins from different evolutionary classes, with a higher sequence similarity between shark gamma S and mammalian gamma-crystallins than that of shark gamma S and carp gamma S or bovine gamma S crystallins. A phylogenetic tree constructed on the basis of the sequence divergence among various beta-, gamma-, and gamma S crystallins corroborates the closer relatedness of shark gamma S to authentic gamma-crystallin than to mammalian and teleostean gamma S crystallins. It further strengthens the supposition that ancestral precursors of gamma S-crystallins were present in the shark lens long before the appearance of present-day teleostean and mammalian gamma S-crystallins.

A genetic linkage map of the bovine genome

A cattle genetic linkage map was constructed which marks about 90% of the expected length of the cattle genome. Over 200 DNA polymorphisms were genotyped in cattle families which comprise 295 individuals in full sibling pedigrees. One hundred and seventy-one loci were found linked to one other locus. Twenty nine of the 30 chromosome pairs are represented by at least one of the 36 linkage groups. Less than a 50 cM difference was found in the male and female genetic maps. The conserved loci on this map show as many differences in gene order compared to humans as is found between humans and mice. The conservation is consistent with the patterns of karyotypic evolution found in the rodents, primates and artiodactyls. This map will be important for localizing quantitative trait loci and provides a basis for further mapping.

A genetic map of DNA loci on bovine chromosome 1

We constructed a genetic map of most of the length of bovine chromosome 1 using the CSIRO and the Texas A&M University cattle reference families. Twelve loci are in a single linkage group, 9 of which are highly polymorphic loci. Four loci are of known biochemical function, alpha-1 crystallin (CRYA1), gamma-s crystallin (CRYG8), superoxide dismutase 1 (SOD1), and uridine monophosphate synthase (UMPS), and these have also been previously mapped in humans. The loci CRYA 1, CSRD 1613, GMBT 7, RM 95, SOD1, and UMPS had been previously assigned to bovine syntenic group U10, while CSRD 1613 and UMPS had also been assigned to chromosome 1 by in situ hybridization. All of the loci show statistically significant linkage to at least one other locus. The conserved loci indicate that there have been major rearrangements during the evolution of bovine chromosome 1 compared to other mammalian chromosomes. The estimate of the total length of the linkage group is 168 cM, which accords well with the predicted length based on chiasmata frequencies for the bovine genome and the relative size of chromosome 1 in the bovine genome.

Characterization of Xenopus laevis gamma-crystallin-encoding genes

In order to gain insight into crystallin (Cry)-encoding gene (cry) evolution and developmental function, we have determined the gene structure and sequence of several Xenopus laevis gamma-cry. These encode the most abundant Cry in the embryonic lens. Four of the X. laevis gamma-cry, which are part of a multigene family, were isolated from a X. laevis genomic library and demonstrated to have the same gene structure as gamma-cry from other vertebrates, thereby providing further evidence that the split between beta and gamma members of the beta gamma cry family occurred relatively early in evolution. Sequence comparisons indicate that these X. laevis genes share 88-90% nucleotide sequence identity in the protein coding regions, which is slightly higher than the identity observed between gamma-cry of other species. The 5' upstream regions of X. laevis gamma-cry contain a few short stretches of homology and one putative promoter element conserved among all cry genes but lack other regions common to gamma-cry promoters from other organisms. The deduced amino acid sequences of all four genes and one cDNA suggest that the structure of X. laevis gamma-Cry is highly conserved with that of other vertebrate gamma-Cry, as deduced from the known three-dimensional structure of bovine gamma B Cry.

A polymorphism in the bovine gamma-S-crystallin gene revealed by allele-specific amplification

A polymorphism was detected in the 3' untranslated region of the bovine gamma-S-crystallin gene by direct sequencing of polymerase chain reaction (PCR) products from genomic DNA of an N'Dama bull and a Boran cow. A set of three PCR primers was designed to detect this difference and thus give allele-specific amplification. The two allele-specific primers differ in length by 20 nucleotides so that the allelic products may be distinguished by simple agarose gel electrophoresis following a single PCR reaction. This provides a simple and rapid assay for this polymorphism.

Identification and typing of members of the protein-tyrosine phosphatase gene family expressed in mouse brain

Protein-tyrosine phosphatases (PTPases) form a novel and important class of cell regulatory proteins. We evaluated the expression of PTPases in mouse brain by polymerase chain amplification of cDNA segments that encode the catalytic domains of these enzymes. Degenerate primer pairs devised on the basis of conserved protein motifs were used to generate a series of distinct PCR-derived clones. In this way, murine homologues of the human PTPases LRP, PTP beta, PTP delta, PTP epsilon and LAR were obtained. Corresponding regions in their catalytic domains were used to reveal the evolutionary relationships between all currently known mammalian PTPase protein family members. Phylogenetic reconstruction displayed considerable differences in mutation rates for closely related PTPases.

Presence of hybridizing DNA sequences homologous to bovine acidic and basic beta-crystallins in all classes of vertebrates

The eye lens beta-crystallins in cow and chicken are encoded by a family of at least six genes. In order to assess the distribution of the corresponding genes among other vertebrates we hybridized beta-crystallin sequences (beta A2, beta A3/A1, beta A4, beta B1, beta B2, beta B3), isolated from a bovine lens cDNA library, to Southern blots on which EcoR1-digested chromosomal DNA was blotted from different vertebrate species. These included human, chimpanzee, calf, rat, pigeon, duck, monitor lizard, toad, trout, and lamprey. Positive hybridization signals were found in the representatives of virtually all classes of vertebrates. The basic beta B-crystallins gave hybridization signals in more species than the acidic beta A ones. In monitor lizard and toad the weakest hybridization signals for basic crystallin probes were found. For acidic crystallin probes the distribution pattern was more simple; among cold-blooded vertebrates a signal for beta A2 was found in trout and lamprey, for beta A4 in trout, and for beta A3/A1 only in toad. The results demonstrate that the duplications leading to the beta-crystallin gene family occurred before or during the earliest stages of vertebrate evolution.

Isolation and characterization of cDNAs encoding beta A2- and beta A4-crystallins: heterologous interactions in the predicted beta A4-beta B2 heterodimer

Except for the two acidic chains, beta A2 and beta A4, the primary structures of all bovine beta-crystallins have previously been elucidated, either by direct protein sequencing or prediction from cDNA sequencing. Both beta A2 and beta A4 were found to be synthesized in half-year-old calf lenses and are therefore likely to be present in a cDNA bovine library constructed from mRNA isolated from lenses of that age. A large number of cDNA clones was screened with all available crystallin, actin, vimentin and lens membrane protein MP26 probes and finally with a randomly primed mRNA probe. Clones positive for the latter, but negative for known lens proteins, were isolated and sequenced. beta A2, comprising 197 aa, and beta A4, comprising 209 aa, were identified. Both proteins have a conserved two-domain structure and an N-terminal extension which is variable. A three-dimensional model of the structure of beta A4 was made based on the coordinates of one subunit from the beta B2 dimer which has recently been solved using x-ray diffraction techniques. The resulting heterodimer structure, together with the compiled bovine beta-crystallin sequences, was used to indicate those regions of the sequences which distinguish acidic from basic beta-crystallins with a view to defining structural features necessary for subunit recognition in beta-crystallin aggregates. With the aid of the present data, the complete evolutionary tree of the bovine beta-crystallin family has been constructed, which confirms the early separation of the genes encoding the three acidic and the three basic beta-crystallins.

Assignment of the human gene for histidine-rich glycoprotein to chromosome 3

Histidine-rich glycoprotein (HRG) is a monomeric plasma glycoprotein involved in the modulation of coagulation and fibrinolysis. Using Southern analysis of human-rodent somatic cell hybrid DNA with a human HRG-specific cDNA probe, the HRG gene was assigned to chromosome 3. One hybrid that was known to contain only a segment of chromosome 3 also reacted positively with the HRG probe. Hybridization analysis with a set of chromosome 3-specific probes showed that the segment of chromosome 3 present in this hybrid is missing the region pter-p14, which indicates that HRG is not located in this region. No restriction fragment length polymorphisms were detected for HRG with 10 commonly used restriction enzymes.

Isolation and characterization of cDNA clones encoding aldose reductase

The action of aldose reductase has been implicated in the etiology of a variety of diabetic complications affecting the visual system. However, very little is known regarding the structure and functional organization of the genes encoding this key enzyme. In the present study, we have isolated and characterized complementary DNA clones encoding bovine lens aldose reductase. Nucleotide sequencing of four independently isolated clones was used to establish a 1154 nucleotide composite cDNA sequence. The cDNA sequence encodes 296 amino acids of the aldose reductase primary structure, and contains an additional 261 nucleotides of apparently untranslated sequence downstream from the coding region. No nucleotide sequence differences were found among the four independently isolated aldose reductase cDNA clones. The aldose reductase amino acid sequence deduced from the cDNA shows high homology to that reported for aldose reductase of the rat lens. Significant similarities are also evident between bovine lens aldose reductase and both human liver aldehyde reductase and frog lens rho-crystallin.