Evidence for neutral and selective processes in the recruitment of enzyme-crystallins in avian lenses

γ-Crystallins of the chicken lens: remnants of an ancient vertebrate gene family in birds

γ-Crystallins, abundant proteins of vertebrate lenses, were thought to be absent from birds. However, bird genomes contain well-conserved genes for γS- and γN-crystallins. Although expressed sequence tag analysis of chicken eye found no transcripts for these genes, RT-PCR detected spliced transcripts for both genes in chicken lens, with lower levels in cornea and retina/retinal pigment epithelium. The level of mRNA for γS in chicken lens was relatively very low even though the chicken crygs gene promoter had lens-preferred activity similar to that of mouse. Chicken γS was detected by a peptide antibody in lens, but not in other ocular tissues. Low levels of γS and γN proteins were detected in chicken lens by shotgun mass spectroscopy. Water-soluble and water-insoluble lens fractions were analyzed and 1934 proteins (< 1% false discovery rate) were detected, increasing the known chicken lens proteome 30-fold. Although chicken γS is well conserved in protein sequence, it has one notable difference in leucine 16, replacing a surface glutamine conserved in other γ-crystallins, possibly affecting solubility. However, L16 and engineered Q16 versions were both highly soluble and had indistinguishable circular dichroism, tryptophan fluorescence and heat stability (melting temperature Tm ~ 65 °C) profiles. L16 has been present in birds for over 100 million years and may have been adopted for a specific protein interaction in the bird lens. However, evolution has clearly reduced or eliminated expression of ancestral γ-crystallins in bird lenses. The conservation of genes for γS- and γN-crystallins suggests they may have been preserved for reasons unrelated to the bulk properties of the lens.

Lys-315 at the Interfaces of Diagonal Subunits of δ-Crystallin Plays a Critical Role in the Reversibility of Folding and Subunit Assembly

δ-Crystallin is the major structural protein in avian eye lenses and is homologous to the urea cycle enzyme argininosuccinate lyase. This protein is structurally assembled as double dimers. Lys-315 is the only residue which is arranged symmetrically at the diagonal subunit interfaces to interact with each other. This study found that wild-type protein had both dimers and monomers present in 2–4 M urea whilst only monomers of the K315A mutant were observed under the same conditions, as judged by sedimentation velocity analysis. The assembly of monomeric K315A mutant was reversible in contrast to wild-type protein. Molecular dynamics simulations showed that the dissociation of primary dimers is prior to the diagonal dimers in wild-type protein. These results suggest the critical role of Lys-315 in stabilization of the diagonal dimer structure. Guanidinium hydrochloride (GdmCl) denatured wild-type or K315A mutant protein did not fold into functional protein. However, the urea dissociated monomers of K315A mutant protein in GdmCl were reversible folding through a multiple steps mechanism as measured by tryptophan and ANS fluorescence. Two partly unfolded intermediates were detected in the pathway. Refolding of the intermediates resulted in a conformation with greater amounts of hydrophobic regions exposed which was prone to the formation of protein aggregates. The formation of aggregates was not prevented by the addition of α-crystallin. These results highlight that the conformational status of the monomers is critical for determining whether reversible oligomerization or aggregate formation occurs.

Expression pattern and biochemical properties of zebrafish N-acetylglutamate synthase

The urea cycle converts ammonia, a waste product of protein catabolism, into urea. Because fish dispose ammonia directly into water, the role of the urea cycle in fish remains unknown. Six enzymes, N-acetylglutamate synthase (NAGS), carbamylphosphate synthetase III, ornithine transcarbamylase, argininosuccinate synthase, argininosuccinate lyase and arginase 1, and two membrane transporters, ornithine transporter and aralar, comprise the urea cycle. The genes for all six enzymes and both transporters are present in the zebrafish genome. NAGS (EC 2.3.1.1) catalyzes the formation of N-acetylglutamate from glutamate and acetyl coenzyme A and in zebrafish is partially inhibited by L-arginine. NAGS and other urea cycle genes are highly expressed during the first four days of zebrafish development. Sequence alignment of NAGS proteins from six fish species revealed three regions of sequence conservation: the mitochondrial targeting signal (MTS) at the N-terminus, followed by the variable and conserved segments. Removal of the MTS yields mature zebrafish NAGS (zfNAGS-M) while removal of the variable segment from zfNAGS-M results in conserved NAGS (zfNAGS-C). Both zfNAGS-M and zfNAGS-C are tetramers in the absence of L-arginine; addition of L-arginine decreased partition coefficients of both proteins. The zfNAGS-C unfolds over a broader temperature range and has higher specific activity than zfNAGS-M. In the presence of L-arginine the apparent Vmax of zfNAGS-M and zfNAGS-C decreased, their Km(app) for acetyl coenzyme A increased while the Km(app) for glutamate remained unchanged. The expression pattern of NAGS and other urea cycle genes in developing zebrafish suggests that they may have a role in citrulline and/or arginine biosynthesis during the first day of development and in ammonia detoxification thereafter. Biophysical and biochemical properties of zebrafish NAGS suggest that the variable segment may stabilize a tetrameric state of zfNAGS-M and that under physiological conditions zebrafish NAGS catalyzes formation of N-acetylglutamate at the maximal rate.

Variation in gene duplicates with low synonymous divergence in Saccharomyces cerevisiae relative to Caenorhabditis elegans

Differences between yeast and worm duplicates result from differences in mechanisms of duplication and effective population size.

Background

The direct examination of large, unbiased samples of young gene duplicates in their early stages of evolution is crucial to understanding the origin, divergence and preservation of new genes. Furthermore, comparative analysis of multiple genomes is necessary to determine whether patterns of gene duplication can be generalized across diverse lineages or are species-specific. Here we present results from an analysis comprising 68 duplication events in the Saccharomyces cerevisiae genome. We partition the yeast duplicates into ohnologs (generated by a whole-genome duplication) and non-ohnologs (from small-scale duplication events) to determine whether their disparate origins commit them to divergent evolutionary trajectories and genomic attributes.

Results

We conclude that, for the most part, ohnologs tend to appear remarkably similar to non-ohnologs in their structural attributes (specifically the relative composition frequencies of complete, partial and chimeric duplicates), the discernible length of the duplicated region (duplication span) as well as genomic location. Furthermore, we find notable differences in the features of S. cerevisiae gene duplicates relative to those of another eukaryote, Caenorhabditis elegans, with respect to chromosomal location, extent of duplication and the relative frequencies of complete, partial and chimeric duplications.

Conclusions

We conclude that the variation between yeast and worm duplicates can be attributed to differing mechanisms of duplication in conjunction with the varying efficacy of natural selection in these two genomes as dictated by their disparate effective population sizes.

The eyes have it: A Problem-Based Learning Exercise in Molecular Evolution

Molecular evolution provides an interesting context in which to use problem-based learning because it integrates a variety of topics in biology, biochemistry, and molecular biology. This three-stage problem for advanced students deals with the structure, multiple functions, and properties of lactate dehydrogenase isozymes, and the related evolutionary trade offs of gene sharing versus gene duplication among their corresponding genes. It has directive elements that require students to find and read classic articles, review thermodynamic principles, and apply their understanding to a mythical world wherein dinosaurs continued to evolve. The science fiction writing assignment that brings closure to the problem transformed the problem with respect to student interest and engagement.

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.

Generation of evolutionary novelty by functional shift

That biological features may change their function during evolution has long been recognized. Particularly, the acquisition of new functions by molecules involved in developmental pathways is suspected to cause important morphologic novelties. However, the current terminology describing functional changes during evolution (co-option or recruitment) fails to recognize important biologic distinctions between diverse evolutionary routes involving functional shifts. The main goal of our work is to stress the importance of an apparently trivial distinction: Whether or not the element that adopts a new function (anything from a morphologic structure to a protein domain) is a single or a duplicated element. We propose that natural selection must act in a radically different way, depending on the historic succession of co-option and duplication events; that is, co-option may provide the selective pressure for a subsequent gene duplication or could be a stabilizing factor that helps maintain redundancy after gene duplication. We review the evidence available on functional changes, focusing whenever possible on developmental molecules, and we propose a conceptual framework for the study of functional shifts during evolution with a level of resolution appropriate to the power of our current methodologies.

Domain exchange experiments in duck delta-crystallins: functional and evolutionary implications

Delta-crystallin, the major soluble protein component of the avian and reptilian eye lens, is homologous to the urea cycle enzyme argininosuccinate lyase (ASL). In duck lenses there are two delta crystallins, denoted delta1 and delta2. Duck delta2 is both a major structural protein of the lens and also the duck orthologue of ASL, an example of gene recruitment. Although 94% identical to delta2/ASL in the amino acid sequence, delta1 is enzymatically inactive. A series of hybrid proteins have been constructed to assess the role of each structural domain in the enzymatic mechanism. Five chimeras--221, 122, 121, 211, and 112, where the three numbers correspond to the three structural domains and the value of 1 or 2 represents the protein of origin, delta1 or delta2, respectively--were constructed and thermodynamically and kinetically analyzed. The kinetic analysis indicates that only domain 1 is crucial for restoring ASL activity to delta1 crystallin, and that amino acid substitutions in domain 2 may play a role in substrate binding. These results confirm the hypothesis that only one domain, domain 1, is responsible for the loss of catalytic activity in delta1. The thermodynamic characterization of human ASL (hASL) and duck delta1 and delta2 indicate that delta crystallins are slightly less stable than hASL, with the delta1 being the least stable. The deltaGs of unfolding are 57.25, 63.13, and 70.71 kcal mol(-1) for delta1, delta2, and hASL, respectively. This result was unexpected, and we speculate that delta crystallins have adapted to their structural role by adopting a slightly less stable conformation that might allow for enhanced protein-protein and protein-solvent interactions.

pH-induced reversible dissociation of tetrameric duck lens delta-crystallin

Animal lenses constitute many soluble proteins, which play a prominent role in eyes' light transparency. delta2-Crystallin, one of the major taxon-specific crystallins in duck lens, is a tetrameric protein consisting of four identical subunits, which contain endogenous argininosuccinate lyase activity. Under a neutral pH environment in this work, the protein was cross-linked with glutaraldehyde as tetrameric and dimeric forms with tetramer as the major form. Under acidic conditions, the protein was time-dependently dissociated into monomers with amino acid residues of pKa values 6.29+/-0.45 and 7.17+/-0.49 being involved in the monomer-monomer interactions and 6.20+/-0.10 and 8.88+/-0.07 in the dimer-dimer interactions. Duck lens delta2-crystallin thus possesses a double dimer structure (alpha2)2 with stronger monomer-monomer interactions than the dimer-dimer interactions. The acidic protein solution's reneutralization caused rapid reassociation of monomers into dimers and tetramers. The tetramer-dimer-monomer dissociation-reassociation thus is a pH-dependent freely interconvertible process.

Evolution of antifreeze glycoprotein gene from a trypsinogen gene in Antarctic notothenioid fish

Freezing avoidance conferred by different types of antifreeze proteins in various polar and subpolar fishes represents a remarkable example of cold adaptation, but how these unique proteins arose is unknown. We have found that the antifreeze glycoproteins (AFGPs) of the predominant Antarctic fish taxon, the notothenioids, evolved from a pancreatic trypsinogen. We have determined the likely evolutionary process by which this occurred through characterization and analyses of notothenioid AFGP and trypsinogen genes. The primordial AFGP gene apparently arose through recruitment of the 5' and 3' ends of an ancestral trypsinogen gene, which provided the secretory signal and the 3' untranslated region, respectively, plus de novo amplification of a 9-nt Thr-Ala-Ala coding element from the trypsinogen progenitor to create a new protein coding region for the repetitive tripeptide backbone of the antifreeze protein. The small sequence divergence (4-7%) between notothenioid AFGP and trypsinogen genes indicates that the transformation of the proteinase gene into the novel ice-binding protein gene occurred quite recently, about 5-14 million years ago (mya), which is highly consistent with the estimated times of the freezing of the Antarctic Ocean at 10-14 mya, and of the main phyletic divergence of the AFGP-bearing notothenioid families at 7-15 mya. The notothenioid trypsinogen to AFGP conversion is the first clear example of how an old protein gene spawned a new gene for an entirely new protein with a new function. It also represents a rare instance in which protein evolution, organismal adaptation, and environmental conditions can be linked directly.

A retinaldehyde dehydrogenase as a structural protein in a mammalian eye lens. Gene recruitment of eta-crystallin

eta-Crystallin is a taxon-specific crystallin, a major component of the eye lens in elephant shrews (Macroscelidea). Sequence analysis of eta-crystallin from two genera of elephant shrews and expression of recombinant eta-crystallin show that the protein is a cytoplasmic (class 1) aldehyde dehydrogenase (ALDH1, EC 1.2.1.3) with activity for the oxidation of retinaldehyde to retinoic acid. Unlike many other mammals, elephant shrews have two ALDH1 genes. One encodes ALDH1/eta-crystallin which, in addition to its very high expression in lens, is also the predominant form of ALDH1 expressed in other parts of the eye. The second gene encodes a "non-lens" ALDH1 (ALDH1-nl) which is the predominant form expressed in liver. This pattern of tissue preference contrasts with other mammals which make use of the same major ALDH1 transcript in both ocular and non-ocular tissues. Thus the gene recruitment of ALDH1/eta-crystallin as a structural protein in elephant shrew lenses is associated with its collateral recruitment as the major form of ALDH1 expressed in other parts of the eye.

The molecular biology of multidomain proteins. Selected examples

The aim of this review is to give an overview of the contribution molecular biology can make to an understanding of the functions and interactions within multidomain proteins. The contemporary advantages ascribed to multidomain proteins include (a) the potential for metabolite channelling and the protection of unstable intermediates; (b) the potential for interactions between domains catalysing sequential steps in a metabolic pathway, thereby giving the potential for allosteric interactions; and (c) the facility to produce enzymic activities in a fixed stoichiometric ratio. The alleged advantages in (a) and (b) however apply equally well to multi-enzyme complexes; therefore, specific examples of these phenomena are examined in multidomain proteins to determine whether the proposed advantages are apparent. Some transcription-regulating proteins active in the control of metabolic pathways are composed of multiple domains and their control is exerted and modulated at the molecular level by protein-DNA, protein-protein and protein-metabolite interactions. These complex recognition events place strong constraints upon the proteins involved, requiring the recognition of and interaction with different classes of cellular metabolites and macromolecules. Specific examples of transcription-regulating proteins are examined to probe how their multidomain nature facilitates a general solution to the problem of multiple recognition events. A general unifying theme that emerges from these case studies is that a basic unitary design of modules provided by enzymes is exploited to produce multidomain proteins by a complex series of gene duplication and fusion events. Successful modules provided by enzymes are co-opted to new function by selection apparently acting upon duplicated copies of the genes encoding the enzymes. In multidomain transcription-regulating proteins, former enzyme modules can be recruited as molecular sensors that facilitate presumed allosteric interactions necessary for the molecular control of transcription.

The structure of avian eye lens delta-crystallin reveals a new fold for a superfamily of oligomeric enzymes

The crystal structure of turkey delta-crystallin, a principal soluble components of the avian lens, has been determined to a resolution of 2.5 A. It is a tetramer, of 200,000 M(r), with 222 symmetry. The subunit has a new fold composed of three mainly alpha-helical domains. One domain is a bundle of five long helices which forms a 20-helix bundle at the core of the tetramer. delta-crystallin shares approximately 90% sequence identity with the enzyme argininosuccinate lyase (EC 4.3.2.1), indicating that it is an example of a 'hijacked' enzyme. It is also distantly related to the class II fumarases, aspartases, adenylosuccinases and 3-carboxy-cis,cis-muconate lactonising enzyme. The structure reveals a putative active-site cleft which is located on the boundary between three subunits of the tetramer. This is the first three-dimensional structure of a representative of this superfamily of enzymes.

Multiple structural types of gap junctions in mouse lens

Gap junctions in the epithelium and superficial fiber cells from young mice were examined in lenses prepared by rapid-freezing, and processed for freeze-substitution and freeze-fracture electron microscopy. There appeared to be three structural types of gap junction: one type between epithelial cells and two types between fiber cells. Epithelial gap junctions seen by freeze-substitution were approximately 20 nm thick and consistently associated with layers of dense material lying along both cytoplasmic surfaces. Fiber gap junctions, in contrast, were 15–16 nm (type 1) or 17–18 nm thick (type 2), and had little associated cytoplasmic material. Type 1 fiber gap junctions were extensive in flat expanses of cell membrane and had a thin, discontinuous central lamina, whereas type 2 fiber gap junctions were associated with the ball-and-socket domains and exhibited a dense, continuous central lamina. Both types of fiber gap junction had a diffuse arrangement of junctional intramembrane particles, whereas particles and pits of epithelial gap junctions were in a tight, hexagonal configuration. The type 2 fiber gap junctions, however, had a larger particle size (approximately 9 nm) than the type 1 (approximately 7.5 nm). In addition, a large number of junctional particles typified the E-faces of both fiber types but not the epithelial type of gap junction. Gap junctions between fiber and epithelial cells had structural features of type 1 fiber gap junctions.(ABSTRACT TRUNCATED AT 250 WORDS)

Inactivation of the endogenous argininosuccinate lyase activity of duck delta-crystallin by modification of an essential histidine residue with diethyl pyrocarbonate

The argininosuccinate lyase activity of duck delta-crystallin was inactivated by diethyl pyrocarbonate at 0 degrees C and pH 7.5. The inactivation followed pseudo-first-order kinetics after appropriate correction for the decomposition of the reagent during the modification period. The plot of the observed pseudo-first-order rate constant versus diethyl pyrocarbonate concentration in the range of 0.17-1.7 mM was linear and went through the origin with a second-order rate constant of 1.45 +/- 0.1 M-1.s-1. The double-logarithmic plot was also linear, with slope of 1.13, which suggested a 1:1 stoichiometry for the reaction between diethyl pyrocarbonate and delta-crystallin. L-Arginine, L-norvaline or L-citrulline protected the argininosuccinate lyase activity of delta-crystallin from diethyl pyrocarbonate inactivation. The dissociation constants for the delta-crystallin-L-arginine and delta-crystallin-L-citrulline binary complexes, determined by the protection experiments, were 4.2 +/- 0.2 and 0.12 +/- 0.04 mM respectively. Fumarate alone had no protective effect. However, fumarate plus L-arginine gave synergistic protection with a ligand binding interacting factor of 0.12 +/- 0.02. The double-protection data conformed to a random Uni Bi kinetic mechanism. Fluorescence-quenching studies indicated that the modified delta-crystallin had minimum, if any, conformational changes as compared with the native delta-crystallin. Inactivation of the enzyme activity was accompanied by an increasing absorbance at 240 nm of the protein. The absorption near 280 nm did not change. Treatment of the modified protein with hydroxylamine regenerated the enzyme activity to the original level. These results strongly indicated the modification of an essential histidine residue. Calculation from the 240 nm absorption changes indicated that only one histidine residue per subunit was modified by the reagent. This super-active histidine residue has a pKa value of approximately 6.8 and acts as a general acid-base catalyst in the enzyme reaction mechanism. Our experimental data are compatible with an E1cB mechanism [Raushel (1984) Arch. Biochem. Biophys. 232, 520-525] for the argininosuccinate lyase with the essential histidine residue close to the arginine-binding domain of delta-crystallin. L-Citrulline, after binding to this domain, might form an extra hydrogen bond with the essential histidine residue.

Puzzle of crystallin diversity in eye lenses

Crystallins have evolved by various mechanisms that are associated with high expression of their genes in the eye lens. The diversity and pattern of crystallins among different species indicate that independent events have occurred at the molecular level during the evolution of the lens in different invertebrates (jellyfish, squid, and octopus) and vertebrates. Although it is possible that different crystallins are needed to fulfill the specific needs of individual species, the unexpectedly large array of proteins that function as crystallins and their abundance in the lens raise the possibility that selective pressures optimizing the function of certain transcription factors in the lens contribute to the recruitment of crystallins.

Comparison of stability properties of lactate dehydrogenase B4/epsilon-crystallin from different species

epsilon-Crystallin occurs as an abundant lens protein in many birds and in crocodiles and has been identified as heart-type lactate dehydrogenase (LDH-B4). Lens proteins have, due to their longevity and environmental conditions, extraordinary requirements for structural stability. To study lens-protein stability, we compared various parameters of LDH-B4/epsilon-crystallin from lens and/or heart of duck, which has abundant amounts of this enzyme in its lenses, and of chicken and pig, which have no epsilon-crystallin. Measuring the thermostability of LDH-B4 from the different sources, the t50 values (temperature at which 50% of the enzyme activity remains after a 20-min period) for LDH-B4 from duck heart, duck lens and chicken heart were all found to be around 76 degrees C, whereas pig heart LDH-B4 was less thermostable, having a t50 value of 62.5 degrees C. A similar tendency was found with urea inactivation studies. Plotting the first-order rate constants obtained from inactivation kinetic plots against urea concentration, it was clear that LDH-B4 from pig heart was less stable in urea than the homologous enzymes from duck heart, chicken heart and duck lens. The duck and chicken enzymes were also much more resistant against proteolysis than the porcine enzyme. Therefore, it is concluded that avian LDH-B4 is structurally more stable than the homologous enzyme in mammals. This greater stability might make it suitable to function as a crystallin, as in duck, but is not necessarily associated with high lens expression, as in chicken.

Carbonyl-metabolizing enzymes and their relatives recruited as structural proteins in the eye lens

The refractive properties of the eye lens are determined by abundant soluble structural proteins known as crystallins. While some crystallins are common to most vertebrates, others are abundant only in groups of related species. These taxon-specific crystallins all turn out to be enzymes, apparently recruited by modification of gene expression without prior gene duplication. They include eta-crystallin, accounting for up to 25% of protein in elephant shrew lenses and apparently identical to cytoplasmic aldehyde dehydrogenase; rho-crystallin from frog lenses, a member of the same superfamily as aldose and aldehyde reductases; and zeta-crystallin, found in guinea pig and camel lenses, which is structurally related to alcohol dehydrogenase (ADH). Unlike ADH, zeta-crystallin requires NADPH rather than NAD+/NADH as cofactor. Molecular modelling of zeta-crystallin shows that amino-acid changes around the co-factor binding site are responsible for this change in affinity. Purified guinea pig lens zeta-crystallin has a substrate preference for orthoquinones which are reduced by a single electron transfer mechanism. cDNA sequencing of zeta-crystallin suggests that the expression in lens as a crystallin depends on a different gene promoter from that used predominantly in liver. The putative guinea pig zeta-crystallin lens promoter has now been assayed for function in transfection studies. Elements with positive and negative effects on transcription, at least one of which has tissue preferred function, have been defined. When introduced into transgenic mice this promoter exhibits tissue-specific expression in the lens. This is the first identification of a lens-specific, alternative promoter in an enzyme crystallin gene.

Taxon-specific recruitment of enzymes as major soluble proteins in the corneal epithelium of three mammals, chicken, and squid

Studies of others have shown that class 3 aldehyde dehydrogenase is a major component of the epithelial cells of the mammalian cornea. Here we demonstrate by peptide sequencing that other major proteins of the corneal epithelium are also identical or related to enzymes in the human, mouse, kangaroo, chicken, and squid. Aldehyde dehydrogenase class 3 was found to be the major protein of human, mouse, and kangaroo corneal epithelial cells. Peptidyl prolyl cis-trans isomerase (cyclophilin) or a homologue thereof is strikingly abundant in the corneal epithelial cells of chicken, but not mammals, and appears to be absent from the cornea of squid. By contrast, enolase or its homologue is relatively abundant in both the mammalian and chicken corneal epithelial cells. In some instances, abundant enzymes are common to cornea and lens in the same species--for example, arginino-succinate lyase/delta 1-crystallin in the chicken and glutathione S-transferase-like protein in the squid; in other cases, the abundant proteins in the cornea have not been found as lens crystallins in any species--for example, aldehyde dehydrogenase class 3 and cyclophilin. These data suggest that enzymes and certain enzyme-crystallins have been recruited as major corneal proteins in a taxon-specific manner and may serve structural rather than, or as well as, enzymatic roles in corneal epithelial cells.

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.

Anomalous behavior of beta B1-crystallin subunits from avian lenses

In soluble extracts of bird lenses, crystallin subunits of apparent sizes between 30kDa and 40kDa show considerable variability in both abundance and mobility in sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS PAGE). This is only partly due to the taxon-specific distribution of lactate dehydrogenase-B (LDH-B)/epsilon-crystallin. Here we show that, in spite of high conservation in primary sequence, beta B1-crystallin subunits of avian lenses have a markedly slower migration in SDS PAGE than those of mammals and that the apparent subunit size of beta B1-crystallin varies among birds. This may be at least partly due to unusual post-translational modifications, since beta B1-crystallins of adult birds react strongly in a glycan detection procedure. No other crystallins in birds or mammals share this strong reaction. Modification of beta B1-crystallin could have interesting consequences for the formation of high molecular weight beta-crystallin aggregates in bird lenses.

Lens protein expression in mammals: taxon-specificity and the recruitment of crystallins

Vertebrate lenses show remarkably taxon-specific patterns of protein composition, most obviously in the recruitment of enzymes as major crystallins. Phylogenetic relationships are particularly apparent in mammals. Here we describe eta-crystallin, which is probably identical to cytosolic aldehyde dehydrogenase, lens-specifically expressed at high abundance in the elephant shrews, primitive eutherians of the family Macroscelidae, and mu-crystallin, a novel lens protein expressed in some marsupials. We have also observed that enzymes that have been recruited as crystallins in some species are also moderately abundant in the lenses of other species. This hints that the origins of enzyme-crystallins may lie in a pool of enzymes widely expressed in lenses at fairly high levels, perhaps because they have important developmental or functional roles in the tissue.