Anterior lens capsule collagens: cyanogen bromide peptides of the C chain

Migratory interaction of amphibian epidermal cells with components of the basement membrane

In adult newts, basal epidermal cells adjacent to a fresh wound move toward the damaged area by migrating over the epidermal basement membrane. In an attempt to determine which basement membrane components mediate this migration, small pieces of glass coated with various natural matrices, purified proteins, or fragments of proteins were implanted into skin wounds such that epidermal cells attempting to form a wound epithelium would encounter the implants. Laminin derived from a cell line (M1536-B3) that produces no type IV collagen was inactive as a migration substrate. Migration on recombinant entactin was somewhat better than on laminin but was still only approximately 14% of that on type I collagen. M15 matrix, a laminin and entactin-containing product of M1536-B3 cells, was no better than entactin alone. Type IV collagen was an excellent substrate, producing slightly more migration than corresponding concentrations of type I collagen at nearly all concentrations tested. Migration on type IV lacking the NC1 domain was at least as good as on intact type IV. All the activity in type IV was present in a 95 kD fragment (alpha 1(IV)95) from the carboxy terminal two-thirds of the alpha 1 chain. Approximately 60% of the activity on alpha 1(IV)95 was obtained on implants coated with a 110 amino acid fragment of the alpha 1 chain derived from the carboxy terminal half of alpha 1(IV)95. Adding the synthetic peptide, arg-gly-asp-ser (RGDS) to the medium, blocked migration on fibronectin-coated implants but had no effect on implants coated with type IV, suggesting that migration on type IV involves different cell surface receptors than those mediating migration over fibronectin. Matrigel, a commercial product containing most basement membrane components, was a poor migration substrate. Thus if type IV mediates basal cell migration toward a wound in vivo, there may have to be some alterations in basement membrane structure to allow epidermal receptors to access type IV active site(s).

Amino acid sequence of the N-terminal aggregation and cross-linking region (7S domain) of the alpha 1 (IV) chain of human basement membrane collagen

The amino acid sequence of the 216-residue-long N-terminal aggregation and cross-linking 7S domain of the alpha 1 (IV) chain of human placental basement membrane collagen is presented. The N terminus of the alpha 1 (IV) chain starts with a non-triple-helical region, which is at least 15 residues long and contains four cysteine and two lysine residues as putative cross-linking sites. This segment is followed by a 120-residue-long triple helical region, which contains the unusual occurrence of a cysteine residue in the Xaa position of a Gly-Xaa-Yaa triplet. Since individual molecules in the 7S domain are associated in an antiparallel manner, this cysteine probably aligns with one of the four cysteines in the amino-terminal end of an adjacent molecule, forming an intermolecular disulfide bridge. The length of the overlap of two adjacent molecules is estimated to be about 110 residues. The triple helix adjacent to the overlap zone is interrupted by a 10-residue-long non-helical area, which is probably responsible for the flexible region of the molecules in the neighbourhood of the overlap zone observed in the electron microscope. The mode of aggregation of the 7S domain, the formation of intermolecular cross-links as well as the relatively high stability of this region against proteolytic attack are discussed in the light of the elucidated amino acid sequence.

Biosynthesis and in vitro translation of type IV procollagens

The present paper describes how epithelial cells, cultured from bovine anterior lens capsule explants, synthesize and secrete procollagen type IV polypeptide chains alpha 1(IV) and alpha 2(IV). Metabolic labeling of these cells with [14C]proline for different time intervals and subsequent analysis by SDS/polyacrylamide gel electrophoresis revealed the presence of two polypeptide chains with apparent molecular masses of 180 kDa and 170 kDa. The procollagens were bacterial-collagenase-sensitive and were specifically immunoprecipitated by antibodies raised against the 7S domain of type IV collagen. Type IV procollagen poly(A)-rich RNA was isolated from cultured lens capsule cells and translated in a reticulocyte lysate cell-free system. Two polypeptides with apparent molecular masses of 152 kDa and 145 kDa were identified as procollagen type IV unmodified chains by gel electrophoresis, collagenase digestion and specific immunoprecipitation. During experiments in which cells were labeled in the presence of alpha, alpha'-bipyridyl, type IV procollagen appeared as one major band comigrating with a 145 kDa polypeptide on SDS-gel electrophoresis.

Isolation and partial characterization of a novel basement membrane collagen

A guanidine-HCl extraction of lens capsule basement membrane dissolves collagenous material. This material was fractionated on an Agarose A-5M column. Fractions 1, 2 and 3 were further purified and partially characterized immunochemically and by amino acid analysis. Fraction 3 has a molecular weight of 55,000 when compared with collagen type I standard. The CNBr peptide pattern and composition of fraction 3 are different from those of alpha 1 (IV) 95K and alpha 2 (IV) 95K chains. The results described suggest the presence of a new chain in lens capsule basement membrane.

Ultrastructural investigation of extracellular structures in subcapsular white corrugated cataract (anterior capsular cataract)

Electron microscopic investigations were performed on six extracted lenses from patients undergoing operative treatment of subcapsular white corrugated cataract. The lens capsule itself was unaltered. There was a pronounced extracellular space under the capsule. In this area collagenous aggregations equivalent in size to collagenous fibres and fibrils could be seen. Collagenous microfibrils and delicate microfilaments were also present. The latter could be observed with and without cross-striations. The fibrous structures are distinguished by considerable variations in shape and diameter. The precursor stages of these fibrous materials are produced by myofibroblast-like cells, probably derived from the lenticular epithelium.

Structural studies of human basement-membrane collagen with the use of a monoclonal antibody

A monoclonal antibody monospecific for human type IV collagen was used as a structural probe to examine aspects of the macromolecular organization of basement-membrane collagen. Electron-microscopic observation of rotary-shadowed antigen-antibody complexes demonstrated a unique binding site for the antibody 55 +/- 6 nm distant from the 7S cross-linking region of tetrameric type IV collagen. This observation allowed a series of studies that showed: (1) the localization of an intramolecular disulphide bridge within the helical domain of the molecule, (2) the alignment of major peptic-digest fragments of the alpha 1 (IV) chain, and (3) confirmation of the postulated antiparallel arrangement of individual molecules within type IV collagen tetramers.

Amino acid sequence of the non-collagenous globular domain (NC1) of the alpha 1(IV) chain of basement membrane collagen as derived from complementary DNA

NC1, the C-terminal non-collagenous globular domain of collagen IV, represents one of the two end regions responsible for the assembly and cross-linking of the extracellular network of basement membrane collagen. Several cDNA clones for the NC1 domain of the alpha 1(IV) collagen chain of mouse have been isolated by using synthetic oligonucleotides as screening probes for mouse libraries. The oligonucleotides were synthesized according to known stretches of the corresponding protein sequence. Sequencing of the overlapping cDNA clones allowed the complete amino acid sequence of the NC1 domain to be deduced as well as the C-terminal 165 amino acid residues of the triple helix. It consists of 229 amino acid residues which comprise two homologous regions with a high content of cysteine. These DNA and protein sequences are compared to the corresponding sequences of other collagens and discussed with respect to their structural and biological significance.

Structure of human-basement-membrane (type IV) collagen. Complete amino-acid sequence of a 914-residue-long pepsin fragment from the alpha 1(IV) chain

The complete amino acid sequence of the 914-residue-long pepsin fragment alpha 1 (IV)95 from the alpha 1 chain of human placental basement membrane (type IV) collagen is presented. This sequence contains 12 interruptions of the collagenous triplet sequence Gly-Xaa-Yaa which varied in length from 1 to 11 residues. The distribution of amino acids between the Xaa and Yaa position was similar to that found in interstitial collagens but the extent of proline and lysine hydroxylation differed. Computer comparisons of the alpha 1 (IV)95 sequence with those of the interstitial collagen chains did not reveal any homology, whereas a comparison with the partial sequences of mouse tumor and bovine lens capsule alpha 1 (IV) showed an approximately 85% identity. The unique sequence characteristics of type IV collagen are discussed in relation to its macromolecular structure and to the interstitial collagens.

Sequence comparison of pepsin-resistant segments of basement-membrane collagen alpha 1(IV) chains from bovine lens capsule and mouse tumour

The C-terminal peptic fragment P1 (about 518 amino acid residues) of bovine lens-capsule collagen alpha 1(IV) chain was cleaved with CNBr and trypsin. The peptides were purified and characterized, allowing their ordering within the P1 fragment by comparison with a corresponding section of mouse collagen alpha 1(IV) chain [Schuppan, Glanville & Timpl (1982) Eur. J. Biochem. 123, 505-512]. About 67% of the sequence of bovine collagen fragment P1 was determined by Edman degradation. Comparison with the sequence of the corresponding mouse collagen fragment P1 showed 76% identity for positions Xaa and Yaa of the triplet structures Gly-Xaa-Yaa. Invariance was found for the positions of two non-triplet interruptions and of 3-hydroxyproline residues, pointing to the functional importance of these structures.

Isolation and characterization of the cyanogen bromide peptides of the collagen 2 alpha chain from a transplantable rat chondrosarcoma

The rat collagen 2 alpha chain was isolated from a transplantable Swarm chondrosarcoma following limited pepsin proteolysis. The chromatographically purified 2 alpha chain when cleaved with cyanogen bromide yields nine peptides which have been isolated and characterized with respect to their molecular weight and their amino acid composition. Eight of these peptides can be rapidly separated by gel-permeation high-performance liquid chromatography and retrieved for further purification by ion-exchange chromatography. The nine peptides are recovered in equimolar amounts and together account for a total of 1,014 amino acid residues. The features of the isolated cyanogen bromide peptides of the 2 alpha chain clearly differentiate it from other known collagen polypeptides. The possible homologies between the 2 alpha chain and other collagen alpha chains are noted. Comparison of the cyanogen bromide peptides indicates a close relationship of the 2 alpha to the alpha 1(V) chain.

Altered expression of collagen phenotype in osteoarthrosis

Immunofluorescence studies have shown the presence of collagen type III in addition to types I and II in osteoarthrotic cartilage. The presence of collagen type III was verified biochemically: collagen was isolated and purified from the pronase digest of osteoarthrotic cartilage. Mixtures of different collagen types were fractionated first by two different DEAE-cellulose runs and then the collagen polymers were isolated by molecular sieve chromatography. Afterwards the collagenous material was reduced, Alkylated, and rechromatographed on an agarose column. Three major peaks corresponding to gamma-, beta- and alpha-chains were observed. Amino acid analyses and the CNBr peptide pattern indicated the identity of the alpha peak as alpha 1 (III). Additional staining of control and disease cartilages for fibronectin revealed the presence of this protein in the diseased tissue.

Precursor-size components are the basic collagenous subunits of murine tumor basement membrane

The acid-insoluble fraction of murine tumor basement membrane produced by the EHS sarcoma was extracted following reduction of disulfide bonds under non-denaturing conditions. Extracts prepared from tumors grown in either lathyritic or normal animals contain two procollagen-like chains as well as cross-linked oligomers of these chains. Extracted monomeric chains behave almost exactly like reduced, radiolabeled EHS collagen precursor chains on SDS-PAGE and SDS-agarose suggesting that they are very similar collagens. Since the collagen extracted after reduction represented a significant proportion of the total tumor collagen, it is likely that tumor basement membrane is assembled from precursor-size Type IV collagen molecules.

A network model for the organization of type IV collagen molecules in basement membranes

Type IV collagen was solubilized from a tumor basement membrane either by acid extraction or by limited digestion with pepsin. The two forms were similar in composition and the size of the constituent chains but differed when examined by electron microscopy and in the fragment pattern produced by bacterial collagenase. The acid-soluble form showed after rotary shadowing strands mainly of a length of 320 nm which terminated in a globule, or two strands connected by a similar globule. The globule was identified as a non-collagenous domain (NC1) which under dissociating conditions could be separated into two peptides showing a monomer-dimer relationship. Higher aggregates of NC1 were visualized under non-dissociating conditions. Some of the acid-extracted molecules have retained the previously 7-S collagen domain. The pepsin-solubilized form lacked domain NC1 and consisted mainly of four triple-helical strands (length 356 nm) joined together at the 7-S domain (length 30 nm). Common to both forms of type IV collagen was a small collagenase-resistant domain NC2 which was composed of collagenous and non-collagenous elements and located between the 7-S domain and the major triple helix. These data indicate that the collagenous matrix of basement membranes consists of a regular network of type IV collagen molecules which is generated by two different interacting sites located at opposite ends of each molecule. The 7-S collagen domain connects four molecules while the NC1 domain connects two molecules. The maximal distance between identical cross-linking sites (7-S or NC1) was estimated to be about 800 nm comprising the length of two molecules.

Type IV collagens' isolation and characterization of 7S collagen from human kidney, liver and lung

7S collagens were isolated after bacterial collagenase treatment of basement membrane material prepared from the pepsin digest of human kidney, liver and lung. The 7S collagens were purified by combination of molecular sieve and ion exchange chromatography. 7S collagen from each of these sources showed similar amino acid composition, electrophoretic patterns of reduced and unreduced samples. Antibodies raised against human placental 7S collagen in rabbits completely cross-reacted with the 7S collagen preparation obtained from kidney, liver and lung in enzyme-immunoassays. Since 7S collagen in known to be a major cross-linked segment of type IV collagens, the data indicate that collagenous proteins of basement membranes are organized in similar networks in a variety of organs.

Basement membrane collagen from bovine lung: its chain associations as observed by two-dimensional electrophoresis

A basement membrane collagen has been isolated from limited pepsin digests of bovine lung alveolar tissue by differential salt fractionation and DEAE-cellulose chromatography. Fractionation and characterization of the peptide chain fragments of the collagen have revealed the presence of two classes of collagen chains: one is 160K-1, 100K-1, and 80K-1 and the other is 100K-2 and 80K-2. Apparent isoelectric points of these chain fragments were 8.4, 8.6, 8.35-8.45, 8.9-9.0 and 8.9-9.0, respectively. Electrophoretic analysis on temperature dependent reductions of the collagen and its denaturation products has revealed the presence of disulfide bonds between collagen chain fragments of 160K-1-80K-2 and possibly 160K-1-160K-1 which require elevated temperatures for reduction and the bonds within 100K-2 fragments (80K-2-small fragments such as 10K-20K) and possibly between 80K-2-80K-2 which are reducible without elevated temperatures.

Immunological characterization of the 7-S domain of type IV collagens

Antisera were raised against the long and short form of mouse and human 7-S collagen and against type IV collagens solubilized by acid extraction or limited digestion with pepsin. All the antisera showed strong binding for 7-S collagen in radioimmunoassays demonstrating that the 7-S domain which serves as a cross-linking region of type IV collagen is the most immunogenic portion of the molecule. Cross-reaction studies and analysis of fragments showed a complex antigenic structure including some determinants common to the long and short form of 7-S collagen and others unique for the long form. Purified antibodies against 7-S collagen reacted in indirect immunofluorescence with almost all basement membranes of the body indicating that the 7-S domain is a common structural element of type IV collagens.

Cell-free synthesis of putative type V procollagen chains programmed by Chinese hamster lung cell mRNA

A messenger RNA fraction isolated from cultured Chinese hamster lung (CHL) cells programs in a cell-free system prepared from wheat germ the efficient incorporation of [14C] proline into newly synthesized protein with a significant fraction of the incorporated substrate being digestible with bacterial collagenase. This reaction requires both subcellular fractions, an energy source, and is inhibited by the antibiotic puromycin. The relative amount of collagenase-digestible to non-digestible cell-free product depends upon the ratio of CHL mRNA to wheat germ lysate, is not affected by either the Mg2+ or K+ concentrations employed, and under optimal condition, approximately 38% of the total incorporated substrate is collagenase-sensitive. Electrophoresis on SDS-polyacrylamide gels of the products programmed by CHL mRNA indicates that the collagenase-digestible material corresponds in size to a procollagen chain with an apparent molecular mass of approximately 170,000 daltons. These studies suggest that the collagen alpha 1 (V) chain is initially synthesized as a precursor procollagen chain and demonstrate that a significant amount of the mRNA in Chinese hamster lung cells codes for this protein.

The structure of acid-soluble basement membrane collagen from bovine anterior lens capsule: molecular parameters and thermal gelation properties

Basement membrane collagen was extracted from bovine anterior lens capsules with acetic acid in the presence of protease and proteinase inhibitors. This undegraded soluble basement membrane has a sedimentation coefficient S0 20,w=7.8 X 10-13 sec-1, a diffusion coefficient D020,w=5.82 X 10-8/ cm2/sec and a zero-shear intrinsic viscosity [eta] = 2200 cm3/g. These data lead to a molecular weight, Mr of 1.1 X 10(6). Electron microscopic observation indicates that the molecules are rod-like in character with a length of 300 nm. Theoretical models describing the intrinsic viscosity in terms of the distribution of mass in such an elongated molecule show that a dumbbell-like structure is the only tenable model. In this model a single 300 nm rod-like region joins two more densely packed globular regions. The globular domains contain hydroxyproline and may has collagenase sensitive regions since digestion with purified bacterial collagenase at 37 degree C for 18 h degrades these domains as well as the central triple-helical domain. The intact acid-soluble basement membrane collagen precipitates from neutral pH solutions upon warming 4 degree C stable solutions. The pH, ionic strength and concentration dependence parameters of this thermal precipitation have been examined. The basement membrane collagen precipitation appears to be dominated by end-region domain hydrophobic interactions. The end-regions are required for precipitation in contrast to the Type I procollagen situation wherein the intact end-regions inhibit thermal gelation. Electron microscopic examination of the basement membrane thermal gels shows networks of very thin filaments which may be related directly to the structure within intact basement membrane.

Biosynthesis of type IV and V (alpha A-alpha B) collagens by human placenta

The collagenous proteins synthesized by placenta in organ culture were characterized. Types I and III collagen accounted for about two-thirds of the collagenous protein produced by the tissue while type IV procollagen comprised around 10%. Type IV collagen contained two chains of 185,000 and 175,000 daltons which are genetically distinct from one another as determined by a peptide mapping procedure. The type IV procollagen was identical to that produced by other tissues based on ratios of 3- to 4-hydroxyproline and hydroxyproline to proline, and on the pattern produced upon polyacrylamide gel electrophoresis before and after peptide mapping procedures. About 20% of the collagen resembled type V collagen in the proportions of 3- and 4-hydroxyproline to proline and of hydroxylysine to lysine, in solubility, and in peptide maps. However, it contained disulfide linked chains larger than those found in the type V collagen solubilized by pepsin. Following pepsin treatment, the disulfide bonds were removed and the mobility of the chains of the labeled protein resembled those in type V collagen. It is likely that the disulfide linked protein represents the intact type V collagen molecule.

Collagen molecules comprised of alpha 1(V)-chains (B-chains): an apparent localization in the exocytoskeleton

Antibodies specific for alpha 1(V) chains and native collagen molecules containing the alpha 1(V) chain have been used to study the localization of alpha 1(V)-containing molecules in differentiating hyaline cartilage. Immunofluorescence data show that the undifferentiated mesenchyme contains significant quantities of these molecules throughout the cell-rich tissue matrix. Examination of fully differentiated hyaline cartilage reveals a unique staining pattern wherein the immunofluorescent material is restricted to the pericellular matrix within the chondrocyte lacunae. We conclude from these data in conjunction with other evidence that the Type V collagens function as components of an exocytoskeleton for connective tissue cells.

Immunochemical distinction between two different chains of type IV collagen

Antisera against mouse and human basement-membrane type IV collagen showed in radioimmunoassays distinct binding with large pepsin fragments obtained from the C-terminal portions of alpha 1 (IV)- and alpha 2 (IV)-chains. These reactions were specific for each constituent polypeptide chain. The data were confirmed by immunoadsorption, allowing the separation of antibodies with restricted chain specificity. Inhibition assays with CNBr peptides demonstrated the different localizations of antigenic determinants, which were either species-specific or shared by the human and mouse antigens.

Chinese hamster lung cells synthesize and confine to the cellular domain a collagen composed solely of B chains

The acid-soluble collagen extracted from cultured Chinese hamster lung (CHL) cell layers has been isolated after limited pepsin digestion and differential salt fractionation. Polyacrylamide gel electrophoresis of this material under denaturing conditions showed the presence of collagen chains with an apparent molecular mass of 120,000 daltons both before and after reduction, indicating the absence of interchain disulfide bonds in the native molecule. When chromatographed on CM-cellulose under denaturing conditions, the majority (> 90%) of the CHL cell layer collagen chains eluted as relatively basic components slightly before the human alpha 2(I) chain and coincident with the human B chain. In addition, the CM-cellulose elution profiles of the cyanogen bromide peptides derived from the human B chain and from the CHL cell layer chain were essentially identical. Examination of CHL cells in culture by using affinity-purified antibody to human B chain revealed this collagen to be localized in an extracellular matrix surrounding the cells. Furthermore, analysis of the culture medium indicated the absence of any comparable collagen chain. These data provide additional evidence for the existence of a molecular form of collagen composed solely of B chains and suggest that this molecular form of collagen has an unusual affinity for the cell layer in this system.

Type IV collagen from chicken muscular tissues. Isolation and characterization of the pepsin-resistant fragments

Type IV collagen has been isolated from adult chicken gizzard after limited pepsin digestion and subsequent differential salt fractionation in acidic and neutral conditions. After denaturation, three fragments (called F1, F2, and F3) were isolated by agarose gel filtration and carboxymethylcellulose chromatography. F1 and F2 possessed apparent molecular weights of 53 000 and 50 000, respectively, and were consistently isolated in a 2:1 proportion. F3 was larger and after reduction of disulfide bonds gave rise to three fragments (called F3A, F3B, and F3C) of apparent molecular weights 68 000, 40 000, and 29 000. No alpha-chain-sized components of Type IV collagen were observed. A native fraction containing F1 and F2, but no F3, was isolated after extraction using less pepsin and an additional salt fractionation in acidic conditions. F1 and F2 in the native form were not separated by carboxymethylcellulose or diethylaminoethylcellulose chromatography performed in nondenaturating conditions or by differential salt precipitation in acidic or neutral conditions; these results suggest that F1 and F2 arise as a single native component of structure (F1)2F2. The fraction containing F1 and F2 also gave rise to a single segment long spacing crystallite pattern and to a circular dichroism spectrum which was typical for a native collagen. F1 and F2 were also isolated from chicken heart, blood vessels, and skeletal muscle, whereas from bovine aorta, using the same isolation procedures, two alpha-chain-sized components were obtained, which appeared to be similar to the two Type IV chains recently described by other groups. The data suggest that (i) pepsin fragmentation of type IV collagen from chicken tissues occurs in a different manner compared to Type IV collagen from mammalian tissues and (ii) for the chicken there must be at least two Type IV chains which are assembled into a single native molecule.

7-S collagen: characterization of an unusual basement membrane structure

A new type of collageneous structure, tentatively named 7-S collagen, was isolated from a mouse tumor basement membrane, mouse and human placenta, bovine lens capsule and human kidney. The protein was solubilized from the tissues by limited digestion with pepsin or trypsin and could easily be separated from other collageneous protein because of its resistance towards further degradation by bacterial collagenase at 20 degrees C. 7-S collagen showed an amino acid composition typical of basement membrane collagen and contained 22% carbohydrate mainly as glucosyl-galactosyl bound to hydroxylysine but also some mannose and glucosamine. Ultracentrifugal analysis demonstrated that the proteins were homogeneous with a sedimentation coefficient of about 7.2 S and with a molecular weight of about 360,000 both in phosphate buffer pH 7 and 6 M guanidine. The peptide was triple helical as shown by circular dichroism and exhibited a biphasic melting profile indicating two conformationally distinct domains with tm = 48 degrees C and 70 degrees C. The more stable domain could be isolated as an homogeneous fragment (Mr = 225,000) after a second digestion with collagenase at 37 degrees C. This fragment contained all the disulfide bonds (42 Cys/1,000 residues) of the original molecule. Electron microscopy showed a rod-like structure in agreement with the hydrodynamic properties of 7-S collagen. The dimensions of these peptides were 3 X 95 nm (long form) and 2.4 X 40-50 nm (short form). Complete reduction of 7-S collagen under denaturing conditions produced several polypeptide chains in the molecular weight range of 27,000-153,000 which differ from each other by Mr increments 25,000-27,000. Separation of the chains on agarose did not reveal any simple stoichiometric relationship indicating that some chains are either cross-linked or represent fragments produced during proteolytic treatments. Complete reduction of 7-S collagen under non-denaturing conditions lowered the thermal transiton of the triple helix to 48 degrees C but did not change its molecular weight except when exposed to dissociating solvents. 7-S collagens were potent immunogens and could be characterized by radioimmunoassays. Antigenicity was slightly reduced by reduction and denaturation while collagenase at 37 degrees C produced a larger decrease. Proteins obtained from various sources showed distinct immunological relationships although interspecies differences in affinity exist. No or only little cross-reaction was observed with type IV and V collagens and some further fragments of basement membrane collagen. The data indicate that 7-S collagen is a unique component of basement membranes which shows a more compact and stable structure than other collageneous proteins.