Gene sharing by delta-crystallin and argininosuccinate lyase

Evidence for the extralenticular expression of members of the beta-crystallin gene family in the chick and a comparison with delta-crystallin during differentiation and transdifferentiation

The beta-crystallins are major water soluble proteins of vertebrate lens fibre cells and have previously been regarded as lens-specific proteins: however beta B2-and beta A3/A1-crystallin RNAs are transcribed and beta-crystallin polypeptides are detectable in the developing chick retina. The beta-crystallin RNA is transcribed in a subpopulation of retina cells and the number of transcribing cells and the level of beta-crystallin polypeptides increase during the differentiation of the retina. Several tissues express beta-crystallin polypeptides, but individual tissues are characterised by qualitative and quantitative differences in the beta- and delta-crystallin polypeptides expressed. The expression of beta-crystallins appears to be non-random as defined by tissue distribution, cellular localisation and ontogeny, implying a function for extralenticular beta-crystallins and a complex mechanism for the regulation of their expression.

Degenerate oligonucleotide sequence-directed cross-species PCR cloning of the BCP 54/ALDH 3 cDNA: priming from inverted repeats and formation of tandem primer arrays

Bovine corneal protein 54 (BCP 54) is the major soluble protein of the bovine cornea, and immunoreactive forms of this protein have been described in a wide range of mammals. Dideoxy sequence determination of a previously synthesized 420-bp cDNA to BCP 54 generated by the novel mixed oligonucleotide primer amplification of cDNA (MOPAC) procedure revealed extensive similarity to the cDNA encoding tumor-associated rat liver (class 3) aldehyde dehydrogenase (RATALD). PCR amplification with additional pairs of degenerate oligonucleotide sequence (DOS) primers derived from both BCP 54-amino-acid sequence and amino acid and nucleotide sequence data from RATALD produced three PCR products that were cloned and subsequently sequenced. The major product was 716-bp BCP 54 cDNA clone encompassing the BCP 54 carboxy-terminal amino acid sequence for which the DOS pair was designed. Sequence alignment of the BCP 54 cDNA and its translation product with RATALD demonstrated 81% and 85% identity at the nucleotide and amino acid levels, respectively. Analysis of the additional two clones established that they were the results of PCR artifactual processes. The first of these was a 552-bp product occurring at elevated primer concentrations that formed through bidirectional amplification from a single DOS annealing to an inverted repeat located in the BCP 54 coding sequence. The second artifactual product was a 212-bp sequence that contained several unreported amplification anomalies, including the formation of a tandem primer array.

Sequence analysis of the ARG7 gene of Schizosaccharomyces pombe coding for argininosuccinate lyase. Expression of the gene in Saccharomyces cerevisiae

The complete nucleotide sequence of the ARG7 gene, coding for argininosuccinate lyase (EC 4.3.2.1), in the fission yeast (Schizosaccharomyces pombe) has been determined. It consists of an open reading frame of 461 codons. The deduced protein has a molecular weight of 51,200 Da. The gene is devoid of introns which is confirmed by the fact that it is expressed in Escherichia coli after spontaneous insertion of a bacterial sequence probably bearing a prokaryotic promoter. A perfect "TATA" box is found at -72 and the major transcription initiation site in Saccharomyces cerevisiae is located at -11 as shown by primer extension experiments. Comparison of the S. pombe lyase with related proteins from other organisms reveals an important degree of conservation except in the carboxyterminal part of the polypeptide. Additionally, a deletion removing 66 amino acids of the carboxy terminus yields an enzyme exhibiting some biological activity. A unique 1,500 b transcript was found in S. cerevisiae when the intact gene was present, but the deleted version of the gene gave rise to at least three transcripts of 1,800, 2,800 and 3,900 b.

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.

Temperature-induced expression of proteins in Leishmania mexicana amazonensis. A 22-kDa protein is possibly localized in the mitochondrion

Temperature increase is an integral part of Leishmania life cycle, and plays a major role in stage transformation. Analysis of the temperature-dependent pattern of protein synthesis on two-dimensional gel electrophoresis shows that, in addition to the conserved heat-shock type of response in which expression of the major 70-kDa and 83-kDa heat-shock proteins is observed, a group of low-molecular-mass (17-40 kDa) proteins is induced in promastigotes of Leishmania mexicana amazonensis at elevated temperatures. Immuno-gold labelling with antibodies raised against the heat-induced 22-kDa proteins was localized mainly in the mitochondrion of Leishmania parasites, though labelling was observed also in the nucleus. The correlation of this finding with various reports on induction of mitochondrial enzymes in response to temperature stress in other organisms is discussed.

Molecular analysis of human argininosuccinate lyase: mutant characterization and alternative splicing of the coding region

Argininosuccinic acid lyase (ASAL) deficiency is a clinically heterogeneous autosomal recessive urea cycle disorder. We previously established by complementation analysis that 28 ASAL-deficient patients have heterogeneous mutations in a single gene. To prove that the ASAL structural gene is the affected locus, we sequenced polymerase chain reaction-amplified ASAL cDNA of a representative mutant from the single complementation group. Fibroblast strain 944 (approximately 1% of residual ASAL activity), from a late-onset patient who was the product of a consanguineous mating, had only a single base-pair change in the coding region, a C-283----T transition at a CpG dinucleotide in exon 3. This substitution converts Arg-95 to Cys (R95C), occurs in a stretch of 13 residues that is identical in yeast and human ASAL, and was present in both of the patient's alleles but not in 14 other mutant or 10 normal alleles. Expression in COS cells demonstrated that the R95C mutation produces normal amounts of ASAL mRNA but little protein and less than 1% ASAL activity. We observed that amplified cDNA from mutant 944 and normal cells (liver, keratinocytes, lymphoblasts, and fibroblasts) contained, in addition to the expected 5' 513-base-pair band, a prominent 318-base-pair ASAL band formed by the splicing of exon 2 from the transcript. The short transcript maintains the ASAL reading frame but removes Lys-51, a residue that may be essential for catalysis, since it binds the argininosuccinate substrate. We conclude (i) that the identification of the R95C mutation in strain 944 demonstrates that virtually all ASAL deficiency results from defects in the ASAL structural gene and (ii) that minor alternative splicing of the coding region occurs at the ASAL locus.

Isolation of cDNAs from a mouse astroglial cell line by a subtracted cDNA library

Astrocytes belong to the glial cell population and represent a major subclass of the CNS. Although different subtypes of astrocytes have been described according to their morphological characteristics, biochemical markers of each subtype of astrocytes are not yet available. We have thus undertaken to compare gene expression pattern of different astroglial subtypes. In this study we have taken advantage of two astroglial cell clones derived from 8 day postnatal mouse cerebellar explants and which might be the in vitro equivalents of the velate protoplasmic (D19) and of the Golgi-Bergmann (C8S) astrocytes (Alliot and Pessac, Brain Res., 306: 283-291, 1984). We have constructed a subtracted cDNA library derived from cytoplasmic poly(A)+ RNAs of the D19 cell line. This library was enriched 12-fold for D19 specific sequences by subtractive hybridization with an excess of cytoplasmic poly(A)+ RNAs purified from the C8S astroglial clone. This subtracted library was differentially screened with cDNA probes derived from D19 and C8S cell lines; both probes were subtracted with C8S poly(A)+ RNAs. Eight cDNA clones corresponding to transcripts overexpressed in D19 were selected. Three cDNAs encode for smooth muscle actin, one for fibronectin and one for polyadenylate binding protein. The three other gene products have not been previously reported. The in vivo distribution pattern of these sequences in various mouse adult tissues shows that all these transcripts are expressed in the cerebellum and/or in the brain.

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

In apparent contrast to most other tissues, the ocular lenses in vertebrates show striking differences in protein composition between taxa, most notably in the recruitment of different enzymes as major structural proteins. This variability appears to be the result of at least partially neutral evolutionary processes, although there is also evidence for selective modification in molecular structure. Here we describe a bird, the chimney swift (Chaetura pelagica), that lacks delta-crystallin/argininosuccinate lyase, usually the major crystallin of avian lenses. Clearly, delta-crystallin is not specifically required for a functionally effective avian lens. Furthermore the lens composition of the swift is more similar to that of the related hummingbirds than to that of the barn swallow (Hirundo rustica), suggesting that phylogeny is more important than environmental selection in the recruitment of crystallins. However differences in epsilon-crystallin/lactate dehydrogenase-B sequence between swift and hummingbird and other avian and reptilian species suggest that selective pressures may also be working at the molecular level. These differences also confirm the close relationship between swifts and hummingbirds.

Kinetic analysis of duck epsilon-crystallin, a lens structural protein with lactate dehydrogenase activity

Biochemical characterization and kinetic analysis of epsilon-crystallin from the lenses of common ducks were undertaken to elucidate the enzyme mechanism of this unique crystallin with lactate dehydrogenase (LDH) activity. Despite the structural similarities between epsilon-crystallin and chicken heart LDH, differences in charge and kinetic properties were revealed by isoenzyme electrophoresis and kinetic studies. Bi-substrate kinetic analysis examined by initial-velocity and product-inhibition studies suggested a compulsory ordered Bi Bi sequential mechanism with NADH as the leading substrate followed by pyruvate. The products were released in the order L-lactate and NAD+. The catalysed reaction is shown to have a higher rate in the formation of L-lactate and NAD+. Substrate inhibition was observed at high concentrations of pyruvate and L-lactate for the forward and reverse reactions respectively. The substrate inhibition was presumably due to the formation of epsilon-crystallin-NAD(+)-pyruvate or epsilon-crystallin-NADH-L-lactate abortive ternary complexes, as suggested by the product-inhibition studies. The significance and the interrelationship of duck epsilon-crystallin with other well-known LDHs are discussed with special regard to its role as a structural protein with some enzymic function in lens metabolism.

Developmental regulation of hypomethylation of delta-crystallin genes in chicken embryo lens cells

Sequences in the two delta-crystallin genes become hypomethylated when they are expressed in the chick lens. This system is particularly advantageous for studying temporal changes in hypomethylation, since lens tissue can be isolated at all developmental stages. In previous work we have shown that most HpaII sites become hypomethylated within the delta 1-crystallin gene long after delta-crystallin gene activation. One site is hypomethylated when crystallin mRNA begins to be synthesized at high levels at 50 h; we show here that this site maps to the 3' end (intron 15) of the delta 1-crystallin gene. In addition, we have examined the methylation status of HpaII and HhaI sites found near the 5' end of the delta 1-crystallin gene. Two HhaI sites adjacent to a viral core enhancer sequence in intron 2 are also first hypomethylated at 50 h. These findings point to regions of the delta 1 gene that should be investigated further for functional significance in regulating delta-crystallin transcription.

Identification of epsilon-crystallin from swan lens as lactate dehydrogenase

Characterization of lens crystallins from black swan, a rare aquatic bird belonging to the family Anatidae, was carried out to search for epsilon-crystallin with lactate dehydrogenase activity. Biochemical comparison of epsilon-crystallins isolated from the swan and duck lenses plus lactate dehydrogenase of chicken heart has also been made in order to establish the structural/functional relatedness of these proteins. Amino acid analyses showed essentially similar overall compositions for these three proteins. Kinetic analysis revealed differences between avian epsilon-crystallins and the authentic heart-type lactate dehydrogenase. The swan lenses similar to duck lenses appeared to contain a thermostable epsilon-crystallin which possesses very high enzymatic activity of lactate dehydrogenase. The characterization of epsilon-crystallins from the available species of aquatic birds may provide some insights into the evolution of this unique crystallin in the Aves and their enzymatic roles inside the lens.

Multiple molecular forms of human lactoferrin. Identification of a class of lactoferrins that possess ribonuclease activity and lack iron-binding capacity

Lactoferrin (Lf), the major iron-binding component of milk, also a major constituent of the specific granules of neutrophils involved in antimicrobial activity and a glycoprotein thought to play a role in regulatory functions in the hematopoietic system as well as other physiologic activities, is shown to occur in three isoforms. One, Lf-alpha, binds iron; the other two, Lf-beta and Lf-gamma, express potent RNase activity, but do not bind iron. The three isoforms are very similar or identical in Mr, pI, partial proteolytic peptide patterns, NH2-terminal amino acid sequence, and reactivity with mAbs and polyclonal antisera against the RNase and Lf, respectively. The finding of structurally similar but enzymatically distinct forms of Lf may be related to the diverse functions of the molecule.

Developmental regulation of hypomethylation of delta-crystallin genes in chicken embryo lens cells

Sequences in the two delta-crystallin genes become hypomethylated when they are expressed in the chick lens. This system is particularly advantageous for studying temporal changes in hypomethylation, since lens tissue can be isolated at all developmental stages. In previous work we have shown that most HpaII sites become hypomethylated within the delta 1-crystallin gene long after delta-crystallin gene activation. One site is hypomethylated when crystallin mRNA begins to be synthesized at high levels at 50 h; we show here that this site maps to the 3' end (intron 15) of the delta 1-crystallin gene. In addition, we have examined the methylation status of HpaII and HhaI sites found near the 5' end of the delta 1-crystallin gene. Two HhaI sites adjacent to a viral core enhancer sequence in intron 2 are also first hypomethylated at 50 h. These findings point to regions of the delta 1 gene that should be investigated further for functional significance in regulating delta-crystallin transcription.

Expression of the murine alpha B-crystallin gene is not restricted to the lens

The murine alpha B-crystallin gene was cloned and its expression was examined. In the mouse, significant levels of alpha B-crystallin RNA were detected not only in lens but also in heart, skeletal muscle, kidney, and lung; low and trace levels were detected in brain and spleen, respectively. The RNA species in lung, brain, and spleen was 400 to 500 bases larger than that in the other tissues. Transcription in lens, heart, skeletal muscle, kidney, and brain initiated at the same position. A mouse alpha B-crystallin mini-gene was constructed and was introduced into the germ line of mice, and its expression was demonstrated to parallel that of the endogenous gene. Transgene RNA was always detected in lens, heart, and skeletal muscle, while expression in kidney and lung was variable; it remains uncertain whether there is transgene expression in brain and spleen. These results demonstrate that regulatory sequences controlling expression of the alpha B-crystallin gene lie between sequences 666 base pairs upstream of the transcription initiation site and 2.4 kilobase pairs downstream of the poly(A) addition site and are not located within the introns. Transfection studies with a series of alpha B-crystallin mini-gene deletion mutants revealed that sequences between positions -222 and -167 were required for efficient expression in primary embryonic chick lens cells; sequences downstream of the poly(A) addition signal were dispensable for expression in this in vitro system.

Characterization of gamma-crystallins from eye lenses of shark: closer structural similarity to mammalian than other piscine gamma-crystallins?

Lens crystallins were isolated and characterized from sharks of the cartilaginous fishes. Four crystallin fractions corresponding to alpha-, beta H-, beta L- and gamma-crystallins, similar to those of mammalian crystallins, were obtained. The native molecular masses and subunit structures of these purified fractions were analyzed by gel permeation chromatography, SDS gel electrophoresis and isoelectric focusing, revealing the typical subunit compositions with various extents of heterogeneity in each orthologous crystallin class. Amino acid and N-terminal sequence analyses corroborate the identification and classification of crystallin classes based on electrophoresis. Unexpectedly, it was found that the amino acid composition and N-terminal sequence of shark gamma-crystallin are more closely related to those of bovine than carp gamma-crystallin. This finding may have some bearing on the divergence and specification of gamma-crystallins between the phylogenetic lines of mammals and fishes.

Evolutionary aspects of urea cycle enzyme genes

The functions and expression pattern of urea cycle enzymes have undergone considerable changes during the course of evolution. Sequence analyses shows that urea cycle enzymes from mammals are homologous to microbial enzymes of the arginine-metabolic pathway. Recently, an unexpected relationship was found between argininosuccinate lyase (EC 4.3.2.1), the fourth enzyme of the cycle, and delta-crystallin, a lens structural protein of birds and reptiles.

Expression of the murine alpha B-crystallin gene is not restricted to the lens

The murine alpha B-crystallin gene was cloned and its expression was examined. In the mouse, significant levels of alpha B-crystallin RNA were detected not only in lens but also in heart, skeletal muscle, kidney, and lung; low and trace levels were detected in brain and spleen, respectively. The RNA species in lung, brain, and spleen was 400 to 500 bases larger than that in the other tissues. Transcription in lens, heart, skeletal muscle, kidney, and brain initiated at the same position. A mouse alpha B-crystallin mini-gene was constructed and was introduced into the germ line of mice, and its expression was demonstrated to parallel that of the endogenous gene. Transgene RNA was always detected in lens, heart, and skeletal muscle, while expression in kidney and lung was variable; it remains uncertain whether there is transgene expression in brain and spleen. These results demonstrate that regulatory sequences controlling expression of the alpha B-crystallin gene lie between sequences 666 base pairs upstream of the transcription initiation site and 2.4 kilobase pairs downstream of the poly(A) addition site and are not located within the introns. Transfection studies with a series of alpha B-crystallin mini-gene deletion mutants revealed that sequences between positions -222 and -167 were required for efficient expression in primary embryonic chick lens cells; sequences downstream of the poly(A) addition signal were dispensable for expression in this in vitro system.

The cellular eye lens and crystallins of cubomedusan jellyfish

The ultrastructure and major soluble proteins of the transparent eye lens of two cubomedusan jellyfish, Tripedalia cystophora and Carybdea marsupialis, have been examined. Each species has two complex eyes (one large and one small) on four sensory structures called rhopalia. The lenses consist of closely spaced cells with few organelles. The lens is situated next to the retina, with only an acellular layer separating it from the photoreceptors. SDS-PAGE showed that the large lens of C. marsupialis has only two crystallin polypeptide bands (with molecular masses of approximately 20,000 and 35,000 daltons), while that of T. cystophora has three bands (two with a molecular mass near 20,000 daltons and one with a molecular mass near 35,000 daltons). Interestingly, the small lens of T. cystophora appears to be markedly deficient in or lack the lower molecular weight proteins. The crystallins behaved as monomeric proteins by FPLC and showed no immunological reaction with antisera of the major squid crystallin, chicken delta-crystallin or mouse gamma-crystallin in western immunoblots. Very weak reactions were found with antimouse alpha- and beta-crystallin sera. The 35,000 dalton crystallin of T. cystophora was purified and called J1-crystallin. It contained relatively high leucine (13%) and tyrosine (9%) and low methionine (2%). Several tryptic peptides were sequenced. Weak sequence similarities were found with alpha- and beta-crystallins, which may account for some of the apparent weak immunological cross-reactivity with these vertebrate crystallins. A polyclonal antiserum made in rabbits from a synthetic peptide of J1-crystallin reacted strongly with J1-crystallin of T. cystophora and C. marsupialis in immunoblots; by contrast, no reaction was obtained with the lower molecular weight crystallins from these jellyfish, with the squid crystallin, or with any crystallins from the frog or human lens. Thus, despite the structural similarities between the cubomedusan, squid and vertebrate lenses, their crystallins appear very different.

Structure of the rat argininosuccinate lyase gene: close similarity to chicken delta-crystallin genes

Argininosuccinate lyase (EC 4.3.2.1) is an enzyme of arginine biosynthesis and is also involved in the urea cycle in the liver of ureotelic animals. A comparison of cDNA-derived amino acid sequences revealed that argininosuccinate lyase is highly homologous with chicken delta-crystallin, a major structural protein of the eye lens. The gene for the rat argininosuccinate lyase was cloned and its structure was determined. This gene is a single-copy gene about 14 kilobases long and is split into 16 exons. A comparison with chicken delta-crystallin genes revealed that all introns interrupt the protein-coding regions at homologous positions. This close similarity in structural organization provides strong evidence for the view of Piatigorsky et al. [Piatigorsky, J., O'Brien, W. E., Norman, B. L., Kalmuck, K., Wistow, G., Borras, T., Nickerson, J. M. & Wawrousek, E. F. (1988) Proc. Natl. Acad. Sci. USA 85, 3479-3483] that chicken delta 1- and delta 2-crystallin genes evolved by recruitment and duplication of the preexisting argininosuccinate lyase gene and that delta 2-crystallin is probably the direct homologue argininosuccinate lyase.

Tau-crystallin/alpha-enolase: one gene encodes both an enzyme and a lens structural protein

tau-Crystallin has been a major component of the cellular lenses of species throughout vertebrate evolution, from lamprey to birds. Immunofluorescence analysis of the embryonic turtle lens, using antiserum to lamprey tau-crystallin showed that the protein is expressed throughout embryogenesis and is present at high concentrations in all parts of the lens. Partial peptide sequence for the isolated turtle protein and deduced sequences for several lamprey peptides all revealed a close similarity to the glycolytic enzyme enolase (E.C. 4.2.1.11). A full-sized cDNA for putative duck tau-crystallin was obtained and sequenced, confirming the close relationship with alpha-enolase. Southern blot analysis showed that the duck genome contains a single alpha-enolase gene, while Northern blot analysis showed that the message for tau-crystallin/alpha-enolase is present in embryonic duck lens at 25 times the abundance found in liver. tau-Crystallin possesses enolase activity, but the activity is greatly reduced, probably because of age-related posttranslational modification. It thus appears that a highly conserved, important glycolytic enzyme has been used as a structural component of lens since the start of vertebrate evolution. Apparently the enzyme has not been recruited for its catalytic activity but for some distinct structural property. tau-Crystallin/alpha-enolase is an example of a multifunctional protein playing two very different roles in evolution but encoded by a single gene.

Squid major lens polypeptides are homologous to glutathione S-transferases subunits

The eye lenses of cephalopods and vertebrates evolved relatively recently and by independent routes. They provide a good experimental model for the study of convergent evolution at the protein level. One proposal is that pre-existing proteins were recruited as structural eye lens proteins during evolution. This has been confirmed for the vertebrate eye lens structural proteins, or crystallins, which have been intensively studied. Despite the limited information about cephalopod eye lenses, it has been suggested that glutathione S-transferases (GSTs) are a possible evolutionary ancestor of the squid major lens proteins. Recently, the N-terminal sequence of the squid major lens protein was shown to be 55% homologous with that of the Ya subunit of the rat GST. Here, we demonstrate that the squid major lens polypeptides are encoded by a gene family of at least three members. We characterize two cDNAs corresponding to these genes and show they probably either are GST subunits themselves, or share an evolutionary ancestor with them.

Duck lens epsilon-crystallin and lactate dehydrogenase B4 are identical: a single-copy gene product with two distinct functions

To investigate whether or not duck lens epsilon-crystallin and duck heart lactate dehydrogenase (LDH) B4 are the product of the same gene, we have isolated and sequenced cDNA clones of duck epsilon-crystallin. By using these clones we demonstrate that there is a single-copy Ldh-B gene in duck and in chicken. In the duck lens this gene is overexpressed, and its product is subject to posttranslational modification. Reconstruction of the evolutionary history of the LDH protein family reveals that the mammalian Ldh-C gene most probably originated from an ancestral Ldh-A gene and that the amino acid replacement rate in LDH-C is approximately 4 times the rate in LDH-A. Molecular modeling of LDH-B sequences shows that the increased thermostability of the avian tetramer might be explained by mutations that increase the number of ion pairs. Furthermore, the replacement of bulky side chains by glycines on the corners of the duck protein suggests an adaptation to facilitate close packing in the lens.