View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Elsevier - Publisher Connector 0022-202X/82/7901-128s$02.00/0 THE JOURNAL OF INVESTIGATIVE DERMATOLOGY, 79:128s-132s, 1982 Vol. 79, Supplement 1 Copyright © 1982 by The Williams & Wilkins Co. Printed in U.S.A. Structure of the Elastic Fiber: An Overview LAWRENCE B. SANDBERG, M.D., PH.D., NORMAN T. SOSKEL, M.D., AND TERRIL B. WOLT, M.S. Departments of Pathology, University of Utah College of Medicine; and Veterans Administration Medical Center; and the Division of Respiratory, Critical Care and Occupational Medicine, Salt Lake City, Utah, U.S.A. Intense research efforts over the past 18 yr have mals, measurable amounts of a soluble protein which we called probed deeply into the structure of the elastic fiber. This tropoelastin could be isolated [9]. The first such animal from began with the elucidation of the demosine crosslinks in which we obtained significant amounts of this protein was the elastin and the description of the elastin precursor, tro­ copper deficient pig. This was described by Smith, Weissman, poelastin, derived from copper-deficient animals. Char­ and Carnes in 1968 [10], and further characterized by Sandberg, acterization of the precursor material indicates that it is Weissman, and Smith a year later [11]. a single polypeptide chain of approximately 800 amino Table II summarizes the amino acid composition of soluble acid residues containing lysine residues in clusters des­ and insoluble or mature elastin from the pig. Tropoelastin is a tined to form the desmosine crosslinks. The molecule single polypeptide chain of about 800 amino acid residues contains large areas of hydrophobic sequence inter­ (68,000 to 70,000 daltons) which makes it about the size of a spersed with shorter stretches of polyalanine and the serum albumin molecule. From the amino acid composition, lysines. The shorter structures may be folded into alpha­ one can see that glycine, proline, alanine, valine, phenylalanine, helices. The larger hydrophobic areas appear to form a isoleucine, and leucine make up the majority of the amino acid unique structure known as the beta spiral which pos­ residues in the protein. These are all nonpolar residues. It can sesses elastometric properties. Inside the hydrophobic also be seen that the polar amino acids, aspartate, glutamate, areas repeating sequences such as the pentapeptide pro­ lysine and arginine account for less than 5% of the total residues gly-val-gly-val have been observed the exact signifi­ in the mature protein. This large predominance of non-polar cance of which is not appreciated, but it appears to be amino acids is fairly unique. In fact, no known protein contains well-conserved between species. Recent studies in the as much alanine as does elastin (24%). Also noted from Table molecular biology of this protein have indicated that it II, is the absence of sulfur containing amino acids in the soluble is synthesized on the rough ER with a short leader material and the presence of desmosines in the mature elastin sequence of about 25 residues. This is lost before the with a very low content of lysine. The lysine residues of tropo­ tropoelastin is exported. Diversity in sequence studies in elastin (the precursor material) have been consumed in the these leaders suggest that there may be two elastins, formation of the desmosine crosslinks. This mechanism is sum­ type A and B, which vary with the maturation of the marized in Fig 1. animal. Part A is a rough sketch of the polypeptide chain before crosslinking as we envisage it. The large loops represent the hydrophobic stretchable areas composed primarily of the dom­ The elastic fiber is ubiquitous to the mammalian system, inant amino acids, glycine, proline, valine, phenylalanine, iso­ found in almost every organ, and is being increasingly appreci­ leucine and leucine. The small coiled areas represent the ala­ ated for its role in imparting elasticity to the organ, a property nine-rich areas which surround the lysine residues. These are necessary for the function of such tissues as skin, aorta, and probably alpha-helical in nature and contain two lysine residues lung. There are at least 4 heritable connective tissue diseases with two or three alanine residues between as shown in parts B affecting the skin in which an elastin defect appears to play an and C. We think that during the crosslinking process, chains important part. These are summarized in Table I. In X-linked are precisely opposed in such as way that one chain with 2 cutis laxa, Menke's kinky hair syndrome, and Ehlers Danlos alanine residues between 2 lysine residues (chain B) is brought syndrome type V, there appears to be a lysyloxidase defect into direct apposition with another chain with 3 alanine residues which results in poorly crosslinked elastin. This poorly cross­ between 2 lysine residues (chain C). This brings the lysine linked material probably has an increased susceptibility to groups (their side chains) into the right spatial relationships so tissue proteases perhaps because of its unusually high lysine that the enzyme lysyloxidase can convert 3 of their epsilon­ content. The nature of elastin's molecular structure and the amino groups to aldehydes as shown in parts D and E, leaving way in which the molecules could be altered in these disease one of the lysine residues with its original epsilon-amino group states have been studied in our laboratory at the University of intact. This sets the stage for a spontaneous condensation as Utah over the past 13 yr. shown in part F to form the stable desmosine crosslink. I say The birth of the molecular biology of elastin, dates back to spontaneous, but actually if one tallys up the protons before 1963 when Thomas, Elsden, and Partridge [8] described the and after the condensation taking into account the formation unusual elastin crosslinks, desmosine and isodesmosine. This of three molecules of water, an oxidation step has to take place description started a flurry of activity into exploration in depth to form the three double bonds of the pyridinium ring. Another of the molecular nature of the protein. Experimental copper enzyme(s) must, therefore, be involved besides lysyloxidase for deficiency and lathyrism in laboratory animals through the this final product to be formed. For this reason and because inactivation of lysyloxidase was soon recognized to produce a lysyloxidase appears to have more than one substrate, we prefer "weak elastin" with a poor crosslink content. From such ani- to think of it as an enzyme system, a whole family dedicated to the formation of connective tissue crosslinks. The desmosine, (79-UT-517) Utah This work was supported by a grant from the as shown in part G, is a tetrafunctional amino acid which Heart Association, by grants (HL 11963 and HL 22446) from the imparts a permanence to the protein not recognized in most National Institutes of Health, and by the Veterans Administration. other body proteins. The half life of elastin probably exceeds Dr. Soskel was formerly a Fellow of the American Lung Association and is the current recipient of a Pulmonary Academic Award Grant (1 that of the life of the individual. This allows a continued K07 00617) from the National Heart, Lung, and Blood Institute. responsiveness to the dynamic changes of the organ in which Reprint requests to: Dr. Lawrence B. Sandberg, Pathology Depart­ the fiber occurs, a responsiveness which must be preserved to ment (113), VA Medical Center, Salt Lake City, UT 84148. maintain a healthful state of that organism. 128s July 1982 STRUCTURE OF THE ELASTIC FIBER 1298 TABLE I. Effects of disease on elastin and related features Histologic Functional Biochemical Etiologic Disease abnormality abnormality abnormality defect Pseudoxan­ Fragmented elastic fibers; i compliance of skin and Polyionic deposits on Speculation: polyionic deposits (pos­ thoma elasti­ granular deposits in place of fragile vessels [2]. elastic fibers [1]. sibly glycoproteins) attract calcium cum amorphous elastin [2]. and cause elastin damage [1]. X-linked cutis In dermis, � numbers of elastic i compliance of skin; occa­ Possible � crosslinks in � lysyloxidase activity [5]. laxa fibers, which are thin, sional pulmonary emphy­ elastin [4]. clumped, granular, and of­ sema [4]. ten fragmented; normal mi­ crofibrils [1-4]. Menke's kinky­ Fragmented vascular elastic Tortuous vessels with occa­ Possible � cross-links in � copper absorption in gut, leading to hair syn­ lamella 4, � amorphous elas­ sional occlusions and hy­ elastin and collagen � lysyloxidase activity [6]. drome tin in skin, with relative i of popigmented brittle skin [4]. microfibrilsin skin and [4]. aorta and clumped elastin in aorta [6]. Ehlers-Danlos No histologic studies pub­ Hyperextensible skin and Possible � cross-links in � lysyloxidase and possibly i elastin syndrome lished. joints [4]. collagen and elastin degradation [7]. [4]. a Adapted from Sandberg, Soskel, Leslie: N Engl J Med 304:566-579, 1981, with permission of the publisher. TABLE II. Amino acid composition of soluble and mature (insoluble) small ones (shown below) contain polyalanine and lysine and elastin from the pig. a represent the repeating structures mentioned Amino acid Soluble elastin Mature elastin Sequence Mol per mol protein No. of residues/BOO residuesb Glycine 245 256 AAAK 6 Alanine 187 181 AAK 6 Proline 91 90 SAK 2 Hydroxyproline 7 8 APGK 2 Valine 103 92 AK 1 Isoleucine 14 14 (K) YGAR 2 Leucine 41 41 Tyrosine 14 12 responsible for the demosine crosslinking portion of the mole­ Phenylalanine 24 25 cule. There are approximately 6 each of the ala-ala-Iys and ala­ Arginine 4 5 ala-ala-Iys peptides per molecule of tropoelastin. The large Lvsine 37 5' fragments elutein the early part of the chromatogram, generally C�osslinksd Very low 25 Aspartic acid and asparagine 3 5 decreasing in size as the chromatogram develops.