Evolutionary and developmental differences in delta-crystallin from bird and reptile lenses. Circular dichroism and fluorescence studies

Characterization and enzyme activity of argininosuccinate lyase/delta-crystallin of the embryonic duck lens

Argininosuccinate lyase (ASL)/delta-crystallin, a major soluble protein of the transparent eye lens of birds and reptiles, is a mixture of tetramers comprising all possible combinations of two similar polypeptides (delta 1 and delta 2). Only the delta 2 polypeptide has ASL activity. In the present investigation we have purified each of the 5 major isoforms (delta A to delta E, pI 5.2 to 5.8) of delta-crystallin tetramers from the embryonic duck lens by isoelectric focussing and established by peptide sequencing that the delta 1 and delta 2 polypeptides are encoded in the previously identified, linked delta 1 and delta 2 genes, respectively. The relative amounts of the different tetramers in the 14-day-old embryonic lens were consistent with equal expression of the 2 delta-crystallin genes and no preference for assembly of the 2 delta polypeptides. The relative amount of ASL activity of the tetramers was a linear function of the relative amount of their delta 2 polypeptides, with delta A (only delta 1) lacking enzymatic activity altogether. delta B (3 delta 1:1 delta 2), delta C (2 delta 1:2 delta 2), delta D (1 delta 1:3 delta 2) and delta E (4 delta 2) all gave normal Michaelis-Menten kinetics for fumarate production from argininosuccinate at 40 degrees C and had a similar Km (average Km for mixture was 0.15 mM). delta E had a Km of 0.187 mM and a Vmax of 9 mumol/min per mg protein. Unlike bovine and like human ASL, both reported previously, embryonic duck ASL/delta-crystallin showed no evidence of cooperativity or activation by GTP. Each isoform had a similar far ultraviolet circular dichroism spectrum and thermal stability between 20 degrees C and 60 degrees C, with denaturation occurring at 65 degrees C. Our data suggest that gene duplication, structural modifications leading to greater thermal stability of the delta 1 and delta 2 polypeptides, and selective loss of ASL activity in the delta 1 polypeptide all occurred during the recruitment of ASL for a refractive role in the duck lens, resulting in the generation of ASL isoenzymes.

A conformational study of corneal dermatan sulfate proteoglycan using fluorescence spectroscopy

DSPG is a major proteoglycan of the corneal stroma and is thought to be important for the transparency of the tissue. We have studied its conformation by exploring the microenvironment and dynamics of its lone Tryptophan (Trp) residue using steady state and time resolved fluorescence spectroscopy. DSPG exhibits a doublet Trp fluorescence emission. Such a doublet emission has been observed earlier in the copper protein azurin and in avian lens delta-crystallin. Unlike the above cases where the doublet emission is thought to arise due to vibronic structure or the location of Trp at the interface of interacting subunits, fluorescence quenching, denaturation studies and ANS binding with DSPG indicate the location of Trp at two different environments. Such a situation could arise from the differential glycosylation of the core protein or due to duplexation and aggregation of the glycosaminoglycan chains.

Ostrich crystallins. Structural characterization of delta-crystallin with enzymic activity

Lens crystallins from the African ostrich (Struthio camelus) were isolated and characterized. Four crystallin fractions corresponding to alpha-, delta/beta- and beta-crystallins similar to those of duck crystallins were isolated, but epsilon-crystallin was found to be absent. The native molecular masses and subunit structures of the purified fractions were analysed by gel filtration. SDS/PAGE and isoelectric focusing, revealing various extents of heterogeneity in each orthologous crystallin class. An ion-exchange chromatographic method was used for the large-scale preparation of delta-crystallin suitable for structural and enzymic studies. It was unexpectedly found that the purified native delta-crystallin of ostrich lens possessed high argininosuccinate lyase activity, in contrast with chicken delta-crystallin. The c.d. spectra indicated a predominant beta-sheet structure in alpha- and beta-crystallins, and a significant contribution of alpha-helical structure in the delta-crystallin fraction. The estimate of secondary structures from c.d. spectroscopy for each crystallin class bears a resemblance to that of duck crystallins, except that ostrich delta-crystallin possesses much less helical content than duck delta-crystallin. Comparison of crystallin compositions and structures from aquatic and terrestrial birds revealed distinct differences.

The conformational status of a protein influences the aerobic photolysis of its tryptophan residues: melittin, beta-lactoglobulin and the crystallins

We have studied the aerobic photolysis of the tryptophan residues of the proteins melittin and beta-lactoglobulin when the proteins are in ordered conformations and when they are in randomly coiled states. The results suggest that the conformational status of the protein is a factor that influences the photolysis of the constituent tryptophan residues. This point appears to be of relevance to the photo-oxidation of the tryptophan residues of the eye lens proteins crystallins.

Zeta-crystallin, a novel protein from the guinea pig lens is related to alcohol dehydrogenases

zeta-Crystallin is a major component of the water-soluble proteins of the guinea pig lens. We have constructed a lens cDNA library from one- to seven-day-old guinea pigs in the plasmid Bluescript KS+ and used the 16 amino acid (aa) sequence of a CNBr peptide to design an oligodeoxyribonucleotide probe. Analysis of two positive clones and direct sequence of the 5' end of the RNA resulted in the completion of a most probably full-length mRNA comprising 1842 nucleotides (nt). The ATG start codon occurs 83 nt downstream from the 5' end. The open reading frame, ending with a stop codon at nt position 1070, predicts a protein of 328 aa with a calculated Mr of 35,071. Comparison of the amino acid sequence with the National Biomedical Research Foundation protein data base reveals a significant similarity of zeta-crystallin with the enzyme of the alcohol dehydrogenase family.

Biochemical comparison of lens crystallins from three reptilian species

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

Conservation of delta-crystallin gene structure between ducks and chickens

A cloned chicken delta-crystallin cDNA was used to identify two putative delta-crystallin genes in the duck by Southern blot hybridization. A DNA fragment containing most of one of these genes was isolated from a library made in bacteriophage lambda Charon 28A containing genomic DNA from 14-day-old embryonic ducks. Electron microscopy, partial gene sequencing, primer extension analysis using duck mRNA, and comparison with the well-characterized chicken delta-crystallin genes suggest that our cloned duck delta-crystallin gene, like the chicken delta-crystallin genes, is 8-10 kb long and contains 17 exons. Hybridization and sequencing data show great similarity between the homologous 5' untranslated and coding exons of the duck and chicken delta-crystallin genes. Overall, the homologous introns also appear to have approximately 30% sequence similarity, and have been subject to deletion/insertion events. Our partial characterization of duck delta-crystallin gene sequences suggests that this avian and reptilian crystallin family has been conserved during evolution, as have the other crystallin gene families that are expressed in the eye lens.

The hydration of protein secondary structures

The hydration of the main-chain carbonyl (CO) groups in proteins have been studied using infra-red spectroscopy, and computer-graphics analysis of high resolution protein crystal structures. The IR measurements indicate that the strength of water binding to the CO groups is lower in beta-sheet proteins compared with alpha-helical ones. Analysis of the protein crystal structures shows that this is due primarily to differences in the geometry of water-CO group interactions in the two types of secondary structure.

tau-Crystallin from the turtle lens: purification and partial characterization

A lens protein with a molecular mass near 46 000 daltons has been purified from the turtle lens. It is a monomeric protein which differs from other lens crystallins in its antigenicity, near-UV circular dichroism spectrum and amino acid composition. Like delta-crystallin, it has appreciable secondary structure (52% alpha-helix). This lens protein has been named tau-crystallin. tau-Crystallin cross-reacts with the novel 48 000 dalton protein identified recently in lenses of the sea lamprey. Although distinct, the possibility that tau-crystallin shares an ancestral relationship with delta-crystallin is discussed.

Characterization and crystallization of delta-crystallin from the eye lens of adult turkey: comparison with delta-crystallin from adult quail and young chick

delta-Crystallin, purified from either adult turkey or quail lenses, and characterized by amino acid composition and circular dichroism spectroscopy, is shown to be similar to delta-crystallin purified from 3-day-old chick lenses. Crystallization of turkey delta-crystallin is reported along with the characterization of the crystal unit cell by X-ray diffraction techniques. Variation in unit cell parameters is observed and is correlated with hydration.

Delta crystallins and their nucleic acids

delta-Crystallin is a major structural protein of avian and reptilian lenses that is absent from the lenses of fish, amphibia and mammals. It appears to be a tetrameric protein with a native molecular weight near 200 000 (200K) and polypeptide molecular weight near 50K and 48K) (see Note added in proof). The alpha-crystallin polypeptides are extremely similar, associate in various combinations of four and are held together by hydrophobic interactions. Although principally cytoplasmic, delta-crystallin may associate with the cell membrane. delta-Crystallin differs from other lens crystallins in its alpha-helical content, native and subunit molecular weights, antigenicity, low wavelength of maximum fluorescence emission (315 nm) after excitation at 280 nm and amino acid composition (high in leucine; low in aromatic residues en no cysteine). Analyses of peptides, native and subunit molecular weights, and circular dichroism spectra indicate that the primary, secondary, tertiary and subunit structures of delta-crystallin have been generally conserved during evolution. There are at least two tandemly arranged delta-crystallin containing 13-15 introns in the chicken; a similar structure exists for a cloned delta-crystallin gene in the duck. Experiments with chicken show that delta-crystallin synthesis occurs principally in the embryo, especially during lens fiber cell differentiation. delta-Crystallin synthesis also takes place during lens fiber cell differentiation in culture. There is evidence for both transcriptional and post-transcriptional regulation of delta-crystallin synthesis. Current studies on the crystallographic and primary structures of delta-crystallin, on the structure, evolution and expression of the delta-crystallin genes, and on the translation of delta-crystallin mRNAs make this specialized lens protein an active area of investigation.

Lamprey 48-kDa lens protein represents a novel class of crystallins

SDS-PAGE revealed a major Mr 48 000 polypeptide of pI around 8 in the water-soluble fraction of lamprey lenses. It occurs as a monomeric protein, and its amino acid composition and tryptic peptides show no resemblances to alpha-, beta-, gamma- or delta-crystallin. Immunoblotting with antiserum against the 48-kDa protein revealed an immunologically related polypeptide of similar Mr in reptiles, several birds and a fish, but showed no cross-reactivity with any other water-soluble lens component. The 48-kDa protein is not detected in many birds and fishes, and in the investigated mammals and amphibians.

Gene and protein structure of a beta-crystallin polypeptide in murine lens: relationship of exons and structural motifs

A 23,000 molecular weight beta-crystallin (beta 23) of the murine eye lens is encoded in a 4.1 +/- 0.3-kilobase gene containing three introns. Each of the four exons seems to code for a separate structural motif of the protein, whose tertiary structure was predicted by an interactive computer graphics technique based on the crystallographic structure of bovine gamma II-crystallin. The first exon also encodes a hydrophobic N-terminal peptide resembling membrane anchor sequences of other proteins. Our results indicate structural homology among the beta- and gamma-crystallin polypeptides, and link gene structure with protein structure in this superfamily of lens proteins.

Age-dependent changes of protein structure. The properties of young and old rabbit aldolase are restored after reversible denaturation

The significantly increased helical content is observed in muscle aldolase molecule of old rabbits. The unfolding and refolding of protein conformation followed by circular dichroism, fluorescence and enzyme activity showed the recovery of initial conformation after the denaturation. The protein folds into the form that existed prior to denaturation--"young" into "young" and "old" into "old"--the conformational differences between them being restored. This suggests that the primary structure modifications prior to the folding of the native protein conformation are the origin of the age-dependent differences of aldolase structure and function.

Physicochemical characterization of lens proteins of the squid Nototodarus gouldi and comparison with vertebrate crystallins

The main water-soluble proteins of squid lens (S-crystallins) have a molecular weight of 60 000, a sedimentation coefficient s020,w of 5.2 S, 20-30% alpha-helical secondary structure, and an unusually high methionine content (12%). The subunits of Mr 30 000 (major) and Mr 27 000 (minor) have related N-terminal amino acid sequences, but a very heterogeneous charge distribution with predominantly basic isoelectric points. Higher-Mr aggregates have similar secondary/tertiary structure and amino acid composition, but contain additional acidic subunits and Mr 35 000-40 000 subunits. S-crystallins resemble vertebrate beta-crystallins in their quaternary structure, and their N-terminal sequence shows analogy with the first 19 residues of calf beta/gamma-crystallin folding units. In the urea-soluble and urea-insoluble lens fractions polypeptides of Mr 58 000 and 80 000, respectively, predominate, which presumably correspond to the main cytoskeleton and membrane proteins. Water-soluble lens components of less than Mr 2000 were isolated which have ultraviolet absorption maxima at 327 and 370 nm.

Structural and functional similarities of delta-crystallin messenger ribonucleic acids from duck and chicken lenses

delta-Crystallin of the embryonic duck lens was compared with that of the embryonic chicken lens with respect to polypeptide composition, synthesis, and messenger ribonucleic acid (mRNA) sequences. Labeling experiments with [35S]methionine revealed that the duck delta-crystallin is composed of minor amounts of polypeptides with molecular weights near 50000 (50K) and 49000 (49K) and much greater amounts of polypeptides with molecular weights near 48000 (48K) and 47000 (47K), as judged by sodium dodecyl sulfate-urea-polyacrylamide gel electrophoresis. All four sizes of polypeptides were synthesized in similar relative proportions as found in vivo in a rabbit reticulocytes lysate supplemented with delta-crystallin mRNA isolated from the embryonic duck lens. Synthesis of the 48K and 47K delta-crystallin polypeptides was differentially reduced in duck lenses cultured in the presence of ouabain. This is similar to the differential reduction of synthesis of the lower molecular weight delta-crystallin peptides in embryonic chicken lenses demonstrated previously. R loops formed between duck or chicken delta-crystallin mRNA and a cloned chicken delta-crystallin cDNA and heteroduplexes formed between duck or chicken delta-crystallin mRNA and cloned chicken genomic DNAs containing delta-crystallin sequences showed that, except for the putative 5' leader sequence, the duck and chicken delta-crystallin mRNAs have extremely similar nucleotide sequences. These data indicate considerable conservation of delta-crystallin throughout the approximately 100 million years of divergence between ducks and chickens. The findings also suggest a possible relationship between the structure of delta-crystallin mRNA and the differential reduction in synthesis of the lower molecular weight delta-crystallin polypeptides in ouabain-treated lenses of ducks and chickens.