Gamma-crystallin family of the mouse lens: structural and evolutionary relationships

α-Crystallin chaperone mimetic drugs inhibit lens γ-crystallin aggregation: potential role for cataract prevention

Γ-Crystallins play a major role in age-related lens transparency. Their destabilization by mutations and physical chemical insults are associated with cataract formation. Therefore, drugs that increase their stability should have anticataract properties. To this end, we screened 2560 Federal Drug Agency–approved drugs and natural compounds for their ability to suppress or worsen H₂O₂ and/or heat-mediated aggregation of bovine γ-crystallins. The top two drugs, closantel (C), an antihelminthic drug, and gambogic acid (G), a xanthonoid, attenuated thermal-induced protein unfolding and aggregation as shown by turbidimetry fluorescence spectroscopy dynamic light scattering and electron microscopy of human or mouse recombinant crystallins. Furthermore, binding studies using fluorescence inhibition and hydrophobic pocket–binding molecule bis-8-anilino-1-naphthalene sulfonic acid revealed static binding of C and G to hydrophobic sites with medium-to-low affinity. Molecular docking to HγD and other γ-crystallins revealed two binding sites, one in the “NC pocket” (residues 50–150) of HγD and one spanning the “NC tail” (residues 56–61 to 168–174 in the C-terminal domain). Multiple binding sites overlap with those of the protective mini αA-crystallin chaperone MAC peptide. Mechanistic studies using bis-8-anilino-1-naphthalene sulfonic acid as a proxy drug showed that it bound to MAC sites, improved T_m of both H₂O₂ oxidized and native human gamma D, and suppressed turbidity of oxidized HγD, most likely by trapping exposed hydrophobic sites. The extent to which these drugs act as α-crystallin mimetics and reduce cataract progression remains to be demonstrated. This study provides initial insights into binding properties of C and G to γ-crystallins.

Crystallins and Their Complexes

The crystallins (α, β and γ), major constituent proteins of eye lens fiber cells play their critical role in maintaining the transparency and refractive index of the lens. Under different stress factors and with aging, β- and γ-crystallins start to unfold partially leading to their aggregation. Protein aggregation in lens basically enhances light scattering and causes the vision problem, commonly known as cataract. α-crystallin as a molecular chaperone forms complexes with its substrates (β- and γ-crystallins) to prevent such aggregation. In this chapter, the structural features of β- and γ-crystallins have been discussed. Detailed structural information linked with the high stability of γC-, γD- and γS-crystallins have been incorporated. The nature of homologous and heterologous interactions among crystallins has been deciphered, which are involved in their molecular association and complex 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.

Eye lens proteomics: from global approach to detailed information about phakinin and gamma E and F crystallin genes

Exploration of the lenticular proteome poses a challenging and worthwhile undertaking as cataracts, the products of a disease phenotype elicited by this proteome, remains the leading cause of vision impairment worldwide. The complete ten day old lens proteome of Mus musculus C57BL/6J was resolved into 900 distinct spots by large gel carrier ampholyte based 2-DE. The predicted amino acid sequences of all 16 crystallins ubiquitous in mammals were corroborated by mass spectrometry (MS). In detailed individual spot analyses, the primary structure of the full murine C57BL/6J beaded filament component phakinin CP49 was sequenced by liquid chromatography/electrospray ionization-tandem MS and amended at two positions. This definitive polypeptide sequence was aligned to the mouse genome, thus identifying the entire C57BL/6J genomic coding region. Also, two murine C57/6J polypeptides, both previously classified as gamma F crystallin, were clearly distinguished by MS and electrophoretic mobility. Both were assigned to their respective genes, one of the polypeptides was reclassified as C57BL/6J gamma E crystallin. Building on these data and previous investigations an updated crystallin reference map was put forth and several non crystallin lenticular components were examined. These results represent the first part of a comprehensive investigation of the mouse lens proteome (http://www.mpiib-berlin.mpg.de/2D-PAGE) with emphasis on understanding genetic effects on proteins and disease development.

Homology models of human gamma-crystallins: structural study of the extensive charge network in gamma-crystallins

The lens is composed of highly stable and long-lived proteins, the crystallins which are divided into alpha-, beta-, and gamma-crystallins. Human gamma-crystallins belong to the betagamma superfamily. A large number of gamma-crystallins have been sequenced and have been found to share remarkable sequence homology with each other. Some of the gamma-crystallins from various sources have also been elucidated structurally by X-ray crystallographic or NMR spectroscopic experiments. Their three-dimensional structures are also similar having consisted of two domains each possessing two Greek key motifs. In this study we have constructed the comparative or homology models of the four major human gamma-crystallins, gammaA-,gammaB-, gammaC-, and gammaD-crystallins and studied the charge network in these crystallins. Despite an overall structural similarity between these crystallins, differences in the ion pair formation do exist which is partly due to the differences in their primary sequence and partly due to the structural orientation of the neighboring amino acids. In this study, we present an elaborate analysis of these charged interactions and their formation or loss with respect to the structural changes.

Transforming growth factor-beta1 negatively regulates crystallin expression in teeth

Previously, we have reported that targeted overexpression of transforming growth factor (TGF) beta1 in the teeth of the transgenic mice (dTGF-beta1) results in a novel tooth phenotype phenomimicking the most prevalent tooth disorders in human. This phenotype was associated with discoloration and attrition of teeth due to defective mineralization. Here, we report a novel expression of crystallin family members in developing mouse teeth and its regulation by TGF-beta1 in these transgenic mice. AlphaB- and beta-crystallins were found to be elevated in dTGF-beta1 mouse teeth, whereas gamma-crystallin (gammaB, gammaC, and gammaF), a marker of cell differentiation, was significantly reduced. Because crystallins are believed to be stress-related proteins, their expression in teeth implicates them in a similar role because teeth are constantly subjected to physical friction and temperature fluctuations.

Chaperone-like activity of a synthetic peptide toward oxidized gamma-crystallin

alphaA-Crystallin can function like a molecular chaperone. We recently reported that the alphaA-crystallin sequence, KFVIFLDVKHFSPEDLTVK (peptide-1, residues 70-88) by itself possesses chaperone-like (anti-aggregating) activity during a thermal denaturation assay. Based on the above data we proposed that the peptide-1 sequence was the functional site in alphaA-crystallin. In this study we investigated the specificity of peptide-1 against gamma-crystallin aggregation in the presence of H2O2 and CuSO4. Peptide-1 was able to completely protect against the oxidation-induced aggregation of gamma-crystallin. Removal of N-terminal Lys or the replacement of Lys with Asp (DFVIFLDVKHFSPEDLTVK, peptide-2) did not alter the anti-aggregation property of peptide-1. However, deletion of KF residues from the N-terminus of peptide-1 resulted in a significant loss of its anti-aggregation property. Bio-gel P-30 size-exclusion chromatography of gamma-crystallin incubated with peptide-2 under oxidative conditions revealed that a major portion of the peptide elutes in the void volume region along with gamma-crystallin, suggesting the binding of the peptide to the protein. Peptide-1 and -2 were also able to prevent the UV-induced aggregation of gamma-crystallin. These data indicate that the same amino acid sequence in alphaA-crystallin is likely to be responsible for suppressing the heat-denatured, oxidatively modified and UV-induced aggregation of proteins.

Mouse models of congenital cataract

Mouse mutants affecting lens development are excellent models for corresponding human disorders. The mutant aphakia has been characterised by bilaterally aphakic eyes (Varnum and Stevens, J Hered 1968;59:147-50); the corresponding gene was mapped to chromosome 19 (Varnum and Stevens, Mouse News Lett 1975;53:35). Recent investigations in our laboratory refined the linkage of 0.6 cM proximal to the marker D19Mit10. Several candidate genes have been excluded (Chuk1, Fgf8, Lbp1, Npm3, Pax2, Pitx3). The Cat3 mutations are characterised by vacuolated lenses caused by alterations in the initial secondary lens fibre cell differentiation. Secondary malformations develop at the cornea and iris, but the retina remains unaffected. The mutation has been mapped to chromosome 10 close to the markers D10Mit41 and D10Mit95. Several candidate genes have been excluded (Dcn, Elk3, Ldc, Mell8, Tr2-11). The series of Cat2 mutations have been mapped close to the gamma-crystallin genes (Cryg; Löster et al., Genomics 1994;23:240-2). The Cat2nop mutation is characterised by a mutation in the third exon of Crygb leading to a truncated gamma B-crystallin and the termination of lens fibre cell differentiation. The Cat2 mutants are interesting models for human cataracts caused by mutations in the human CRYG genes at chromosome 2q32-35.

Cataract mutations and lens development

The lens plays an essential role for proper eye development. Mouse mutants affecting lens development are excellent models for corresponding human disorders. Moreover, using mutations in particular genes the process of eye and lens development can be dissected into distinct steps. Therefore, three mouse mutants will be described in detail and discussed affecting three essential stages: formation of the lens vesicle, initiation of secondary lens fiber cell formation, and terminal differentiation of the secondary fiber cells. The mutant aphakia (ak) has been characterized by bilaterally apakic eyes [Varnum and Stevens (1968) J. Hered. 59, 147-150], and the corresponding gene was mapped to chromosome 19 [Varnum and Stevens (1975) Mouse News Letters 53, 35]. Recent investigations in our laboratory refined the linkage 0.6 +/- 0.3 N cm proximal to the microsatellite marker D19Mit10. The linked gene Pax2, responsible for proper development of the posterior part of the eye and the optic nerve, was excluded as candidate gene by sequence analysis. Histological analysis of the homozygous ak mutants revealed a persisting lens stalk and subsequently the formation of lens rudiments. The lens defects led to irregular iris development and retinal folding. Congenital aphakia is known as a rare human anomaly. Besides a corneal dystrophy (CDTB), no corresponding disease is localized at the homologous region of human chromosome 10q23. The Cat3 mutations are characterized by vacuolated lenses caused by alterations in the beginning of secondary lens fiber cell differentiation at embryonic day 12.5. Secondary malformations develop at the cornea and the iris, but the retina remains unaffected. Two mutant alleles of the Cat3 locus have been mapped to mouse chromosome 10 very close to the microsatellite markers D10Mit41 and D10Mit95 (less than 0.3 cM). Since Cat3 is mapped to a position, which is homologous to human chromosome 12q21-24, the disorder cornea plana congenita can be considered as a candidate disease. The series of Cat2 mutations have been mapped close to the locus encoding the gamma-crystallin gene cluster Cryg [Löster et al. (1994) Genomics 23, 240-242]. The Cat2nop mutation is characterized by a deletion of 11 bp and an insertion of 4 bp in the 3rd exon of Crygh leading to a truncated gamma B-crystallin. The defect in the Crygh gene is causative for the stop of lens fiber cell differentiation from embryonic day 15.5 onward. Besides the lens, no further ocular tissue is affected. The Cat2 mouse mutants are interesting models for human cataracts caused by mutations in the gamma-crystallin genes at human chromosome 2q32-35. The ak, Cat3 and Cat2 mutants are discussed in the context of other mutants affecting early eye and lens development. Additionally, human congenital cataracts are discussed, which have been characterized similar to the mouse models. The overview of the three types of mutants demonstrates that genes, which affect the early eye development, e.g. at the lens vesicle stage, have consequences for the development of the whole eye. In contrast, if the mutation influences later steps of lens differentiation, the consequences are restricted to the lens only. These data indicate a decreasing effect of the lens for the regulation of eye development during embryogenesis.

Three murine cataract mutants (Cat2) are defective in different gamma-crystallin genes

A number of murine cataract mutations have been localized to chromosome 1 close to the gamma-crystallin gene cluster (Cryg) (Everett et al., 1994, Genomics 20: 429-434; Löster et al., 1994, Genomics 23: 240-242). Based on the size of the mapping or allelism tests they have not been shown to be genetically distinct and have been assigned to locus symbol Cat2. Here we assign three mutations to the respective gamma-crystallin gene. Using a systematic candidate gene approach to analyze the entire Cryg cluster, an A-->G transition was found in exon 2 of Cryga for the ENU-436 mutation and is designated Cryga1Neu. The mutant allele Crygbnop (formerly Cat2(nop)) is caused by a replacement of 11 bp by 4 bp in the third exon of Crygb, while a C-->G transversion in exon 3 of Cryge has been found for the Cryget (formerly Cat2(t)) mutation. For the mutation Cryga1Neu, an Asp-->Gly exchange is deduced, whereas the mutations Crygbnop and Cryget lead to the formation of in-frame stop codons and give rise to truncated proteins of 144 and 143 amino acids, respectively. The effects of the mutations upon gamma-crystallin structure are likely to be quite different. The Cryga1Neu mutation is expected to affect the link between Greek-key motifs 2 and 3, whereas both Crygbnop and Cryget mutations are supposed to truncate the fourth Greek-key motif. All three mutations are predicted to alter protein folding of the gamma-crystallins and result in lens cataract, but the phenotype for each is quite distinctive.

Identification, partial characterization, and genetic mapping of kinesin-like protein genes in mouse

Microtubule-dependent motors of the kinesin superfamily have undergone structural and functional diversification during evolution and play crucial roles in cell division and intracellular transport. Degenerate oligonucleotides homologous to highly conserved regions of sequence within the motor domain were used in a polymerase chain reaction to isolate five new members (KIF3C, KIFC2, KIFC3, KIFC4, and KIF22) of the kinesin superfamily from a mouse brain cDNA library. Northern analysis showed that KIF3C and KIFC2 are expressed mainly in neural tissues, that KIFC4 and KIF22 are expressed primarily in proliferative tissues and cell lines, and that KIFC3 is apparently ubiquitous. To elucidate the organization of genes encoding kinesin-like motors in the mouse genome and to explore the potential associations of these genes with classical mouse mutations or human genetic diseases, these new genes as well as genes encoding the previously reported KIF3A and KIF3B motors were mapped to mouse chromosomes by using an interspecific backcross panel of DNAs from The Jackson Laboratory. The data indicate that the gene KIFC4 is present in three copies in the mouse genome on chromosomes 13 (KIFC4A), 10 (KIFC4B), and 17 (KIFC4C). The gene KIF22 is present in two copies on chromosomes 7 (KIF22A) and 1 (KIF22B). The genes KIF3A, KIF3B, KIF3C, KIFC2, and KIFC3 are each single loci and map to chromosomes 11, 2, 12, 15, and 8, respectively.

Functional interactions between retinoic acid receptor-related orphan nuclear receptor (ROR alpha) and the retinoic acid receptors in the regulation of the gamma F-crystallin promoter

We have previously demonstrated that an everted repeat of the hexamer PuGGTCA located within the gamma F-crystallin promoter mediates activation of the murine gamma F-crystallin gene by retinoic acid and thyroid hormone receptors. Here, we show that the recently identified retinoic acid receptor-related orphan nuclear receptor (ROR alpha) is expressed in the murine lens and activates the gamma F-crystallin promoter. In contrast to the retinoic acid and thyroid hormone receptors, activation of the gamma F-crystallin promoter by ROR alpha requires binding to the single 3' half-site and spacer sequences of gamma F-crystallin hormone response element (gamma F-HRE). We further demonstrate that ROR alpha-dependent activation is repressed by the competitive binding of retinoic acid receptor/retinoid X receptor heterodimers to the gamma F-HRE in the absence of all-trans-retinoic acid. These studies suggest that the interplay of retinoid receptors and ROR alpha on the gamma F-HRE may constitute an important mechanism regulating gamma F-crystallin gene expression in the murine lens.

Developmental regulation and cell type-specific expression of the murine gamma F-crystallin gene is mediated through a lens-specific element containing the gamma F-1 binding site

The mouse gamma F-crystallin gene, one of six differentially regulated members of the gamma-crystallin gene family, is expressed exclusively in central nuclear fiber cells of the adult lens. The expression of this gene is controlled through regulatory elements contained in two upstream enhancers and the proximal promoter. Here we show that while the upstream enhancers and the proximal promoter could each direct gene expression in fiber cells formed at early stages of lens growth and development, cooperation between these elements is required to achieve expression in fiber cells formed at later stages. Evidence is provided that cooperative interaction between these elements modulates gene expression by increasing promoter strength. We also show that sequences within the proximal promoter region that bind lens cell nuclear factor gamma F-1 are sufficient to elicit gene expression in central nuclear fiber cells of the adult lens.

A linkage map of mouse chromosome 1 using an interspecific cross segregating for the gld autoimmunity mutation

An interspecific backcross was used to define a high resolution linkage map of mouse Chromosome (Chr) 1 and to analyze the segregation of the generalized lymphoproliferative disease (gld) mutation. Mice homozygous for gld have multiple features of autoimmune disease. Analysis of up to 428 progeny from the backcross [(C3H/HeJ-gld x Mus spretus)F1 x C3H/HeJ-gld] established a map that spans 77.6 cM and includes 56 markers distributed over 34 ordered genetic loci. The gld mutation was mapped to a less than 1 cM segment on distal mouse Chr 1 using 357 gld phenotype-positive backcross mice. A second backcross, between the laboratory strains C57BL/6J and SWR/J, was examined to compare recombination frequency between selected markers on mouse Chr 1. Significant differences in crossover frequency were demonstrated between the interspecific backcross and the inbred laboratory cross for the entire interval studied. Sex difference in meiotic crossover frequency was also significant in the laboratory mouse cross. Two linkage groups known to be conserved between segments of mouse Chr 1 and the long arm of human Chrs 1 and 2 where further defined and a new conserved linkage group was identified that includes markers of distal mouse Chr 1 and human Chr 1, bands q32 to q42.

Differential localization by in situ hybridization of specific crystallin transcripts during mouse lens development

The embryonic development of the mammalian lens is well known at the biochemical and histological level. However few data are available at the molecular level concerning gene expression during the continuous differentiation of the lens. In the present study, we have investigated by in situ hybridization the changes in the distribution of mouse crystallin mRNA as a marker of differentiated lens cells, during development of the lens primordium, when tissue interactions are known to be essential. The transcripts of alpha and beta crystallins are first detected at the early elongation stage of primary fibres; gamma-crystallin-transcripts do not appear until the late elongation phase. All areas of the lenses exhibited crystallin mRNA until the beginning of secondary fiber formation at 18 days of development. Hybridization for alpha and beta crystallin is confined at that time to the equatorial part of the lens. The gamma crystallin transcripts are no longer found in the equatorial region after 1 post-natal day, but remain in the lens core, decreasing gradually. A possible mechanism is discussed.

Interaction of a lens cell transcription factor with the proximal domain of the mouse gamma F-crystallin promoter

The elements regulating lens-specific expression of the mouse gamma F-crystallin gene were examined. Here we show that mouse gamma F-crystallin sequences -67 to +45 contain a low basal level of lens-specific promoter activity and that sequences -67 to -25, which are highly conserved among different gamma-crystallin genes, are able to function as a strong transcriptional activator when duplicated and placed upstream of the TATA box. We also show that nuclear factors from lens and nonlens cells are able to form different complexes with sequences centered at -46 to -36 and demonstrate that binding of the factor from lens cells correlates with lens-specific promoter activity of the mouse gamma F-crystallin gene.

Isolation and characterization of irradiation fusion hybrids from mouse chromosome 1 for mapping Rmc-1, a gene encoding a cellular receptor for MCF class murine retroviruses

An irradiation-reduced somatic cell hybrid mapping panel was constructed of BALB/c mouse Chromosome 1. Nineteen hybrids were selected from a pool of 292 clones to generate a fine structure physical map of the distal 40 cM of the chromosome. The hybrids contain mouse DNA fragments only from Chromosome 1, ranging from approximately 5 cM to approximately 20 cM. Utilizing a viral infectibility assay, a cellular receptor gene, Rmc-1, for the MCF class of murine retroviruses was found to be linked to Lamb2, in the region between the Lamb2 and Bxv-1 loci. In addition, analysis of the hybrid mapping panel resulted in the remapping of three loci, Atpb, Ly-5, and Pmv-24, as compared to the mouse linkage map. Two previously unmapped endogenous proviruses are also putatively assigned positions on the chromosome.

Interaction of a lens cell transcription factor with the proximal domain of the mouse gamma F-crystallin promoter

The elements regulating lens-specific expression of the mouse gamma F-crystallin gene were examined. Here we show that mouse gamma F-crystallin sequences -67 to +45 contain a low basal level of lens-specific promoter activity and that sequences -67 to -25, which are highly conserved among different gamma-crystallin genes, are able to function as a strong transcriptional activator when duplicated and placed upstream of the TATA box. We also show that nuclear factors from lens and nonlens cells are able to form different complexes with sequences centered at -46 to -36 and demonstrate that binding of the factor from lens cells correlates with lens-specific promoter activity of the mouse gamma F-crystallin gene.

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.

Multiple regulatory elements of the murine gamma 2-crystallin promoter

Crystallins are the major water-soluble proteins of the vertebrate eye lens. These lens-specific proteins are encoded by several multi-gene families whose expression is differentially regulated during development. Our previous studies showed that the mouse gamma 2-crystallin promoter is active on transfection into lens-explant cultures derived from 14-day-old chick embryos but not on transfection into a variety of non-lens cells. In this study, transient expression data show that a sequence of 226 nucleotides upstream from the transcription start site is sufficient for activity of this promoter in the chicken lens cells. This sequence can be further divided into two domains, A and B, both of which are required for promoter function. Domain A (nucleotide -68 to -18) contains the TATA box and sequence motifs that are conserved in all gamma-crystallin promoters. Domain B (-226 to -120) consists of three regions. One of these regions contains an element with dyad symmetry and a sequence similar to the octamer motif. The second region contains an enhancer core consensus sequence. Two "enhancer-like" activities have been detected, one in Domain B and a second in a more distal region (-392 to -278) that does not appear to be required for promoter activity in transfection assays.

The ontogeny of gamma-crystallin mRNAs in CatFraser mice

Mice which are either homozygous or heterozygous for the CatFraser mutation have ocular cataracts accompanied by selective reduction of the γ-crystallins, a homologous family of proteins present in the lens and encoded by a family of tightly linked genes. We measured the concentrations of four different mRNAs, each encoding a different γ-crystallin, in the lenses of homozygous CatFraser mice and in normal controls at various stages of development by preparing Northern blots from lens RNA, probing with RNAs complementary to each of the four messages and densitometry of the bands thus generated. The results show that, for each of these messages, the ontogenetic patterns observed in normal mice are retained in the mutant, but at much lower concentrations.

Optimal resolution of eye lens gamma-crystallins by cation-exchange high-performance liquid chromatography on SynChropak CM300

Cation-exchange high-performance liquid chromatography on SynChropak CM300 in Tris-acetate buffers of pH 5-7, using sodium acetate gradients, produces an excellent separation of the various gamma-crystallin gene products and their post-synthetically modified forms from eye lens. With a single analysis of total lens extract, the gamma-crystallins can be resolved, quantified and collected for amino acid analysis. Experimental conditions are presented for optimal resolution of individual human, rat, bovine and dogfish shark gamma-crystallins. Applications presented include determinations of different synthesis of gamma-crystallins and chemical modification (oxidation by hydrogen peroxide) in situ.

The gamma-crystallin gene families: sequence and evolutionary patterns

The gamma-crystallin proteins consist of two topologically equivalent domains, each built up out of two similar motifs. They are encoded by a gene family, which already contained five members before the divergence of rodents and primates. A further gene duplication took place in each lineage. To analyze the pattern of evolution within this gene family, the coding sequences of six human genes, six rat genes, and four mouse genes were compared. Between species, a uniform rate of evolution of all regions of the protein is seen. The ratio of synonymous to nonsynonymous substitution in the human/rat or human/mouse comparison is much lower than the ratio when rat and mouse are compared indicating that the gamma-crystallin proteins are better conserved in the rodent lineage. Within species, the regions encoding the two external motifs I and III of the protein show a greater extent of nonsynonymous substitution than the regions encoding the two internal protein motifs II and IV. The low extent of synonymous substitution between the second exons (encoding motifs I and II) of the rat gamma-crystallin genes suggests the frequent occurrence of gene conversion. In contrast, a high extent of synonymous substitution is found in exon 3 (encoding motifs III and IV) of the rat genes. The same phenomenon is seen within the human gene family. The frequencies of occurrence of the various dinucleotides deviate less from those predicted from the frequencies of occurrence of each individual nucleotide in the second exons than in the third exons. The sequences of the third exons are significantly depleted in CpG, ApA, and GpT and enriched in CpT and GpA.

The murine cataractogenic mutation, Cat Fraser, segregates independently of the gamma crystallin genes

The murine mutation, Cat Fraser (CatFr), causes dominantly inherited ocular cataracts. Lenses of adult mice bearing this mutation contain reduced amounts of all seven γ-crystallin proteins and their corresponding transcripts. Levels of other lens proteins and transcripts appear normal and no extra-ocular effects of the mutation have been observed. The selective effect of this mutation on the γ-crystallins is consistent with the possibility that the site at which it occurs is involved in the coordinated regulation of the family of genes which encodes them. We have shown that several restriction fragment length polymorphisms in the γ-crystallin genes segregate independently of the CatFr mutation. Therefore, despite its selective effect on the expression of the γ-crystallin genes, the mutation is not linked to them. This observation rules out the possibility that the mutation is in a cis-acting regulatory site.

Physicochemical characterization of gamma-crystallins from bovine lens--hydrodynamic and biochemical properties

A detailed hydrodynamic study has been made on the gamma-crystallin of the bovine lens. Sedimentation study indicates that gamma-crystallin shows a nearly gaussian peak throughout the course of sedimentation at high speed, using a synthetic boundary cell. The diffusion and sedimentation coefficients are 10.3 x 10(-7) cm2/sec and 2.51 S, respectively. The weight-average molecular weight of the unfractionated gamma-crystallin calculated from sedimentation equilibrium is 21,800. The four major subfractions of gamma-crystallin show similar hydrodynamic properties with an intrinsic viscosity of 2.50 ml/g and a Stokes radius of 21 A. The distinct electrophoretic mobilities exhibited by the four subfractions show gel-concentration dependence and similar slopes in the Ferguson plot, indicative of being charge isomers of the same molecular species. Amino acid analysis of these four subfractions corroborated the conclusions that these gamma-crystallin polypeptides are closely related and comprise a multigene family of crystallins. Based on the sedimentation and intrinsic viscosity data, gamma-crystallin can be modeled as a prolate ellipsoid with an axial ratio of approximately 3.0 and a hydration factor of 0.27 g water per gram protein. The circular dichroism data for gamma-crystallins showed a minimum at about 217 nm, characteristic of a beta-sheet conformation. These structural characteristics are in good accord with those derived from X-ray diffraction data for gamma-crystallin II.

Surface interactions of gamma-crystallins in the crystal medium in relation to their association in the eye lens

A comparative study of intermolecular interactions in crystals of two homologous low molecular weight proteins, gamma-II and gamma-IIIb crystallins, from calf eye lens was carried out. Crystal packings for these proteins are very different: intermolecular contact areas compose about 33% of the total accessible surface area of gamma-II as compared with 13% in gamma-III. Two key residues seem to be mainly responsible for the differences in protein association in the crystal medium. These are Ser 103 and Leu 155 in gamma-II, which are replaced by Met 103 and His 155 in gamma-IIb. A similar substitution of these residues is observed in different gene products of gamma-crystallins from a number of vertebrates. This is consistent with the existence of a genetically controlled mechanism for determining intermolecular association of gamma-crystallins in the native medium of the lens.

Gamma-crystallins of the human eye lens: expression analysis of five members of the gene family

While only two gamma-crystallins have been identified in the human eye lens, molecular studies indicate that the human gamma-crystallins are encoded in a multigene family comprising at least seven closely related members. Sequence analysis of five of these genes has suggested that three (gamma 1-2, G3, and G4) are potentially active, while two (G1 psi and G2 psi) correspond to closely related pseudogenes. Here we report on the detailed structure of a sixth gamma-crystallin gene, G5, and our results obtained with transient expression assays to characterize both the promoter activity and translation products of five members of the gene family. We show that 5'-flanking sequences of G1 psi and G2 psi lacked detectable promoter activity, while the corresponding sequences of G3, G4, and G5 were able to direct high levels of expression of the bacterial chloramphenicol acetyltransferase gene in primary lens epithelia, but not in cultures of nonlens origin. Detailed sequence comparisons indicated that active genes contained several conserved sequence tracts 5' of the TATA box which may constitute functional elements of a lens-specific gamma-crystallin promoter. Expression of the gamma-crystallin coding sequences from the human metallothionein IIA promoter in nonlens cells facilitated characterization of the polypeptides encoded by individual gamma-genes and, in future studies, should permit comparison of these proteins with distinct gamma-crystallins in the human lens.

The mouse eye lens obsolescence (Elo) mutant: studies on crystallin gene expression and linkage analysis between the mutant locus and the gamma-crystallin genes

Previous studies showed that the mouse Elo (eye lens obsolescence) mutation is located on chromosome 1, at a site near the Len-1 locus, which is defined by a set of polymorphic gamma-crystallin proteins. To investigate further the relationship between Elo and the gamma-crystallins, we have examined the steady-state levels of gamma-crystallin transcripts in normal and mutant eyes and analyzed the linkage relationship between the Elo locus and the gamma-crystallin genes. Our data showed that, while gamma-crystallin mRNA levels are preferentially reduced in the mutant eyes, the mutation does not seem to map within the gamma-crystallin gene cluster. The distance between Elo and the gamma 6 gene (the most proximal gamma-crystallin gene member) is estimated to be 1.4 +/- 0.9 cM, whereas that between gamma 6 and the distantly linked gamma 2 gene is 2.7 +/- 1.3 cM. Our data also suggest the possibility of recombination hot spots with the gamma-crystallin gene cluster.

Human lens gamma-crystallins: isolation, identification, and characterization of the expressed gene products

We have isolated the individual gamma-crystallins expressed in young human lenses and identified with which of the six known human gamma-crystallin genes they each correspond. We find that at least 90% of the gamma-crystallins synthesized in the young human lens are the products of genes gamma G3 and gamma G4. We demonstrate that gamma G4-crystallin undergoes a temperature-dependent phase separation, and we have measured the low-concentration branch of its coexistence curve (phase separation temperature vs. concentration) up to about 40 mg/ml. By comparison, we found no evidence of gamma G3-crystallin phase separating, even at lower temperatures and higher concentrations. This is consistent with predictions based on sequence homology between human and rat gamma-crystallins. The implications of these findings for human inherited and senile cataracts are considered.

Gamma-crystallins of the human eye lens: expression analysis of five members of the gene family

While only two gamma-crystallins have been identified in the human eye lens, molecular studies indicate that the human gamma-crystallins are encoded in a multigene family comprising at least seven closely related members. Sequence analysis of five of these genes has suggested that three (gamma 1-2, G3, and G4) are potentially active, while two (G1 psi and G2 psi) correspond to closely related pseudogenes. Here we report on the detailed structure of a sixth gamma-crystallin gene, G5, and our results obtained with transient expression assays to characterize both the promoter activity and translation products of five members of the gene family. We show that 5'-flanking sequences of G1 psi and G2 psi lacked detectable promoter activity, while the corresponding sequences of G3, G4, and G5 were able to direct high levels of expression of the bacterial chloramphenicol acetyltransferase gene in primary lens epithelia, but not in cultures of nonlens origin. Detailed sequence comparisons indicated that active genes contained several conserved sequence tracts 5' of the TATA box which may constitute functional elements of a lens-specific gamma-crystallin promoter. Expression of the gamma-crystallin coding sequences from the human metallothionein IIA promoter in nonlens cells facilitated characterization of the polypeptides encoded by individual gamma-genes and, in future studies, should permit comparison of these proteins with distinct gamma-crystallins in the human lens.

Biochemical characterization of crystallins from frog lenses

Lens crystallins were isolated from the homogenate of frog (Rana catesbeiana) eye lenses by gel permeation chromatography and characterized by gel electrophoresis, amino acid analysis and circular dichroism. Four well-defined fractions corresponding to alpha/beta-, beta-, frog 39.5 kDa and gamma-crystallins comprising the relative weight percentages in the total soluble cytoplasmic proteins of 18%, 15%, 14% and 48% respectively were obtained. The native molecular masses for each purified fraction were determined to be 432, 207, 40 and 23 kDa, respectively. The polypeptide compositions as determined by SDS-gel electrophoresis revealed the typical subunit structures of mammalian crystallins with the exception of 39.5 kDa monomeric crystallin, which has not been shown in other classes of vertebrate lenses. The spectra of circular dichroism indicate a predominant beta-sheet structure in all four fractions, which also bears a resemblance to the secondary structure of mammalian crystallins. Comparison of the amino acid compositions of frog crystallins with those of mammalian and fish crystallins suggests that gamma-crystallin from the frog is more closely related to that of porcine than fish crystallins, and the frog 39.5 kDa, frog beta- and lamprey 48 kDa crystallins are probably mutually interrelated.

The eye lens crystallins: ambiguity as evolutionary strategy

Comparative studies of the different families of lens-specific proteins of the vertebrates, the crystallins, and their genes reveal several interesting evolutionary features. The origin of alpha-crystallin can be traced back to the small heat shock proteins, while the superfamily of beta gamma-crystallins shows structural similarities with a bacterial spore coat protein. The crystallins display a great diversity within and between species, as well as during development. Ambiguous transcription, mRNA-processing, and translation contribute to this diversity of the crystallins and their expression. These mechanisms include the occurrence of atypical poly-A addition signals, alternative splicing, and the use of two initiation codons on a single mRNA.

The amino-terminal sequences of four major carp gamma-crystallin polypeptides and their homology with frog and calf gamma-crystallins

Four major gamma-crystallin subfractions have been isolated from the carp (Cyprinus carpio) and their N-terminal sequences determined by Edman protein sequencing. Extensive homologies indicative of close relatedness in their primary structure were found in these four gamma-crystallin polypeptides. Comparison of the carp N-terminal sequences with those of mammalian and amphibian gamma-crystallins also showed a high degree of homology present in their N-terminal segments despite the dissimilarity of amino acid compositions of fish gamma-crystallins to those of higher classes of vertebrates. The distinct yet closely-related partial sequences of carp gamma-crystallins could account for the profound microheterogeneity detected in the characterization of carp crystallins, suggesting the presence of a multigene family for gamma-crystallin in the lowest class of vertebrates, i.e. the fish.

Lens-specific promoter activity of a mouse gamma-crystallin gene

Crystallins are the major water-soluble proteins in vertebrate eye lenses. These lens-specific proteins are encoded by several gene families, and their expression is differentially regulated during lens cell differentiation. Here we show that a cloned mouse gamma-crystallin promoter is active in lens explants derived from 14-day-old chicken embryos but inactive in a variety of cells of non-lens origin. We also show that sequences required for proper utilization of this promoter are contained between nucleotide positions -392 and +47 relative to the transcription initiation site; deletion of sequences from positions -392 to -171 completely abolishes promoter activity. Since chickens do not have gamma-crystallin genes, the expression of a mouse gamma-crystallin promoter in chicken lens cells suggests that different classes of crystallin genes may be regulated by common lens tissue-specific mechanism(s) independent of species.

Structural and evolutionary relationships among five members of the human gamma-crystallin gene family

We have characterized five human gamma-crystallin genes isolated from a genomic phage library. DNA sequencing of four of the genes revealed that two of them predict polypeptides of 174 residues showing 71% homology in their amino acid sequence; the other two correspond to closely related pseudogenes which contain the same in-frame termination codon at identical positions in the coding sequence. Two of the genes and one of the pseudogenes are oriented in a head-to-tail fashion clustered within 22.5 kilobases. All three contain a TATA box 60 to 80 base pairs upstream of the initiation codon and a highly conserved segment of 44 base pairs in length immediately preceding the TATA box. The two genes and the two pseudogenes are similar in structure: each contains a small 5' exon encoding three amino acids followed by two larger exons that correspond exactly to the two similar structural domains of the polypeptide. The first intron varies from 100 to 110 base pairs, and the second intron ranges from 1 to several kilobases, rendering an overall gene size of 1.7 to 4.5 kilobases. At least one of the two pseudogenes appears to have been functional before inactivation, suggesting that their identical mutation was generated by gene conversion.

Lens-specific promoter activity of a mouse gamma-crystallin gene

Crystallins are the major water-soluble proteins in vertebrate eye lenses. These lens-specific proteins are encoded by several gene families, and their expression is differentially regulated during lens cell differentiation. Here we show that a cloned mouse gamma-crystallin promoter is active in lens explants derived from 14-day-old chicken embryos but inactive in a variety of cells of non-lens origin. We also show that sequences required for proper utilization of this promoter are contained between nucleotide positions -392 and +47 relative to the transcription initiation site; deletion of sequences from positions -392 to -171 completely abolishes promoter activity. Since chickens do not have gamma-crystallin genes, the expression of a mouse gamma-crystallin promoter in chicken lens cells suggests that different classes of crystallin genes may be regulated by common lens tissue-specific mechanism(s) independent of species.

Structural and evolutionary relationships among five members of the human gamma-crystallin gene family

We have characterized five human gamma-crystallin genes isolated from a genomic phage library. DNA sequencing of four of the genes revealed that two of them predict polypeptides of 174 residues showing 71% homology in their amino acid sequence; the other two correspond to closely related pseudogenes which contain the same in-frame termination codon at identical positions in the coding sequence. Two of the genes and one of the pseudogenes are oriented in a head-to-tail fashion clustered within 22.5 kilobases. All three contain a TATA box 60 to 80 base pairs upstream of the initiation codon and a highly conserved segment of 44 base pairs in length immediately preceding the TATA box. The two genes and the two pseudogenes are similar in structure: each contains a small 5' exon encoding three amino acids followed by two larger exons that correspond exactly to the two similar structural domains of the polypeptide. The first intron varies from 100 to 110 base pairs, and the second intron ranges from 1 to several kilobases, rendering an overall gene size of 1.7 to 4.5 kilobases. At least one of the two pseudogenes appears to have been functional before inactivation, suggesting that their identical mutation was generated by gene conversion.

Selective loss of a family of gene transcripts in a hereditary murine cataract

The eye lens of the Fraser mouse contains a dominantly inherited cataract with reduced amounts of seven distinct but homologous gamma crystallins encoded by a family of gamma-crystallin genes. The results of experiments with cultured lenses, cell-free RNA translation, and Northern blot hybridization indicated a specific loss of the family of gamma-crystallin messenger RNA's in the Fraser mouse lens. Southern blot hybridization of genomic DNA's from normal and Fraser mice showed no differences in gamma-crystallin coding sequences.