Structural and immunological characterization of type IV collagen isolated from chicken tissues

Previously, a type IV collagen fraction was isolated from chicken gizzard and further fractionated into three components called F1, F2 and F3 [Mayne, R. and Zettergren, J.G. (1980) Biochemistry, 19, 4065-4072]. F1 and F2 were consistently isolated in a 2:1 proportion, and the existence of a small native fragment of structure (F1)2F2 was proposed. In the present series of experiments, a type IV collagen fraction was isolated from the chicken kidney and shown to consist almost entirely of F1 and F2 which were again present in a 2:1 proportion. Identical one-dimensional peptide maps for F1 and F2 from both sources were obtained by polyacrylamide gel electrophoresis of peptides obtained after cleavage with cyanogen bromide or Staphylococcus aureus V8 protease. The denaturation temperature of a preparation containing F1 and F2 in native form was determined by optical rotatory dispersion and a single melting curve was observed with a melting temperature of 33 degrees C. This result provides further supportive evidence that F1 and F2 exist as a native fragment (F1)2F2. Antibodies were prepared in rabbits against a type IV collagen fraction isolated from chicken gizzard, and immunofluorescent staining of a wide variety of basement membranes was demonstrated. Experiments were performed in which various type IV collagen fractions were observed in the electron microscope after rotary shadowing. The lengths of (F1)2F2 and F3 were 147 nm and 174 nm respectively, the sum of these lengths (321 nm) corresponding closely to the length of the major triple-helical domain of type IV collagen (326-328 nm). A specific cleavage site was located at a distance of 215 nm from the 7-S domain which, together with the length of (F1)2F2, gives a total length of 362 nm. This value corresponds closely to the maximum length of the arms which originate from the 7-S domain (355 nm) when type IV collagen was solubilized with a low concentration of pepsin. The results suggest that (a) type IV collagen isolated from the chicken gizzard is closely related, if not identical, to type IV collagen isolated from other tissues; (b) there is a single type IV collagen molecule of chain organization[alpha 1(IV)]2 alpha2(IV); (c) the order of the pepsin-resistant fragments within a type IV molecule is 7S-F3-(F1)2F2.     

Monoclonal antibodies against chicken type IV and V collagens: electron microscopic mapping of the epitopes after rotary shadowing

The location of the epitopes for monoclonal antibodies against chicken type IV and type V collagens were directly determined in the electron microscope after rotary shadowing of antibody/collagen mixtures. Three monoclonal antibodies against type IV collagen were examined, each one of which was previously demonstrated to be specific for only one of the three pepsin-resistant fragments of the molecule. The three native fragments were designated (F1)2F2, F3, and 7S, and the antibodies that specifically recognize each fragment were called, respectively, IA8 , IIB12 , and ID2 . By electron microscopy, monoclonal antibody IA8 recognized an epitope located in the center of fragment (F1)2F2 and in tetramers of type IV collagen at a distance of 288 nm from the 7S domain, the region of overlap of four type IV molecules. Monoclonal antibody IIB12 , in contrast, recognized an epitope located only 73 nm from the 7S domain. This result therefore provides direct visual evidence that the F3 fragment is located closest to the 7S domain and the order of the fragments must be 7S-F3-(F1)2F2. The epitope for antibody ID2 was located in the overlap region of the 7S domain, and often several antibody molecules were observed to binding to a single 7S domain. The high frequency with which antibody molecules were observed to bind to fragments of type IV collagen suggests that there is a single population of type IV molecules of chain organization [alpha 1(IV)]2 alpha 2(IV), and that four identical molecules must form a tetramer that is joined in an antiparallel manner at the 7S domain. The monoclonal antibodies against type V collagen, called AB12 and DH2 , were both found to recognize epitopes close to one another, the epitopes being located 45-48 nm from one end of the type V collagen molecule. The significance of this result still remains uncertain, but suggests that this site is probably highly immunoreactive. It may also be related to the specific cleavage site of type V collagen by selected metalloproteinases and by alpha-thrombin. This cleavage site is also known to be located close to one end of the type V molecule.     

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.     

Domain and basement membrane specificity of a monoclonal antibody against chicken type IV collagen

A monoclonal antibody, IV-IA8, generated against chicken type IV collagen has been characterized and shown to bind specifically to a conformational-dependent site within a major, triple helical domain of the type IV molecule. Immunohistochemical localization of the antigenic determinant with IV-IA8 revealed that the basement membranes of a variety of chick tissues were stained but that the basement membrane of the corneal epithelium showed little, if any, staining. Thus, basement membranes may differ in their content of type IV collagen, or in the way in which it is assembled. The specificity of the antibody was determined by inhibition ELISA using purified collagen types I-V and three purified molecular domains of chick type IV collagen ([F1]2F2, F3, and 7S) as inhibitors. Only unfractionated type IV collagen and the (F1)2F2 domain bound the antibody. Antibody binding was destroyed by thermal denaturation of the collagen, the loss occurring at a temperature similar to that at which previous optical rotatory dispersion studies had shown melting of the triple helical structure of (F1)2F2. Such domain-specific monoclonal antibodies should prove to be useful probes in studies involving immunological dissection of the type IV collagen molecule, its assembly within basement membranes, and changes in its distribution during normal development and in disease.     

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.     

The role of the main noncollagenous domain (NC1) in type IV collagen self-assembly

Type IV collagen incubated at elevated temperatures in physiologic buffers self-associates (a) via its carboxy-terminal (NC1) domain, (b) via its amino-terminal (7S) domain, and (c) laterally; and it forms a network. When examined with the technique of rotary shadowing, isolated domain NC1 was found to bind along the length of type IV collagen to four distinct sites located at intervals of approximately 100 nm each. The same 100-nm distance was observed in domain NC1 of intact type IV collagen bound along the length of the collagen molecules during initial steps of network formation and in complete networks. The presence of anti-NC1 Fab fragments in type IV collagen solutions inhibited lateral association and network formation in rotary shadow images. During the process of self-association type IV collagen develops turbidity; addition of isolated domain NC1 inhibited the development of turbidity in a concentration-dependent manner. These findings indicate that domain NC1 of type IV collagen plays an important role in the process of self-association and suggest that alterations in the structure of NC1 may be partially responsible for impaired functions of basement membranes in certain pathological conditions.     

The structure of type IX collagen

We present a detailed analysis both of tryptic peptides and amino-terminal sequences of the subunits of two collagenous fragments (HMW and LMW) previously isolated from pepsin extracts of chicken cartilage (Reese, C.A., and Mayne, R. (1981) Biochemistry 20, 5443-5448). This analysis and a comparison with the nucleotide sequence of the cDNApYN1738 (Ninomiya, Y., and Olsen, B.R. (1984) Proc. Natl. Acad. Sci. U. S. A. 81, 3014-3018) shows that HMW and LMW are pepsin-resistant fragments of a unique collagen composed of molecules with three different polypeptide chains (alpha-chains). This collagen has been assigned the type number IX, and the alpha-chain encoded by pYN1738 has been given the designation alpha 1 (IX). Type IX collagen contains three triple-helical domains and at least two sets of interchain disulfide bridges. At the amino and carboxyl ends are noncollagenous domains which do not appear to be homologous to amino and carboxyl propeptides of interstitial collagens.     

Degradation of native type IV procollagen by human neutrophil elastase. Implications for leukocyte-mediated degradation of basement membranes

Leukocyte-derived proteases may contribute to the destruction of basement membranes during inflammation. We have, therefore, examined the degradation of human type IV procollagen (PC) by purified human neutrophil elastase (HLE). Native [14C]proline-labeled type IV PC was isolated from cultures of human HT-1080 cells and incubated with HLE for various times at 25 or 37 degrees C. Cleavage products were resolved by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and identified by CNBr peptide mapping. Incubation of type IV PC with HLE (less than 1:10 HLE:type IV weight ratio) resulted in cleavage of the pro alpha 1 (IV) and pro alpha 2 chains (Mr 180,000 and 175,000) to discrete components of Mr greater than 140,000. Peptide mapping indicated that the carboxy-terminal collagenase-resistant domains of both chains were rapidly and preferentially degraded. Longer incubations or incubations at higher enzyme:substrate ratios resulted in extensive and asymmetric internal cleavage with the generation of fragments similar in size distribution to the major pepsin-resistant fragments of type IV collagen. Our findings indicate that soluble, native human type IV PC is a substrate for HLE and is preferentially cleaved within the globular carboxy-terminal domains of the pro alpha 1 and pro alpha 2 chains. We suggest that even limited cleavage of type IV PC by HLE may disrupt intermolecular carboxy-terminal interactions believed to be important for basement membrane assembly and for maintaining basement membrane structure in vivo.     

Binding of laminin to type IV collagen: a morphological study

A mixture of laminin and type IV collagen was analyzed by rotary shadowing using carbon/platinum and electron microscopy. Laminin was found to form distinct complexes with type IV collagen: one site of interaction is located 140 nm from the COOH-terminal, noncollagenous (NC1) domain and the other is located within the NH2-terminal region. The isolated NC1 fragment of type IV collagen does not appear to interact with laminin, while pepsin-treated type IV collagen, which lacks the NC1 domain, retains its ability to form complexes with laminin. Analysis of the laminin-type IV complexes indicates that laminin binds to type IV collagen via the globular regions of either of its four arms. This finding is supported by experiments using fragment P1 of laminin which lacks the globular regions and which does not bind to type IV collagen in a specific way. In addition, after heat-denaturation of laminin no specific binding is observed.     

Ultrastructural morphology and domain structure of a unique collagenous component of basement membranes

A disulfide cross-linked collagenous fragment (7 S) has been isolated by pepsin solubilization from several tissues rich in basement membranes including bovine lung, human placenta, and the murine EHS tumor. Examination of this material by the rotary shadowing technique indicates that these fragments are similar to but not identical with the 7S collagen described recently [Risteli, J., B?chinger, H.P., Engel, J., Furthmayr, H., & Timpl, R. (1980) Eur. J. Biochem. 108, 239-250]. The central rodlike portion of the particles was found to be similar in length; however, the peripheral four arms of 7S particles from bovine and murine sources are 10 nm longer in comparison to those from human sources. In addition, about 5-7% of all the particles contain a fifth arm. Specific antibodies to bovine 7 S cross-react with murine 7 S but only to a rather limited extent with human 7 S. These antibodies react with antigenic sites located at the ends of the peripheral arms of the fragment as visualized directly with rotary shadowing techniques. The data are consistent with a structural difference in type IV collagens from bovine, human, and mouse which leads to pepsin cleavage at different sites in a particular noncollagenous region adjacent to 7 S. However, since bovine 7S antibodies cross-react with human and murine tissues by immunofluorescence despite the lack of complete serological cross-reactivity, it is suggested that type IV collagens from all three species have some degree of homology in this region.     

A unique, pepsin-sensitive collagen synthesized by aortic endothelial cells in culture

A unique collagen, designated EC, has been isolated from the culture medium of adult bovine aortic endothelial cells. After diethylaminoethylcellulose chromatography of [3H]proline-labeled culture medium, three non-disulfide-bonded bacterial collagenase-sensitive components with apparent Mr of 177000 (EC 1), 125000 (EC 2), and 100000 (EC 3) were demonstrated. Molecular sieve chromatography, cyanogen bromide cleavage, and two-dimensional peptide mapping of radioiodinated EC fragments produced by protease digestion suggest that the lower molecular weight components originate from EC 1. Both EC 1 and EC 2 were digested by pepsin within 10 min to products of less than 60000 molecular weight, under conditions which supported only limited proteolysis of other native collagens. A pepsin-resistant fragment of Mr 50000, derived from a digest of EC 2, contained equal amounts of hydroxyproline and proline, suggesting that at least a portion of the endothelial collagen contains a stable, collagen-like triple helix. Comparative mapping using mast cell protease and cyanogen bromide cleavage, followed by polyacrylamide gel electrophoresis, indicates that the primary structure of this collagen differs from that of other known collagen types.     

Type VIII collagen from bovine Descemet's membrane: structural characterization of a triple-helical domain

Bovine corneal Descemet's membrane (DM) was subjected to limited pepsin digestion. Soluble native collagens were fractionated by differential salt precipitation, and a mixture of type V collagen and collagenous fragments with a chain Mr of 50,000 (50K) was obtained at a concentration of 1.5 M NaCl. Further purification of the 50K collagen by molecular sieve and high-performance liquid chromatography resulted in the isolation of two-non-disulfide-bonded polypeptides, 50K-A and 50K-B, which were susceptible to several neutral proteases, including bacterial collagenase. By the criteria of peptide mapping, amino acid composition, and N-terminal sequence analysis, 50K-A and 50K-B were structurally dissimilar, although both chains contained Gly-X-Y repeats. 50K-A and 50K-B were immunologically and structurally distinct from collagen type I, III, IV, V, and VI. Immunohistochemical studies of bovine ocular tissue showed preferential distribution of the collagen containing the 50K fragment in the DM, with a more disperse arrangement of apparently interconnecting fibrils in the corneal stroma. Type VIII collagen isolated from the culture medium of metabolically radiolabeled bovine corneal endothelial (BCE) cells and its pepsin-resistant Mr 50 000 domain(s) both cross-reacted with antisera to 50K polypeptides from the corneal DM. Additionally, the CNBr peptide maps of pepsin-resistant Mr 50 000 polypeptides of type VIII collagen isolated from BCE cells and bovine corneal DM were highly similar.(ABSTRACT TRUNCATED AT 250 WORDS)     

Localization of binding sites for laminin, heparan sulfate proteoglycan and fibronectin on basement membrane (type IV) collagen

Rotary shadowing electron microscopy was used to examine complexes formed by incubating combinations of the basement membrane components: type IV collagen, laminin, large heparan sulfate proteoglycan and fibronectin. Complexes were analyzed by length measurement from the globular (COOH) domain of type IV collagen, and by examination of the four arms of laminin and the two arms of fibronectin. Type IV collagen was found to contain binding sites for laminin, heparan sulfate proteoglycan and fibronectin. With laminin the most frequent site was centered approximately 81 nm from the carboxy end of type IV collagen. Less frequent sites appeared to be present at approximately 216 nm and approximately 291 nm, although this was not apparent when the sites were expressed as a fraction of the length of type IV collagen to which they were bound. For heparan sulfate proteoglycan the most frequent site occurred at approximately 206 nm with a less frequent site at approximately 82 nm. For fibronectin, a single site was present at approximately 205 nm. Laminin bound to type IV collagen through its short arms, particularly through the end of the lateral short arms and to heparan sulfate proteoglycan mainly through the end of its long arm. Fibronectin bound to type IV collagen through the free end region of its arms. Using a computer graphics program, the primary laminin binding sites of two adjacent type IV collagen molecules were found to align in the "polygonal" model of type IV collagen, whereas with the "open network" model, a wide meshed matrix is predicted. It is proposed that basement membrane may consist of a lattice of type IV collagen coated with laminin, heparan sulfate proteoglycan and fibronectin.     

Characterization of pepsin-resistant collagen-like tail subunit fragments of 18S and 14S acetylcholinesterase from Electrophorus electricus

Digestion of 18S and 14S acetylcholinesterase from eel electric organ with pepsin at 15 degrees C for 6 h results in extensive degradation of the catalytic subunits, but a major portion of the collagen-like tail structure associated with these enzyme forms resists degradation. The pepsin-resistant structures partially aggregate and can be isolated by gel exclusion chromatography on Sepharose CL-6B in buffered 1 M sodium chloride. The largest structure, denoted F3, has a molecular weight of 72 000 according to gel electrophoresis in sodium dodecyl sulfate and is composed of three 24 000 molecular weight polypeptides linked by intersubunit disulfide bonds. This structure is largely, but not completely, a collagen-like triple helix as indicated by a circular dichroism spectrum typical of triple-helical collagen and an amino acid composition characterized by 27% glycine, 5% hydroxyproline, and 5% hydroxylysine. Continued pepsin action results in degradation of the disulfide linkage region such that disulfide-linked dimers F2 and finally F1 monomers become the predominant forms in sodium dodecyl sulfate. Digested samples in which either F3 or F2 predominate have virtually identical circular dichroic spectra and amino acid compositions and generate similar diffuse 24 000 molecular weight polypeptides following disulfide reduction. Thus the intersubunit disulfide linkages in F3 must occur close to the end(s) of the fragment polypeptide chains. Pepsin conversion of F3 to F2 is particularly accelerated between 25 and 30 degrees C, suggesting that the triple-helical structure in the disulfide linkage region undergoes thermal destabilization in this temperature range. Digestion at 40 degrees C yields presumably triple-helical F1 structures devoid of disulfide linkages, although their degradation to small fragments can be detected at this temperature. The question of whether the three tail subunits that give rise to F1 polypeptides are identical remains open.     

Characterization of the precursor form of type VI collagen

Well characterized monospecific antisera against pepsin-extracted bovine type VI collagen were used to identify and characterize the intact form of type VI collagen. In immunoblotting experiments the antisera reacted with the pepsin-resistant fragments of the alpha 1(VI) and alpha 3(VI) chains, but not with the fragment of the alpha 2(VI) chain. Extracts obtained from uterus and aorta with 6 M guanidine HCl contained two immunoreactive polypeptides of Mr = 190,000 and 180,000 based on globular protein standards. Cleavage of extracts with pepsin generated the previously characterized pepsin-resistant fragments of alpha 1(VI) and alpha 3(VI), indicating that the higher molecular weight polypeptides represent the intact parent chains, alpha 1(VI) and alpha 3(VI). Digestion of extracts with bacterial collagenase released an Mr = 100,000 noncollagenous fragment from the alpha 1(VI) chain. Thus, intact type VI collagen in tissues contains a relatively short triple helical domain and at least one very large globular domain which is sensitive to pepsin but resistant to collagenase digestion. Immunoblotting revealed a polypeptide of Mr = 240,000, which we suggest represents the pro-alpha 1(VI) chain, in the culture medium of bovine fibroblasts. Bands intermediate in molecular weight between 240,000 and 190,000 were identified in cell layers. These findings establish type VI collagen as a protein with very large nontriple helical domains, a property that undoubtedly plays an important role in its function.     

Production of both interstitial and basement membrane procollagens by fibroblastic WI-38 cells from human embryonic lung

The synthesis and secretion of type IV procollagen, in addition to that of procollagen types I and III, was detected in cells derived from human embryonic lung (WI-38) by immunofluorescence, metabolic labeling, immunoprecipitation, collagenase digestion and the characteristic polypeptide sizes of both intact procollagen type IV chains and their initial pepsin-resistant fragments as determined by polyacrylamide gel electrophoresis. Locally obtained human embryonic lung cells secreted the same procollagens, but neither embryonic nor adult human skin fibroblasts were found to secrete type IV procollagen in amounts detectable by the same methods.     

Type XII collagen. A large multidomain molecule with partial homology to type IX collagen

Three overlapping cDNAs encoding alpha 1 (XII) collagen have been isolated and sequenced. The DNAs define five sequence domains within the chain. Three domains are nontriple-helical; two are relatively short triple-helical regions. The amino acid sequences of tryptic peptides derived from 16- and 10-kDa pepsin-resistant fragments isolated from tendon extracts are in full agreement with the deduced sequences of the triple-helical regions. Two of the five sequence domains in alpha 1 (XII), one triple-helical and one nontriple-helical, show a high degree of similarity to regions in type IX collagen chains. In addition, examination of seven exons in the alpha 1 (XII) gene shows that the gene is, in part, similar to the structure of type IX collagen genes. Therefore, collagen types IX and XII are partially homologous. The alpha 1 (XII) sequence data predict an asymmetric structure for type XII collagen molecules, fully consistent with the rotary shadowing images. These images show a triple-helical 75-nm tail attached through a central globule to three finger-like structures, each 60 nm long (Dublet, B., Oh, S., Sugrue, S. P., Gordon, M. K., Gerecke, D. R., Olsen, B. R., and van der Rest, M. (1989) J. Biol. Chem. 264, 13150-13156).     

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.     

Biosynthetic and structural properties of endothelial cell type VIII collagen

A highly unusual endothelial cell collagen (Sage, H., Pritzl, P., and Bornstein, P., (1980) Biochemistry 19, 5747-5755) has been characterized in greater detail. Pulse-chase experiments with bovine aortic endothelial cells revealed two nondisulfide-bonded collagens, of apparent chain Mr = 177,000 and 125,000, with an estimated synthesis and secretion time of 75 min. Stepwise, quantitative processing to stable lower molecular weight forms as described for type I procollagen was not observed. Endothelial collagen was secreted over a temperature range of 24-37 degrees C and, prior to heat denaturation, did not display affinity for a gelatin-binding fragment of fibronectin coupled to Sepharose. The presence of a pepsin-resistant domain (Mr = 50,000) in both the soluble and cell layer-associated forms of this protein was shown by ion exchange chromatography and sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Endothelial collagen was cleaved by vertebrate collagenase into several discrete fragments that differed in molecular weight from the characteristic alpha A and alpha B fragments generated from the interstitial collagens. Nontriple helical domains corresponding to the NH2- and COOH-terminal propeptides of other procollagen types were not found after incubation of endothelial collagen with bacterial collagenase. Additional evidence for the lack of extended noncollagenous sequences was provided by studies with mast cell proteases, which convert native procollagen to collagen but are unreactive toward native interstitial collagens. Endothelial collagen was not cleaved by these enzymes at 37 degrees C, but, as observed for interstitial collagen alpha chains, required prior heating at elevated temperatures for cleavage to occur. In view of this unique set of structural characteristics, and a distribution that is not restricted to the endothelium, we have designated this protein as type VIII collagen.     

Shape and assembly of type IV procollagen obtained from cell culture.

Type IV procollagen was isolated from the culture medium of the teratocarcinoma cell line PYS-2 by affinity chromatography on heparin-Sepharose. Immunological studies showed that type IV procollagen is composed of pro-alpha 1(IV) and pro-alpha 2(IV) chains and contains two potential cross-linking sites which are located in the short triple-helical 7S domain and the globular domain NC1 . The 7S domain was also identified as the heparin binding site. Rotary shadowing visualized type IV procollagen as a single triple-helical rod (length 388 nm) with a globule at one end. Some of the procollagen in the medium, however, had formed aggregates by alignment of 2-4 molecules along their 7S domains. After deposition in the cell matrix, non-reducible cross-links between the 7S domains are formed while the globules of two procollagen molecules connect to each other. The latter may require a slight proteolytic processing of the globular domains NC1 . The shape of type IV procollagen and the initial steps in its assembly are compatible with a recently proposed network of type IV collagen molecules in basement membranes. Since both type IV collagen and laminin bind to heparin, the formation of higher ordered structures by interaction of both proteins with heparan-sulfate proteoglycan may occur in situ.