Structure of the 3' portion of the bovine elastin gene

Tropoelastin: a versatile, bioactive assembly module

Elastin provides structural integrity, biological cues and persistent elasticity to a range of important tissues, including the vasculature and lungs. Its critical importance to normal physiology makes it a desirable component of biomaterials that seek to repair or replace these tissues. The recent availability of large quantities of the highly purified elastin monomer, tropoelastin, has allowed for a thorough characterization of the mechanical and biological mechanisms underpinning the benefits of mature elastin. While tropoelastin is a flexible molecule, a combination of optical and structural analyses has defined key regions of the molecule that directly contribute to the elastomeric properties and control the cell interactions of the protein. Insights into the structure and behavior of tropoelastin have translated into increasingly sophisticated elastin-like biomaterials, evolving from classically manufactured hydrogels and fibers to new forms, stabilized in the absence of incorporated cross-linkers. Tropoelastin is also compatible with synthetic and natural co-polymers, expanding the applications of its potential use beyond traditional elastin-rich tissues and facilitating finer control of biomaterial properties and the design of next-generation tailored bioactive materials.

Matrikines and the lungs

The extracellular matrix is a complex network of fibrous and nonfibrous molecules that not only provide structure to the lung but also interact with and regulate the behaviour of the cells which it surrounds. Recently it has been recognised that components of the extracellular matrix proteins are released, often through the action of endogenous proteases, and these fragments are termed matrikines. Matrikines have biological activities, independent of their role within the extracellular matrix structure, which may play important roles in the lung in health and disease pathology. Integrins are the primary cell surface receptors, characterised to date, which are used by the matrikines to exert their effects on cells. However, evidence is emerging for the need for co-factors and other receptors for the matrikines to exert their effects on cells. The potential for matrikines, and peptides derived from these extracellular matrix protein fragments, as therapeutic agents has recently been recognised. The natural role of these matrikines (including inhibitors of angiogenesis and possibly inflammation) make them ideal targets to mimic as therapies. A number of these peptides have been taken forward into clinical trials. The focus of this review will be to summarise our current understanding of the role, and potential for highly relevant actions, of matrikines in lung health and disease.

Structural disorder and dynamics of elastin

Elastin is a self-assembling, extracellular-matrix protein that is the major provider of tissue elasticity. Here we review structural studies of elastin from over four decades, and draw together evidence for solution flexibility and conformational disorder that is inherent in all levels of structural organization. The characterization of disorder is consistent with an entropy-driven mechanism of elastic recoil. We conclude that conformational disorder is a constitutive feature of elastin structure and function.

Cellular interactions with elastin

Elastin is a key structural component of the extracellular matrix. Tropoelastin is the soluble precursor of elastin. In addition to providing elastic recoil to various tissues such as the aorta and lung, elastin, tropoelastin and elastin degradation products are able to influence cell function and promote cellular responses. These responses include chemotaxis, proliferation and cell adhesion. The interaction of elastin products with cells has been attributed to the elastin receptor. However, additional cell-surface receptors have also been identified. These include G protein-coupled receptors and integrins. The potential roles of these receptors in cell-elastin interactions, with particular focus on elastin formation are discussed.

The role of the carboxy terminus of tropoelastin in its assembly into the elastic fiber

Tropoelastin, the soluble precursor protein of insoluble amorphous elastin, contains repeating segments that are important for the characteristic elasticity and crosslinking sites of mature elastin. In addition, there is a unique carboxy terminal domain that is encoded by exon 36 of the elastin gene, and it has been suggested that this region may play a role in the process of insolubilization. The contribution of exon 36 to the maturation of tropoelastin into insoluble elastin was probed in these studies. Neonatal rat aortic smooth muscle cells were cultured and the fate of [3H] Lys labeled human recombinant tropoelastin (hrTE) molecules added to the cultures was monitored. In comparison to the hrTE containing the region encoded by exon 36, hrTE molecules lacking this domain were less efficiently incorporated into elastin, as evidenced by a decrease in NaOH insoluble radioactivity. Specific residues within the domain encoded by exon 36 were targeted for further study in experiments in which the two Cys residues were reduced and alkylated, and/or the four basic Arg-Lys-Arg-Lys residues at the carboxy terminus were removed. Both of these modifications resulted in decreased incorporation into elastin equivalent to the complete removal of the carboxy terminus. Prior treatment of the cell layer with elastase reduced the efficiency of insolubilization of hrTE containing the domain encoded by exon 36, but had no effect on the processing of molecules lacking this region. These data suggest that exon 36 of the elastin gene contributes to normal efficient incorporation of tropoelastin into the elastin fiber.

Quantum molecular modeling of the elastinic tetrapeptide Val-Pro-Gly-Gly

The free Val-Pro-Gly-Gly tetrapeptide belonging to the Proline-rich sequences of elastin has been studied both theoretically and experimentally. The molecular modelisation was carried out using AM1 and ab initio quantum computations while the conformation in solution was ascertained by circular dichroism spectroscopy performed on the synthesized tetrapeptide. Experimental and theoretical investigations lead to the conclusion that the most probable structure is constituted by a type II beta-turn.

An aggregating elastin-like pentapeptide

Synthetic VGGVG, a "monomeric" unit of the glycine-rich regions of elastin, has been investigated for its molecular and supramolecular properties. In aqueous solution the pentapeptide showed conformational features strongly concentration-dependent. CD and NMR studies suggested a partial unfolding on increasing the concentration. Electron microscopy, on the other hand, evidenced extensive aggregation of the pentapeptide yielding elastin-like supramolecular structures constituted either by twisted ropes or by banded fibrils.

The cysteine residues in the carboxy terminal domain of tropoelastin form an intrachain disulfide bond that stabilizes a loop structure and positively charged pocket

Analysis of purified bovine tropoelastin with Ellman's reagent and [14C]iodoacetamide demonstrated that the only two cysteine residues in the molecule form an intrachain disulfide bond. Molecular modeling suggests that the cysteine residues are juxtaposed as the result of a tight turn that produces an antiparallel beta structure. Protruding from the C-terminal end of the turn is the sequence Arg-Lys-Arg-Lys which forms the floor of a positively charged pocket created by the extension of the arginine and lysine side chains on opposite sides of the peptide chain perpendicular to the plane of the turn. The side chain of a conserved lysine residue in the disulfide-bonded loop forms the top of the pocket. This positively charged pocket may define a binding site for acidic microfibrillar proteins that mediate elastic fiber assembly.

Molecular dynamics study of the conformational behavior of a representative elastin building block: Boc-Gly-Val-Gly-Gly-Leu-OMe

The conformational behavior of the synthetic peptide, Boc-Gly-Val-Gly-Gly-Leu-OMe, containing the X-Gly-Gly and Gly-Gly-X (X = Val or Leu) repeating sequences and constituting a fragment of elastin was investigated by molecular mechanics and molecular dynamics (MD) simulation. The results suggest that, irrespective of the approximations used, the molecule shows a manifold of low energy conformations characterized by gamma-turns and type II beta-turns. Furthermore, MD simulations point out a conformational floppiness due to very low barriers between different conformations. Experimental CD measurements in a virtually apolar medium (dioxane--epsilon = 2.209), which better mimics the vacuum conditions of the simulation, support the theoretical results. The general emerging picture, indicating the molecule as characterized by a combination of flexibility with conformational preferences, is in agreement with previous experimental findings and enriches of new aspects the description of the microscopic behavior of this molecule suggesting more detailed interpretation of previous data.

Alternative splicing of rat tropoelastin mRNA is tissue-specific and developmentally regulated

Sequence analysis of cDNA clones coding for rat tropoelastin previously has identified two variants that potentially corresponded to alternatively spliced tropoelastin mRNAs (Pierce et al., 1990). We have now used S1 nuclease protection analysis of total RNA from aorta, skin and lungs of 10-day and 6-week old rats to localize all sites of alternative splicing in the tropoelastin mRNA and to examine tissue-specific and developmental regulation of the use of these sites. This analysis revealed multiple sites of alternative splicing involving rat tropoelastin coding sequences corresponding to exons 12 through 15 of the bovine tropoelastin gene and a single site of alternative splicing at sequences corresponding to exon 33. Messenger RNAs from all three tissues at both developmental stages were alternatively spliced at the same sites; there was no evidence for the use of an alternative splice site unique to a particular tissue or developmental stage. However, both tissue-specific and developmentally regulated differences were apparent in the proportion of rat tropoelastin mRNA alternatively spliced at exon 33. Tropoelastin mRNA from the aorta and lungs of neonatal rats was alternatively spliced at exon 33 ten time more frequently than tropoelastin mRNA from skin. Between 10 days and 6 weeks of development, the use of this site of alternative splicing decreased by twenty-fold in RNA from skin, ten-fold in RNA from lungs and two-fold in RNA from aorta. In contrast, alternative splicing at exons 12 through 15 occurred in a small percentage of the mRNA and use of these sites exhibited minimal tissue-specific differences or developmental regulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Mammalian tropoelastin: multiple domains of the protein define an evolutionarily divergent amino acid sequence

We have recently derived the complete amino acid sequence of rat tropoelastin from a series of overlapping cDNA clones. Comparison of this protein sequence to bovine and human tropoelastin has revealed significant differences in the rates of evolutionary divergence of the various domains of tropoelastin. The overall rate of divergence of the hydrophobic domains of tropoelastin was twice as fast as the cross-link domains of the protein. Certain hydrophobic domains, however, are as conserved as cross-link regions, particularly the hydrophobic sequence coded for by exon 33, the only exon subject to alternate usage in all three mammalian species and the most conserved domain in rat, bovine and human tropoelastin. This conservation of sequence strongly suggests a more complex function of the hydrophobic region encoded by exon 33, beyond the elastic recoil characteristic of all hydrophobic domains of tropoelastin. A comparison of average rates of divergence of hydrophobic and cross-link domains of tropoelastin to functionally-defined domains of other structural proteins, such as collagen, has also revealed that overall, tropoelastin is a highly divergent amino acid sequence, comparable to proteins such as globin and the fibrino-peptides.

Regulation of elastin gene expression

Recent isolation and characterization of cDNAs encompassing the full length of chicken, cow, and human elastin mRNA have led to the elucidation of the primary structure of the respective tropoelastins. Comparison of the tropoelastin from the different species has revealed that large segments of the sequence are conserved, but considerable variation also exists, ranging in extent from relatively small alterations, such as conservative amino acid substitutions, to large-scale deletions and insertions. Several distinct approaches have yielded compelling evidence of a single elastin gene per haploid genome. Analysis of the bovine and human elastin genes revealed that functionally distinct hydrophobic and cross-link domains of the protein are encoded in separate exons which alternate in the genes. The human gene contains 34 exons, the intron/exon ratio is unusually large (20:1), and the introns contain large amounts of repetitive sequences that may predispose to genetic instability. Comparison of the cDNA and genomic sequences has demonstrated that the primary transcript of both species is subject to considerable alternative splicing, which can account for the presence of multiple tropoelastin isoforms. It is likely that the conformation of elastin is, at least in part, that of a random coil, and therefore it might be expected that the stringency for conservation of the amino acid sequence would be less than that for other proteins with unique conformations. This suggests that functional elastin molecules that vary in their sequence and fitness may exist in the human population and be compatible with a normal life. Potentially though, these variations could have profound consequences on the properties of vital tissues found in the cardiovascular and pulmonary systems over the lifetime of the individual. Consequently, analysis of the structure of the elastin gene and its variation in what is regarded as the normal human population, rather than in those individuals with clearly heritable diseases, assumes greater importance. The 5'-flanking region of the gene is G + C rich and contains several SP-1 and AP2 binding sites, as well as putative glucocorticoid, cAMP, and TPA responsive elements, but no consensus TATA box or functional CAAT box. Primer extension and S1 mapping of the elastin mRNA indicated that transcription was initiated at multiple sites. Transfection experiments using promoter elements/reporter gene constructs demonstrated that the basic promoter element was found within region -128 to -1. In addition, three distinct up-regulatory and two down-regulatory regions were delineated. Taken together, these findings suggest that the regulation of elastin gene expression is complex and takes place at several levels.

Exons--original building blocks of proteins?

In a recent paper, Walter Gilbert's group has estimated the number of original exons from which all extant proteins might have been constructed. The approach used is subjected to a critical analysis here. It is shown that there are flawed assumptions about both the mechanism and generality of exon-shuffling and in the sequence comparison procedures employed, the latter failing to distinguish chance similarity from similarity due to common ancestry. These methodological errors lead to the omission of many known cases of exon-shuffling and the inclusion of others which may not be genuine. In consequence, the analysis from the Gilbert group cannot give a reliable estimate of those modules that actually participated in exon-shuffling and provides no information on the number of protein archetypes that did not participate in these processes.

Tropoelastin heterogeneity: implications for protein function and disease

The organization of the tropoelastin gene is similar to that of other genes coding for matrix proteins in that the exons code for distinct domains of the protein. An unusual feature of tropoelastin expression is that the primary transcript of the gene coding for tropoelastin undergoes extensive, developmentally regulated alternative splicing, resulting in numerous protein isoforms. Although the significance of this heterogeneity is unknown, the multiple sequence variations may affect the function of tropoelastin. Without an understanding of the importance of the domains of tropoelastin and the process of fibrillogenesis, characterization of defects resulting in aberrant elastin production will be hindered. In this update, we review recent findings on tropoelastin and speculate as to the structural and regulatory role of various regions of this matrix protein.

Synthetic fragments and analogues of elastin. II. Conformational studies

Conformational studies on synthetic repetitive sequences and analogues of elastin are described. CD and nmr measurements gave evidence of flexible beta-turns as the dominant structural feature whose stability was found to decrease by increasing the number of repetitive units. The sequences comprised the structural unit Gly-X-Gly (X = Val, Leu, Ala), with X-Gly or Gly-Gly located at the corners of the bend. Based on that, it is proposed that these regions of elastin, unlike the proline-containing sequences, contribute to the elasticity of the protein through a classical mechanism in terms of the rotational isomeric state theory.

Synthetic fragments and analogues of elastin. I. The synthesis

The synthesis of some repetitive sequences of elastin and their simplified analogues, all comprising the structural unit Gly-X-Gly (X = Val, Leu, Ala), is described. In particular, the following peptides and polypeptides were synthesized and characterized: Boc-Gly-Val-Gly-Gly-Leu-OMe, Boc-Gly-Leu-Gly-Gly-Val-OMe, Boc-(Gly-Val-Gly-Gly-Leu)2-OMe, Boc-(Gly-Val-Gly-Gly-Leu)3-OMe, Boc-Gly-Val-Gly-Gly-OEt, Boc-Leu-Gly-Gly-Leu-OMe, Boc-Val-Gly-Gly-Val-OMe, poly(Ala-Gly-Gly), poly(Val-Gly-Gly), and poly(Leu-Gly-Gly). In every case, the synthesis was accomplished by classical procedures in solution, by using the p-nitrophenyl ester method for the polycondensation step, and the mixed anhydride or the azide methods for the coupling steps.

Structure of the elastin gene and alternative splicing of elastin mRNA: implications for human disease

The protein elastin is largely responsible for the elastic properties of vertebrate lungs, large blood vessels, and skin. The structure of the human, bovine, and chick elastin gene and protein monomer, tropoelastin, has recently been elucidated by using techniques of molecular biology. Extensive homology of amino acid sequence exists among the mammalian species and there is in addition strong conservation of nucleotide sequences in the 3' untranslated region of the gene. The translated exons are small and embedded in large expanses of introns. Sequences coding for the hydrophobic regions, responsible for the elastic properties of the molecule, and the alanine-lysine rich regions, responsible for crosslink formation between molecules, reside in separate exons and alternate for the most part in the elastin gene. S1 analyses and sequence analysis of cDNA and genomic clones have indicated that there is substantial alternative splicing of the primary elastin transcript. Variations in the structure of mRNAs resulting from alternative splicing could explain the existence of the multiple forms of tropoelastin observed electrophoretically in several species. Different kinds of splicing patterns could occur in human populations and may contribute to aging and pathological situations in the cardiovascular and pulmonary systems.

Smooth muscle isoactin and elastin in fetal bovine lung

The formation of elastic fiber network in the lung is developmentally regulated. In this study we first demonstrated that tropoelastin mRNA per unit total RNA in the fetal bovine lung increased from 110 to 250 days of gestation (270 day term) as measured by Northern blot analysis. To examine the extent that smooth muscle (SM) type cells contribute to this gestational increase in elastin phenotype, we utilized a dual immunofluorescent staining technique on lung sections with anti-elastin polyclonal and anti-SM isoactin monoclonal antibodies. Elastin staining was always found to localize in proximity to SM isoactin-positive cells at various stages of prenatal lung parenchymal development. Minimal, if any, elastin was seen at interstitial fibroblasts, which were negative for the SM isoactin staining. Distribution of SM (type) cells and elastic fiber together along the airways became sparse and discontinuous distally, and it seemed that formation of air sacs was between the discontinuous elastic fibers. We speculate that smooth muscle (type) cells may be the major elastogenic cells of distal airways and may play an important role in alveolar formation.

Structure of the bovine elastin gene and S1 nuclease analysis of alternative splicing of elastin mRNA in the bovine nuchal ligament

Genomic clones encompassing all the translated sequences, the 3' untranslated sequence, and 1 kb flanking the ATG translation initiation codon of bovine tropoelastin have been obtained and characterized by restriction enzyme analysis and extensive DNA sequencing. These analyses demonstrated that functionally distinct hydrophobic and cross-linking domains of the protein are segregated into separate exons throughout the gene. The putative promoter region lacks a TATA box, has an extremely high G+C content, and contains several SP1 binding sites. Comprehensive S1 analyses using probes covering the entire mRNA and RNA isolated from the nuchal ligament of bovine fetuses of different ages, neonate calves, and adult cows demonstrated that while only a single exon is alternatively spliced at high frequency, many exons are alternatively spliced at limited, variable frequencies. The results also suggest that such limited splicing is increased in the adult tissue relative to fetal and neonate tissues.

Chemotaxis of fibroblasts toward nonapeptide of elastin

Bovine ligamentum fibroblasts, which produce elastin, migrate towards a positive chemical gradient of human platelet-derived growth factor and of the tropoelastin repeat hexapeptide Val-Gly-Val-Ala-Pro-Gly, as previously shown. They are also responsive to two permutations of a nonapeptide that repeats in tropoelastin, i.e., Ala-Gly-Val-Pro-Gly-Phe-Gly-Val-Gly and Gly-Phe-Gly-Val-Gly-Ala-Gly-Val-Pro. Concentration curves and checkerboard assays prove that the nonapeptides are chemoattractants. The component pentapeptide, Gly-Phe-Gly-Val-Gly, is chemotactic, while the component tetrapeptide Ala-Gly-Val-Pro is not. The hexapeptide competitively suppresses the nonapeptide chemotaxis suggesting the involvement of a common cell receptor. The results support the concept that elastin has multiple cell recognition sites as measured by the chemotactic response and that among the hydrophobic repeating sequences of elastin chemotacticity is selectively and multiply localized.

Sequence variation of bovine elastin mRNA due to alternative splicing

Poly A+ RNA, isolated from a single 210 day fetal bovine nuchal ligament, was used to synthesize cDNA by the RNase H method, using AMV reverse transcriptase for first strand synthesis and DNA polymerase I for the second strand. The cDNA was inserted into lambda gt10 using EcoRI linkers, and recombinant phage containing elastin sequences were identified by hybridization with a 1.3 kb sheep elastin cDNA clone, pcSELI (Yoon, K. et al., Biochem. Biophys. Res. Comm. 118: 261-265, 1984). Three clones containing the largest inserts of 2.9, 2.8, and 2.6 kb were selected for further study. The complete sequence analysis of the 3 clones was correlated with the sequence of 10.2 kb of the bovine elastin gene. The analyses: (i) showed that the cDNA encompassed the great majority of the translated sequence, (ii) ordered the tryptic peptides of porcine tropoelastin, (iii) determined new amino acid sequences not previously found in the porcine peptides and (iv) demonstrated that alternative splicing of the primary transcript leads to significant variation in the sequence of the translated portion of the mRNA.

Alternative splicing of human elastin mRNA indicated by sequence analysis of cloned genomic and complementary DNA

Poly(A)+ RNA, isolated from a single 7-mo fetal human aorta, was used to synthesize cDNA by the RNase H method, and the cDNA was inserted into lambda gt10. Recombinant phage containing elastin sequences were identified by hybridization with cloned, exon-containing fragments of the human elastin gene. Three clones containing inserts of 3.3, 2.7, and 2.3 kilobases were selected for further analysis. Three overlapping clones containing 17.8 kilobases of the human elastin gene were also isolated from genomic libraries. Complete sequence analysis of the six clones demonstrated that: the cDNA encompassed the entire translated portion of the mRNA encoding 786 amino acids, including several unusual hydrophilic amino acid sequences not previously identified in porcine tropoelastin, exons encoding either hydrophobic or crosslinking domains in the protein alternated in the gene, and a great abundance of Alu repetitive sequences occurred throughout the introns. The data also indicated substantial alternative splicing of the mRNA. These results suggest the potential for significant variation in the precise molecular structure of the elastic fiber in the human population.

Sequence analysis of elastin cDNA from chick aorta and tissue-specific transcription of the elastin gene in developing chick embryo

A portion of elastin cDNA with a size of 1.5 kilobase pairs (kb) was cloned from chick aorta. Sequence analysis revealed that the cDNA consists of 0.9 kb of coding region and 0.6 kb of 3'-untranslatable region. The primary structure of the peptide deduced from the coding sequence exhibited a strong homology with the published data from sheep and bovine elastin cDNA. The abundance of elastin mRNA in the aorta and skin was studied in developing chick embryo by Northern analysis using the cDNA as a probe. The elastin mRNA level in the aorta gradually decreased in the late half of development, while the elastin mRNA level in the skin was dramatically elevated between the 18th and 21st days. These results strongly suggest that the transcription of the elastin gene was controlled specifically in the respective organ during development.

Intron-dependent evolution: preferred types of exons and introns

Exon insertions and exon duplications, two major mechanisms of exon shuffling, are shown to involve modules that have introns of the same phase class at both their 5'- and 3'-ends. At the sites of intronic recombinations exon insertions and duplications create new introns which belong to the same phase class as the recipient introns. As a consequence of repeated exon insertions and exon duplications introns of a single phase class predominate in the resulting genes, i.e. gene assembly by exon shuffling is reflected both by this nonrandom intron phase usage and by the correlation between the domain organization of the proteins and exon-intron organization of their genes. Genes that appeared before the eukaryote-prokaryote split do not show these diagnostic signs of exon shuffling. Since ancestral introns (e.g. self-splicing introns) did not favour intronic recombination, exon shuffling may not have been significant in the early part of protein evolution.

The gene coding for tropoelastin is represented as a single copy sequence in the haploid sheep genome

The identity of the primary in vitro translation products of fetal sheep nuchal ligament elastin mRNA was confirmed as two distinct polypeptides of 63 Kdal and 65 Kdal in both rabbit reticulocyte and wheat germ extract cell-free translation systems. Both polypeptides were co-translationally processed by a microsomal membrane signal peptidase, with the removal of 20-25 amino acid residues. A single (3,5 kb) RNA species encodes both tropoelastin polypeptides. Restriction endonuclease mapping of sheep genomic DNA by hydridization with two radiolabelled genomic DNA fragments containing sequences coding for sheep tropoelastin (pSE1-1,3 and pSE1-0.7,) indicated the presence of a single elastin gene. The elastin gene copy number was further quantitated by comparison of hybridisation of pSE1-1.3 and pSE1-0.7 to slot-blots and Southern transfers of sheep genomic DNA and to standard curves constructed with each clone. These results clearly demonstrate that each of these sequences is represented only once per haploid genome, suggesting that the two tropoelastin polypeptides are products of a single elastin gene.

Repeating structure of chick tropoelastin revealed by complementary DNA cloning

A cDNA library was constructed from chick aorta poly(adenylic acid)-containing RNA in the expression vector pEX1. Several clones were identified by screening the library with a polyclonal antiserum raised against chick tropoelastin and confirmed by DNA sequencing. Analysis of the deduced amino acid sequence, corresponding to the mature tropoelastin and most of the signal peptide, revealed that the molecule is composed of at least 8, and possibly 13, repeating units. The common features of each unit include an N-terminal region composed largely of alanines and lysines and ending with an aromatic amino acid, followed by a GAG span and then a C-terminal region consisting mostly of valines, prolines, and glycines often present in several copies of the sequence (VPGV). This structure is discussed in terms of the functional properties of the molecule.

Structure of the 3' region of the human elastin gene: great abundance of Alu repetitive sequences and few coding sequences

Two overlapping clones encompassing 8.5 kb of the human elastin gene were isolated from two genomic libraries constructed by partial digestion with either HaeIII/AluI or Sau3A and contained in lambda Charon 4A or EMBL3, respectively. The 6 kb of DNA comprising the most 3' portion of the gene were sequenced demonstrating an extremely low coding ratio since only three exons containing a total of 134 translated nucleotides were identified. Two exons totaling 78 bp of translated sequences which were previously found in the bovine gene were absent in the human gene. The 3' most exon encoded the unusual amino acid sequence, GGACLGKACGRKRK. The human gene was terminated by 1.2 kb of untranslated sequence which contained two polyadenylation attachment signals. The remainder of the 6 kb was composed of intervening sequences which were abundantly rich in Alu family repetitive sequences found in both orientations. This first report of the characterization of the human elastin gene suggests that significant variation in the gene may exist between species and raises the possibility of consequential polymorphism, mediated by recombination between Alu sequences, in the human population.

Temperature dependence of length of elastin and its polypentapeptide

Comparison of the temperature dependence of elastomer length of the cross-linked protein, elastin, and of gamma-irradiation cross-linked poly(VPGVG), the polypentapeptide of elastin, with that of latex rubber demonstrate markedly dissimilar behaviors between a classical rubber and the protein and polypeptide elastomers. In the absence of a load latex rubber expands with increasing temperature as is known for classical rubbers comprised of a network of random chains whereas the protein and polypeptide elastomers markedly decrease in length. When under load with a constant applied force, as a classical rubber, latex linearly decreases length with increasing temperature whereas the decrease in length is very non-linear with temperature increase for the protein and polypeptide elastomers. The protein and polypeptide elastomers examined here do not exhibit the characteristic and fundamental temperature dependence of length considered typical of networks of random chains. Accordingly the more complex and even inverse behavior of elastin and the polypentapeptide of elastin in the absence of load require consideration of structural perspectives different from those of a random chain network with negligible interchain interactions.

Newly determined carboxy terminal sequences in tropoelastin: immunologic identification in insoluble elastin

The carboxy terminal sequence of sheep, bovine and human tropoelastin (GFPGGACLGKA/SCGRKRK) has been inferred in earlier studies from sequencing of cloned complementary and genomic DNA. However, this putative carboxy terminal sequence was not found previously in peptides recovered from tryptic digests of tropoelastin. In order to determine whether the amino acid sequence described above is found in insoluble elastin, antibodies were raised against the chemically synthesized peptides with the appropriate sequences and the antibodies were shown to react with peptides derived from human, bovine, porcine, dog and hamster insoluble elastins. These results strongly suggest that the sequence (GFPGGACLGKA/SCGRKRK) at the carboxy terminus of tropoelastin is found in the elastins of many species.

Characterization of biologically active domains on elastin: identification of a monoclonal antibody to a cell recognition site

Monoclonal antibodies to bovine alpha-elastin were characterized with solid-phase ELISA, Western blot, immunoprecipitation, and immunoaffinity chromatography. One monoclonal antibody, BA-4, bound to insoluble elastin, alpha-elastin, and tropoelastin and to peptide fragments generated by proteolytic digestion of insoluble elastin. Immunoaffinity chromatography of elastin fragments released from insoluble elastin with pancreatic elastase demonstrated that BA-4 was specific for a chemotactically active epitope composed of valine, glycine, alanine, and proline in a molar ratio of approximately 2:2:1:1. This composition matches the Val-Gly-Val-Ala-Pro-Gly repeating sequence in elastin that has been shown to be a chemoattractant for fibroblasts and monocytes. Specific ablation of the chemotactic activity of synthetic Val-Gly-Val-Ala-Pro-Gly by BA-4 IgG confirmed the identity of the epitope recognized by the monoclonal antibody and suggests that, despite its hydrophobic nature, this cell recognition domain is accessible on the surface of elastin and is strongly immunogenic. BA-4 should prove useful for investigating cell surface receptors for elastin.