The host-cell restriction factor SERINC5 restricts HIV-1 infectivity without altering the lipid composition and organization of viral particles

The host-cell restriction factor SERINC5 potently suppresses the infectivity of HIV, type 1 (HIV-1) particles, and is counteracted by the viral pathogenesis factor Nef. However, the molecular mechanism by which SERINC5 restricts HIV-1 particle infectivity is still unclear. Because SERINC proteins have been suggested to facilitate the incorporation of serine during the biosynthesis of membrane lipids and because lipid composition of HIV particles is a major determinant of the infectious potential of the particles, we tested whether SERINC5-mediated restriction of HIV particle infectivity involves alterations of membrane lipid composition. We produced and purified HIV-1 particles from SERINC5293T cells with very low endogenous SERINC5 levels under conditions in which ectopically expressed SERINC5 restricts HIV-1 infectivity and is antagonized by Nef and analyzed both virions and producer cells with quantitative lipid MS. SERINC5 restriction and Nef antagonism were not associated with significant alterations in steady-state lipid composition of producer cells and HIV particles. Sphingosine metabolism kinetics were also unaltered by SERINC5 expression. Moreover, the levels of phosphatidylserine on the surface of HIV-1 particles, which may trigger uptake into non-productive internalization pathways in target cells, did not change upon expression of SERINC5 or Nef. Finally, saturating the phosphatidylserine-binding sites on HIV target cells did not affect SERINC5 restriction or Nef antagonism. These results demonstrate that the restriction of HIV-1 particle infectivity by SERINC5 does not depend on alterations in lipid composition and organization of HIV-1 particles and suggest that channeling serine into lipid biosynthesis may not be a cardinal cellular function of SERINC5.

Sphingosine-1-Phosphate Lyase Deficient Cells as a Tool to Study Protein Lipid Interactions

Cell membranes contain hundreds to thousands of individual lipid species that are of structural importance but also specifically interact with proteins. Due to their highly controlled synthesis and role in signaling events sphingolipids are an intensely studied class of lipids. In order to investigate their metabolism and to study proteins interacting with sphingolipids, metabolic labeling based on photoactivatable sphingoid bases is the most straightforward approach. In order to monitor protein-lipid-crosslink products, sphingosine derivatives containing a reporter moiety, such as a radiolabel or a clickable group, are used. In normal cells, degradation of sphingoid bases via action of the checkpoint enzyme sphingosine-1-phosphate lyase occurs at position C2-C3 of the sphingoid base and channels the resulting hexadecenal into the glycerolipid biosynthesis pathway. In case the functionalized sphingosine looses the reporter moiety during its degradation, specificity towards sphingolipid labeling is maintained. In case degradation of a sphingosine derivative does not remove either the photoactivatable or reporter group from the resulting hexadecenal, specificity towards sphingolipid labeling can be achieved by blocking sphingosine-1-phosphate lyase activity and thus preventing sphingosine derivatives to be channeled into the sphingolipid-to-glycerolipid metabolic pathway. Here we report an approach using clustered, regularly interspaced, short palindromic repeats (CRISPR)-associated nuclease Cas9 to create a sphingosine-1-phosphate lyase (SGPL1) HeLa knockout cell line to disrupt the sphingolipid-to-glycerolipid metabolic pathway. We found that the lipid and protein compositions as well as sphingolipid metabolism of SGPL1 knock-out HeLa cells only show little adaptations, which validates these cells as model systems to study transient protein-sphingolipid interactions.

The effect of solvent dynamics on the low frequency collective motions of DNA in solution and unoriented films

Infrared spectroscopy is used to probe the dynamics of in vitro samples of DNA prepared as solutions and as solid unoriented films. The lowest frequency DNA mode identified in the far-infrared spectra of the DNA samples is found to shift in frequency when the solvent influence in the hydration shell is altered. The lowest frequency mode also has characteristics that are similar to beta-relaxations identified in other glass forming polymers.

The influence of chain dynamics on the far-infrared spectrum of liquid methanol-water mixtures

Far-infrared-absorption spectroscopy has been used to study the low-frequency (<or=100 cm-1) intermolecular modes of methanol in mixtures with water. With the aid of a first-principles molecular-dynamics simulation on an equivalent system, a detailed understanding about the origin of the low-frequency IR modes has been established. The total dipole spectrum from the simulation suggests that the bands appearing in the experimental spectra at approximately 55 and 70 cm-1 in methanol and methanol-rich mixtures arise from both fluctuations and torsional motions occurring within the methanol hydrogen-bonded chains. The influence of these modes on both the solvation dynamics and the relaxation mechanisms in the liquid is discussed within the context of recent experimental and theoretical results that have emerged from studies focusing on the short-time dynamics in the methanol hydrogen bond network.

The influence of chain dynamics on the far-infrared spectrum of liquid methanol

Far-infrared-absorption spectroscopy is used to investigate the low-frequency (<or=100 cm-1) intermolecular interactions in liquid methanol. Using an intense source of far-infrared radiation, modes are elucidated at approximately 30 and 70 cm-1 in the absorption spectrum. These modes are believed to arise from intermolecular bending and librational motions, respectively, and are successfully reproduced in an ab initio molecular-dynamics simulation of methanol.

Binding of fibulin-1 to nidogen depends on its C-terminal globular domain and a specific array of calcium-binding epidermal growth factor-like (EG) modules

The calcium-binding basement membrane protein fibulin-1C was shown to bind nidogen in a calcium-dependent fashion. Fibulin-1C consists of small N (domain 1) and C-terminal (domain III) globular structures connected by a central rod (domain II) composed of nine epidermal growth factor (EG) modules, eight of which possess a consensus sequence for calcium binding. Several point and deletion mutants and chimeric protein constructs were used to define the nidogen binding epitope of fibulin-1C by surface plasmon resonance and solid phase assays. All recombinant products were obtained from transfected kidney cells in a folded form as shown by CD spectroscopy, electron microscopy and proteolysis. They were used to demonstrate that calcium-binding is essentially due to the EG modules possessing the consensus binding sequence. Deletion of domain III caused a 30-fold reduction in nidogen binding, whereas deletion of domain I had no effect, yet domain III alone was also inactive. Successive deletions of two to seven EG modules of domain II also caused partial of complete inactivation of binding depending on how many were deleted or their position relative to domain III. Site-directed mutagenesis within the calcium binding consensus sequences demonstrated a similar dependence. Replacement of seven of the calcium-binding modules by a similar tandem array from a related protein showed a distinct (fibulin-2) to almost complete loss of binding (fibrillin-1). This indicates a complex epitope structure involving domains II and III, which each may provide binding epitopes or stabilize each other.

Risks of beryllium disease related to work processes at a metal, alloy, and oxide production plant

OBJECTIVES

To describe relative hazards in sectors of the beryllium industry, risk factors of beryllium disease and sensitisation related to work process were sought in a beryllium manufacturing plant producing pure metal, oxide, alloys, and ceramics.

METHODS

All 646 active employees were interviewed; beryllium sensitisation was ascertained with the beryllium lymphocyte proliferation blood test on 627 employees; clinical evaluation and bronchoscopy were offered to people with abnormal test results; and industrial hygiene measurements related to work processes taken in 1984-93 were reviewed.

RESULTS

59 employees (9.4%) had abnormal blood tests, 47 of whom underwent bronchoscopy. 24 new cases of beryllium disease were identified, resulting in a beryllium disease prevalence of 4.6%, including five known cases (29/632). Employees who had worked in ceramics had the highest prevalence of beryllium disease (9.0%). Employees in the pebble plant (producing beryllium metal) who had been employed after 1983 also had increased risk, with a prevalence of beryllium disease of 6.4%, compared with 1.3% of other workers hired in the same period, and a prevalence of abnormal blood tests of 19.2%. Logistic regression modelling confirmed these two risk factors for beryllium disease related to work processes and the dependence on time of the risk at the pebble plant. The pebble plant was not associated with the highest gravimetric industrial hygiene measurements available since 1984.

CONCLUSION

Further characterisation of exposures in beryllium metal production may be important to understanding how beryllium exposures confer high contemporary risk of beryllium disease.

Dimer model for the microfibrillar protein fibulin-2 and identification of the connecting disulfide bridge

Fibulin-2 is a novel extracellular matrix protein frequently found in close association with microfibrils containing either fibronectin or fibrillin. The entire protein and its predicted domains were obtained as recombinant products and examined by ultracentrifugation and electron microscopy. This demonstrated a disulfide-linked homodimer of 175 kDa subunits. Partial reduction to monomers identified specifically an odd Cys574 residue responsible for dimer formation in one of three anaphylatoxin-like modules that constitute the central globular domain I (13 kDa) of fibulin-2. Furthermore, a Cys574-Ser mutation abolished disulfide connection but not non-covalent dimerization of fibulin-2. The C-terminal region (85 kDa) was shown to represent a 35-nm-long rod consisting of 11 calcium-binding EGF-like modules (domain II) and a small terminal globe (domain III). The unique N-terminal domain N (55 kDa) was also rod-shaped (approximately 38 nm) and rich in galactosamine indicating extensive O-glycosylation. A dimer model is proposed indicating mainly a rod-like shape of 80 nm length based on an anti-parallel association of two subunits through their domains I. This model also implies alignment of domains II and N between different subunits. This was demonstrated by surface plasmon resonance assay which showed a distinct interaction between domains N and II with a Kd of approximately 0.7 microM.

Expression of fibulin-2 by fibroblasts and deposition with fibronectin into a fibrillar matrix

The extracellular matrix protein fibulin-2 was shown to be a typical product of cultured human and mouse fibroblasts by several immunological assays. It is secreted and deposited in cells and tissues as a disulfide-bonded oligomer identical in size to the previously described recombinant fibulin-2. Most of the fibroblast fibulin-2 is deposited into a dense fibrillar meshwork which requires treatment with EDTA and/or 6 M urea for solubilization. Fibulin-2 and fibronectin are synthesized at equivalent levels and both colocalize in the fibrils as shown by immunofluorescence. Metabolic labelling and pulse-chase studies demonstrated fibulin-2 oligomers in detergent extracts of cells and their rapid translocation to extracellular EDTA-sensitive assembly forms. Unlike for fibronectin and fibulin-1 only a little fibulin-2 was found in the cell culture medium. Immunogold staining of confluent human fibroblasts showed localization of fibulin-2 to a fine meshwork or bundles of amorphous microfibrils in the matrix. This also demonstrated a distinct colocalization of fibulin-2 and fibronectin at the electron microscope level, indicating that the interaction between these two protein shown in in vitro assays may also exist in situ. No distinct colocalization of both proteins could, however, be observed with cross-striated fibrils of collagen I and collagen VI microfibrils.

Structural and electron microscopic analysis of neurocan and recombinant neurocan fragments

Neurocan, a nervous tissue-specific chondroitin sulfate proteoglycan of the aggrecan family which has been shown to interact with neural cell adhesion molecules and tenascin, could be visualized by rotary shadowing electron microscopy as two globular domains interconnected by an extended flexible filament of 60-90 nm. Several recombinant neurocan fragments generated in the human embryonic kidney cell line 293 represent as observed by electron microscopy the expected parts of this structure, which indicates a correct folding of these molecules. Biological activity of the recombinant N-terminal globular domain could be demonstrated by its coelution with hyaluronan in gel permeation chromatography. In addition, the modification of the recombinant fragments with certain carbohydrate structures was analyzed. High mannose oligosaccharides could be mapped to the N-terminal globular domain of the brain-derived molecule. Only recombinant fragments containing parts of the central region of the molecule were modified with chondroitin sulfate chains and with the HNK-1 epitope, and could be considerably altered in their migratory behavior on SDS-polyacrylamide gel electrophoresis by neuraminidase treatment. These findings and the electron microscopic shape indicate a mucin-like character for the central neurocan region.

Characterization of mutant forms of recombinant human properdin lacking single thrombospondin type I repeats. Identification of modules important for function

Properdin is a serum glycoprotein that up-regulates the alternative pathway of complement by stabilizing the C3b-Bb complex. It also binds sulfated glycoconjugates, such as sulfatide, in vitro. Properdin is composed of cyclic dimers, trimers, and tetramers of a 53-kDa monomeric subunit. The monomer contains an N-terminal region of no known homology and six thrombospondin type 1 repeats (TSRs) of approximately 60 amino acids. To identify the regions of properdin important for function, we have expressed human properdin, and mutant forms each lacking a single TSR, in Chinese hamster ovary cells. In addition, limited tryptic digestion yielded "nicked" properdin by the cleavage of one peptide bond in TSR5. The structural and functional properties of these altered forms of properdin were investigated. Properdin "nicked" in TSR5 is unable to bind C3b but retains its overall structure and its ability to bind sulfatide. The removal of TSR5 prevents C3b and sulfatide binding. Properdin lacking TSR4 is unable to stabilize the C3b-Bb complex but is able to bind C3b and sulfatide, and shows the presence of monomers and dimers in an electron microscope. Properdin without TSR3 is able to stabilize the C3b-Bb complex, to bind C3b and sulfatide, and forms dimers, trimers, and tetramers. Properdin lacking TSR6 is unable to form oligomers. The N-linked carbohydrate of properdin is not required for oligomerization or stabilization of the C3b-Bb complex. The results implicate TSR5 in both C3b and sulfatide binding, and suggest that TSR4 may also be involved in stabilization of the C3b-Bb complex.

Characterization of mutant forms of recombinant human properdin lacking single thrombospondin type I repeats. Identification of modules important for function

Properdin is a serum glycoprotein that up-regulates the alternative pathway of complement by stabilizing the C3b-Bb complex. It also binds sulfated glycoconjugates, such as sulfatide, in vitro. Properdin is composed of cyclic dimers, trimers, and tetramers of a 53-kDa monomeric subunit. The monomer contains an N-terminal region of no known homology and six thrombospondin type 1 repeats (TSRs) of approximately 60 amino acids. To identify the regions of properdin important for function, we have expressed human properdin, and mutant forms each lacking a single TSR, in Chinese hamster ovary cells. In addition, limited tryptic digestion yielded "nicked" properdin by the cleavage of one peptide bond in TSR5. The structural and functional properties of these altered forms of properdin were investigated. Properdin "nicked" in TSR5 is unable to bind C3b but retains its overall structure and its ability to bind sulfatide. The removal of TSR5 prevents C3b and sulfatide binding. Properdin lacking TSR4 is unable to stabilize the C3b-Bb complex but is able to bind C3b and sulfatide, and shows the presence of monomers and dimers in an electron microscope. Properdin without TSR3 is able to stabilize the C3b-Bb complex, to bind C3b and sulfatide, and forms dimers, trimers, and tetramers. Properdin lacking TSR6 is unable to form oligomers. The N-linked carbohydrate of properdin is not required for oligomerization or stabilization of the C3b-Bb complex. The results implicate TSR5 in both C3b and sulfatide binding, and suggest that TSR4 may also be involved in stabilization of the C3b-Bb complex.

Structural properties of recombinant domain III-3 of perlecan containing a globular domain inserted into an epidermal-growth-factor-like motif

A fragment comprising approximately domain III-3 of the basement membrane heparan sulfate proteoglycan perlecan was prepared in recombinant form from kidney cell clones. This fragment was predicted to contain a cysteine-free globular domain inserted within an epidermal-growth-factor(EGF)-like motif (L4 module) and three additional EGF-like motifs (LE module) without large inserts. This prediction was confirmed by electron microscopy, which demonstrated a globule joined to a very short rod-like segment. The globule was selectively destroyed by pepsin, which also demonstrated that its insertion into an EGF-like motif did not prevent the typical disulfide connections known for such motifs. Yet the globule was more stable against neutral proteinases. The fragment showed a distinct content (55-60%) of alpha helical and beta structure and a partially reversible melting of the conformation in 6 M guanidine. Antibodies raised against recombinant domain III-3 demonstrated a complete cross-reaction with tissue-derived perlecan but not with laminin and a distinct basement membrane staining of tissue sections. Most of the epitopes were lost after reduction and alkylation. Together the data demonstrated a proper folding of recombinant domain III-3 similar to its structure in the native protein and provided the first structural evidence for a novel globular protein motif L4 based on an EGF-like scaffold.

Structural comparison of cuticle and interstitial collagens from annelids living in shallow sea-water and at deep-sea hydrothermal vents

Two types of annelid collagens of different sizes were purified, one from acetic acid extracts of the cuticle (length 2.5 microns) and the other, after pepsin digestion, from interstitial spaces of the body wall (0.3 micron). They were obtained from Alvinella pompejana, Alvinella caudata and Paralvinella grasslei collected at 2600 m depth around anoxic hydrothermal vents and from Arenicola marina and Nereis diversicolor living in shallow sea-water habitats. The length of the corresponding collagens from different species and their amino acid compositions including the hydroxylation of proline were remarkably similar. The melting point of the triple helix, however, differed between the Alvinella species (approximately 45 degrees C), Paralvinella (approximately 35 degrees C) and the shallow sea-water annelids (approximately 28 degrees C), indicating adaption to habitats with different temperatures. The cuticle collagens of the annelids possess a globular domain, which is apparently involved in oligomer formation, and show similar fragment pattern. Almost identical cross-striation patterns of segment-long-spacing segments of the interstitial collagens indicated sequence similarity, which was confirmed by partial Edman degradation of alpha-chains. These data showed almost complete identity between the two Alvinella species and a lower sequence identity with Paralvinella (approximately 95%), Arenicola (67 to 72%) and the vent vestimentiferan Riftia pachyptila (64 to 71%). The data suggest a close evolutionary relationship between these worms, despite a clear separation of habitat preference and thermal stability of the collagens.

Comparative study of the structural and functional properties of a bovine plasma C-type lectin, collectin-43, with other collectins

Collectin-43 (CL-43) is a recently described bovine plasma protein containing both collagenous regions and C-type-lectin domains [Holmskov, Teisner, Willis, Reid and Jensenius (1993) J. Biol. Chem. 268, 10120-10125; Lim, Willis, Reid, Lu, Laursen, Jensenius and Holmskov (1994) J. Biol. Chem. 269, 11820-11824]. CL-43 was purified by affinity chromatography on mannan-Sepharose. On SDS/PAGE under reducing conditions the purified lectin showed a double band at about 43 kDa, with the upper band representing the intact molecule and the lower band a truncated form that lacked the N-terminal nine amino acid residues. Under non-reducing conditions, only one band was seen at 120 kDa. Analytical gel chromatography and sucrose-density-gradient centrifugation of the purified molecule, showed a Stokes radius of 9.1 +/- 0.3 nm (91 +/- 3 A) and a sedimentation coefficient (s20,w) of 3.6 +/- 0.1 S. These values correspond to a molecular mass of 119-138 kDa under non-denaturing condition in solution. The frictional coefficient (f/f0) was 2.7, indicating extreme elongation due to the collagenous segment. Only monomer subunits, with 37.4 +/- 1.7-nm-long rods, were seen by electron microscopy. These findings indicate that CL-43, in contrast with the other circulating collectins, is found only as a single subunit composed of three polypeptide chains. Two-dimensional gel electrophoresis showed that CL-43 has two isoforms, with pI values of 4.9 and 5.3, corresponding to the native form and the truncated form of the molecule respectively. CL-43, like conglutinin, lung surfactant protein A and mannan-binding protein (MBP), was shown to bind to the collectin receptor. Bovine MBP caused the activation of the complement system as revealed by the deposition of complement component C4 upon incubation of diluted serum in wells containing MBP bound to solid-phase mannan. CL-43, lung surfactant protein D (SP-D) and conglutinin showed no complement-activating properties under the same conditions. Conglutinin binds fluid- and solid-phase iC3b, while CL-43 and MBP do not show such reactivity.

Structural characterization of two variants of fibulin-1 that differ in nidogen affinity

Two C-terminal variants C and D of mouse fibulin-1 were purified from the culture medium of stably transfected human kidney cell clones. They showed, after rotary shadowing, a dumbbell-like structure of about 33 nm in length. Pepsin digestion demonstrated stability of the disulfide-bonded domains 1 (anaphylatoxin-like) and II (multiple EGF-like motifs) but not for domain III which is different in the variants. A close similarity of the variants was observed in immunochemical assays indicating that domain III epitopes are not very antigenic. Binding analysis in solid phase assays demonstrated for variant C a 100-fold stronger binding to the basement membrane protein nidogen than for variant D. Both interactions were sensitive to EDTA. Surface plasmon resonance assays confirmed this difference and showed KD = 60 nM for variant C and KD > 1 microM for variant D. Lower binding activities and smaller differences between both variants were observed for the calcium-dependent binding to fibronectin, laminin-1 and collagen IV. Self aggregation into nest-like oligomers was observed at high concentrations of fibulin-1 which was not sensitive to EDTA.

Recombinant expression and structural and binding properties of alpha 1(VI) and alpha 2(VI) chains of human collagen type VI

Full-length alpha 1(VI) and alpha 2(VI) cDNAs in an eukaryotic expression vector were used to obtain stably transfected human kidney cell clones and to purify these collagen-VI chains in substantial quantities from the culture medium. Both chains appeared mainly as monomers together with some dimers that were disulfide linked through their C-terminal globular domains. Despite sufficient hydroxylation of proline and lysine residues, the chains did not form a triple-helix, as shown by electronmicroscopy, CD spectra and pepsin sensitivity. Digestion of the chains with bacterial collagenase released the N-terminal and C-terminal globular domains, which were identified by their size and partial sequences. They showed a substantial content of alpha-helical conformation and a distinct globular structure after rotary shadowing. Antibodies could be raised that distinguished between the two chains and reacted with the globular domains. The alpha 2(VI) but not the alpha 1(VI) chain showed binding to a heparan sulfate proteoglycan (perlecan), fibronectin and pepsin-solubilized collagen VI. Purified globular domains did not bind these ligands indicating the localization of binding sites within the triple-helical domain. Both chains showed a distinct affinity for heparin but failed to bind to various collagen types.

Protein binding and cell adhesion properties of two laminin isoforms (AmB1eB2e, AmB1sB2e) from human placenta

Two isoforms of laminin were extracted from human placenta by neutral buffer containing EDTA, copurified through several steps and finally separated by Mono Q anion exchange chromatography. One variant consisted of disulphide-linked 340, 230 and 190 kDa subunits, which were identified by immunoblotting as Am, B1e and B2e chains. In the other variant, the B1e chain was replaced by B1s of 180 kDa. After rotary shadowing, both variants showed a similar cross-shaped structure. The nidogen content of these laminins was substoichiometric and variable (3-70%), indicating loss by endogenous proteolysis. Yet both human isoforms were able to bind mouse nidogen with an affinity (Kd approximately 0.5 nM) comparable to that of AeB1eB2e laminin from a mouse tumour. Since the binding site is known to be contributed by a single EGF-like motif of the B2e chain, this demonstrates that activity of this site is independent of chain assembly. Binding activity of both isoforms to collagen IV and the heparan sulphate proteoglycan perlecan was correlated to the nidogen content and could be enhanced by adding nidogen. Binding to heparin was only partial and heparin did not inhibit perlecan binding. This indicated a crucial role for nidogen in mediating the integration of these laminin isoforms into basement membranes. Variant AmB1sB2e showed calcium-dependent binding to fibulin-1, while only a little activity was found for AmB1eB2e. Both isoforms promoted adhesion and spreading of several cell lines. Adhesion could be completely inhibited by antibodies to the integrin beta 1 subunit but not, or only weakly, by antibodies against beta 3, alpha 2, alpha 3, alpha 5 and alpha 6 subunits. No inhibition was observed with an Arg-Gly-Asp-containing peptide.

Similarity in structure between C1q and the collectins as judged by electron microscopy

The collectins are carbohydrate binding proteins which, like C1q, contain collagen-like sequences. The collectins belong to group III of the family of lectins containing C-type carbohydrate recognition domains (CRDs). The structural similarity between the collectins and C1q is clearly demonstrated by electron microscopy in that they all contain multiple polypeptides which are organised into subunits containing triple-helical stalks throughout their collagen-like regions and globular 'heads' in the C-terminal regions. Four, or six, of these structures are associated via distinct, short, N-terminal regions to form the oligomeric molecules seen in the electron microscope. The overall structural similarity between C1q and the collectins, however, does not extend to similarity in amino acid sequences over the C-terminal regions. The C-terminal regions of C1q, unlike those of the collectins, do not contain the conserved residues found in the CRDs present in the C-type lectins. Instead, C1q has a high degree of homology to collagen sequences (Type VIII and X) and this is consistent with the fact that, unlike the collectins, C1q binds to protein motifs in IgG, or IgM, rather than to carbohydrate structures. Also, despite sometimes showing interruptions in their collagen-like regions, the collectins do not always display a 'bend' in their collagen-like 'stalks' similar to that which is seen in C1q. Therefore, C1q may be more closely related to collagens than to the collectins. The collectins can be classed into two distinct group, with MBP and SP-A being hexamers and SP-D, conglutinin and collectin-43 (CL-43) being tetramers, with proteins in the latter group also having significantly larger dimensions with respect to the length of their collagen-like 'stalks'.

Structural similarity between lung surfactant protein D and conglutinin. Two distinct, C-type lectins containing collagen-like sequences

Preparations of bovine lung surfactant D (SP-D) and conglutinin were examined by electron microscopy, gel-filtration and SDS/PAGE. SP-D is composed of non-covalently linked subunits, of 160 kDa, which each contain three, disulphide-linked, 44-kDa polypeptide chains. In the electron microscope a single 160-kDa subunit of SP-D appears as a 45.8 +/- 3-nm-long rod connected to a small globular 'head'. Particles were also seen which correspond to non-covalently linked dimers, trimers and tetramers of the 160-kDa monomer subunit of SP-D. The tetramer structure contains 12 polypeptide chains and is very similar to the electron microscopy images and model reported by Strang et al. [Strang, C. J., Slayter, US., Lachmann, P. J. and Davis, A. E. (1986) Biochem. J. 236, 3811-389] for bovine conglutinin in which four 160-kDa subunits are disulphide-linked to give a molecule of expected molecular mass of 528 kDa. This study confirmed the findings by Strang et al. in the above paper for intact conglutinin and also emphasised that the rod-like structures, of length 37.6 +/- 3.7 nm, seen in the conglutinin subunits were significantly shorter than those in SP-D despite the close similarity in amino acid sequence (79% identify) and chain length between the two proteins. In addition, a truncated form of conglutinin was found in the conglutinin preparations, due to limited proteolysis of the Arg-Ala bond at position 54 in the 44-kDa chains. These truncated conglutinin chains yield a subunit composed of three shortened, non-disulphide-linked, chains and this subunit appears as a monomer with a rod length of 34.2 +/- 2.8 nm in the electron microscope. On gel-filtration, a proportion of the SP-D preparation behaved, as expected, as a molecule with an apparent molecular mass of 600 kDa. The remainder of the SP-D preparation behaved as aggregated material with a molecular mass greater than 900 kDa which yielded no distinct structures in the electron microscope. Intact conglutinin was eluted at a position greater than 900 kDa but yet provided clear electron microscopy images of the tetramer structure described above.(ABSTRACT TRUNCATED AT 400 WORDS)

Structure and binding properties of collagen type XIV isolated from human placenta

Collagen XIV was isolated from neutral salt extracts of human placenta and purified by several chromatographic steps including affinity binding to heparin. The same procedures also led to the purification of a tissue form of fibronectin. Collagen XIV was demonstrated by partial sequence analysis of its Col1 and Col2 domains and by electron microscopy to be a disulphide-linked molecule with a characteristic cross-shape. The individual chains had a size of approximately 210 kD, which was reduced to approximately 180 kD (domain NC3) after treatment with bacterial collagenase. Specific antibodies mainly to NC3 epitopes were obtained by affinity chromatography and used in tissue and cell analyses by immunoblotting and radioimmunoassays. Two sequences from NC3 were identified on fragments obtained after trypsin cleavage. They were identical to cDNA-derived sequences of undulin, a noncollagenous extracellular matrix protein. This suggests that collagen XIV and undulin may be different splice variants from the same gene. Heparin binding was confirmed in ligand assays with a large basement membrane heparan sulphate proteoglycan. This binding could be inhibited by heparin and heparan sulphate but not by chondroitin sulphate. In addition, collagen XIV bound to the triple helical domain of collagen VI. The interactions with heparin sulphate proteoglycan and collagen VI were not shared by the NC3 domain, or by reduced and alkylated collagen XIV. No or only low binding was observed for collagens I-V, pN-collagens I and III, and several noncollagenous matrix proteins, including laminin, recombinant nidogen, BM-40/osteonectin, plasma and tissue fibronectin, vitronectin, and von Willebrand factor. Insignificant activity was also shown in cell attachment assays with nine established cell lines.

Recombinant nidogen consists of three globular domains and mediates binding of laminin to collagen type IV

Recombinant mouse nidogen and two fragments were produced in mammalian cells and purified from culture medium without resorting to denaturing conditions. The truncated products were fragments Nd-I (positions 1-905) comprising the N-terminal globule and rod-like domain and Nd-II corresponding mainly to the C-terminal globule (position 906-1217). Recombinant nidogen was indistinguishable from authentic nidogen obtained by guanidine dissociation from tumor tissue with respect to size, N-terminal sequence, CD spectra and immunochemical properties. They differed in protease stability and shape indicating that the N-terminal domain of the more native, recombinant protein consists of two globules connected by a flexible segment. This established a new model for the shape of nidogen consisting of three globes of variable mass (31-56 kDa) connected by either a rod-like or a thin segment. Recombinant nidogen formed stable complexes (Kd less than or equal to 1 nM) with laminin and collagen IV in binding assays with soluble and immobilized ligands and as shown by electron microscopy. Inhibition assays demonstrated different binding sites on nidogen for both ligands with different specificities. This was confirmed in studies with fragment Nd-I binding to collagen IV and fragment Nd-II binding to laminin fragment P1. In addition, recombinant nidogen but not Nd-I was able to bridge between laminin or P1 and collagen IV. Formation of such ternary complexes implicates a similar role for nidogen in the supramolecular organization of basement membranes.

Molecular characterization of cuticle and interstitial collagens from worms collected at deep sea hydrothermal vents

Two different collagens were isolated and characterized from the body walls of the vestimentiferan tube worm Riftia pachyptila and the annelid Alvinella pompejana, both living around hydrothermal vents at a depth of 2600 m. The acid-soluble cuticle collagens consisted of a long triple helix (2.4 microns for Alvinella, 1.5 microns for Riftia) terminating into a globular domain. Molecular masses of 2600 and 1700 kDa, respectively, were estimated from their dimensions. The two cuticle collagens were also quite different in amino acid composition, in agreement with their different supramolecular organizations within tissues. Interstitial collagens corresponding to cross-striated fibrils underneath the epidermal cells could be solubilized by digestion with pepsin and consisted of a single alpha-chain. They were similar in molecular mass (340 kDa) and length (280 nm) but differed in composition and banding patterns of segment-long-spacing fibrils. This implicates significant sequence differences also in comparison to fibril-forming vertebrate collagens, although all form typical quarter-staggered fibrils. The thermal stability of the worm collagens was, with one exception (interstitial collagen of Riftia), in the range of mammalian and bird collagens (37 to 46 degrees C), and thus distinctly above that of shallow sea water annelids. Yet, their 4-hydroxyproline contents were not directly correlated to this stability. About 20% of Riftia collagen alpha-chain sequence was elucidated by Edman degradation and showed typical Gly-X-Y repeats but only a limited homology (45 to 58% identity) to fibril-forming vertebrate collagens. A single triplet imperfection and the variable hydroxylation of proline in the X position were additional unique features. It suggests that this collagen represents an ancestral form of fibril-forming collagens not directly corresponding to an individual fibril-forming collagen type of vertebrates.

Identification of a major antigenic epitope on CNBr-fragment 11 of type II collagen recognized by murine autoreactive B cells

Immunization of certain strains of mice with native type II collagen (CII) induces both development of arthritis and an antibody response to autologous CII. The autoantibody response in a high-responder strain, the DBA/1 mouse, has been described earlier, and a number of monoclonal antibodies have been characterized for arthritogenicity and autoreactive binding to cartilage in vivo and in vitro. Here we map the antigenic epitope of one of these arthritogenic monoclonal antibodies (CII-C1). It belongs to a group of antibodies recognizing the CNBr fragment alpha 1(II)-CB11 of CII. Using the enzyme-linked immunosorbent assay technique, we show that the antibody reacts only with native, triplehelical CII, but not with other collagens. The antibody is able to stain specifically the CB11 fragment by immunoblotting, suggesting some partial renaturation of the CNBr fragment into triple-helical structures after blotting. The binding site of CII-C1 on CB11 was further focused by rotary shadowing of antibody-labeled CII to a site 89 +/- 8 nm from the amino end of CII, corresponding to the middle of CB11. This location was confirmed by cleavage of CB11 with trypsin, separation of the tryptic peptides by high-performance liquid chromatography and dot-blot analysis of the antigenic peptides with the CII-C1 antibody. Sequencing of the single positive peptide located the antigenic epitope within the sequence GFAGQAGPAGATGAPGRP (residues 316-333). Assuming 0.29 nm per residue, this corresponds to a position within 92-96.5 nm from NH2 terminal end of CII. Apart from glycine residues, which are not exposed on the triple-helical structure, only two amino acid residues (F-x-y-Q) are conserved in CII from different species but are not found in the triple-helix of other collagens except type IV collagen. Therefore, this structure is likely to be of critical importance for the binding of the CII-C1 antibody. Of potential importance is that this structure is also found in certain other arthritogenic proteins such as 65-kDa mycobacterial protein, in CMV and EBV.

Cell adhesion, spreading and neurite stimulation by laminin fragment E8 depends on maintenance of secondary and tertiary structure in its rod and globular domain

The cell adhesion, spreading and neurite-promoting properties of mouse tumor laminin fragment E8, which contains major site(s) responsible for laminin-cell interactions, were probed by proteolytic degradation, denaturation, synthetic peptides and antibody inhibition. Removal of more than half of the N-terminal portion contributing to the rod-like domain did not effect cell attachment or spreading although neurite-promoting activity was reduced. More extensive degradation of the rod or of the globular domains of E8, or separation of the globule from the rod, also resulted in loss of cell spreading activity although weak attachment was found to an A chain subfragment comprising the globular domain and a short piece of the rod. Exposure of E8 to increasing concentrations of dissociating agents produce an apparently reversible denaturation but an irreversible loss of both attachment and neurite-promoting activities, as did reduction and alkylation of disulfide bonds in the globular domain. Although cell adhesion and spreading were blocked by antibodies to an alpha 6 integrin subunit, neurite outgrowth was unaffected, indicating two distinct receptors for these two activities. Furthermore, a synthetic peptide, the sequence of which is found in the vicinity of adhesion and neurite-promoting sites and previously implicated in neurite growth and cell attachment activities, was found to be inactive. These results indicate that the major cell attachment and neurite-promoting sites of laminin are distinct although both require the native conformation of parts of the rod and the terminal globular domain of the long arm of laminin.

Binding of the pentamer/hexamer forms of mannan-binding protein to zymosan activates the proenzyme C1r2C1s2 complex, of the classical pathway of complement, without involvement of C1q

The serum lectin, mannan binding protein (MBP), was isolated in a yield of 40 micrograms/liter from pooled normal human serum by affinity chromatography on mannan-Sepharose, followed by gel-filtration and ion-exchange chromatography and finally by passage down an anti-IgM Sepharose column. A rabbit antiserum was prepared against the purified MBP and an enzyme-linked immunoassay developed that used both the specificity of the polyclonal antibody and the Ca+(+)-dependent carbohydrate binding property of MBP. Assay of the sera from 103 blood-donors showed a wide range of MBP levels, ranging from 0 to 870 micrograms/liter. MBP, after interaction with zymosan, caused efficient activation of a C1r2 125I-C1s2 complex that was prepared by incubation of 125I-C1s2 with serum, from a patient with a complete genetic deficiency of C1q, followed by gel-filtration on Sepharose 6B. The purified MBP is composed of a mixture of trimers, tetramers, pentamers, and hexamers of an approximate 90-kDa structural unit as judged by chromatography, SDS-PAGE and electron microscopy studies. Only the molecules in the pentamer/hexamer fraction, which have a similar overall structure to that of C1q, appeared to cause efficient, zymosan-dependent, activation of C1s within the C1r2C1s2 complex. The pentamer/hexamer form of MBP may therefore play an important role in antibody-independent activation of the C system during the early stages of certain infections.

Binding of the pentamer/hexamer forms of mannan-binding protein to zymosan activates the proenzyme C1r2C1s2 complex, of the classical pathway of complement, without involvement of C1q

The serum lectin, mannan binding protein (MBP), was isolated in a yield of 40 micrograms/liter from pooled normal human serum by affinity chromatography on mannan-Sepharose, followed by gel-filtration and ion-exchange chromatography and finally by passage down an anti-IgM Sepharose column. A rabbit antiserum was prepared against the purified MBP and an enzyme-linked immunoassay developed that used both the specificity of the polyclonal antibody and the Ca+(+)-dependent carbohydrate binding property of MBP. Assay of the sera from 103 blood-donors showed a wide range of MBP levels, ranging from 0 to 870 micrograms/liter. MBP, after interaction with zymosan, caused efficient activation of a C1r2 125I-C1s2 complex that was prepared by incubation of 125I-C1s2 with serum, from a patient with a complete genetic deficiency of C1q, followed by gel-filtration on Sepharose 6B. The purified MBP is composed of a mixture of trimers, tetramers, pentamers, and hexamers of an approximate 90-kDa structural unit as judged by chromatography, SDS-PAGE and electron microscopy studies. Only the molecules in the pentamer/hexamer fraction, which have a similar overall structure to that of C1q, appeared to cause efficient, zymosan-dependent, activation of C1s within the C1r2C1s2 complex. The pentamer/hexamer form of MBP may therefore play an important role in antibody-independent activation of the C system during the early stages of certain infections.

Binding of nidogen and the laminin-nidogen complex to basement membrane collagen type IV

The laminin-nidogen complex and purified nidogen both bind collagen IV but not other collagens, as shown by solid-state ligand-binding and inhibition assays. Laminin purified from the dissociated complex and a variety of laminin proteolytic fragments failed to bind collagen IV. Complexes formed in solution between nidogen or laminin-nidogen and collagen IV were visualized by rotary shadowing which identified one major binding site about 80 nm away from the C-terminus of the collagen triple helix. A second, weaker binding site may exist closer to its N-terminus. Binding sites of nidogen were assigned to its C-terminal globular domain which also possesses laminin-binding structures. A more diverse collagen-IV-binding pattern was observed for the laminin nidogen complex, whereby interactions may involve both nidogen and short-arm structures of laminin.

Extended and globular protein domains in cartilage proteoglycans

Electron microscopy after rotary shadowing and negative staining of the large chondroitin sulphate proteoglycan from rat chondrosarcoma, bovine nasal cartilage and pig laryngeal cartilage demonstrated a unique multidomain structure for the protein core. A main characteristic is a pair of globular domains (diameter 6-8 nm), one of which forms the N-terminal hyaluronate-binding region. They are connected by a 25 nm-long rod-like domain of limited flexibility. This segment is continued by a 280 nm-long polypeptide strand containing most chondroitin sulphate chains (average length 40 nm) in a brush-like array and is terminated by a small C-terminal globular domain. The core protein showed a variable extent of degradation, including the loss of the C-terminal globular domain and sections of variable length of the chondroitin sulphate-bearing strand. The high abundance (30-50%) of the C-terminal domain in some extracted proteoglycan preparations indicated that this structure is present in the cartilage matrix rather than being a precursor-specific segment. It may contain the hepatolectin-like segment deduced from cDNA sequences corresponding to the 3'-end of protein core mRNA [Doege, Fernandez, Hassell, Sasaki & Yamada (1986) J. Biol. Chem. 261, 8108-8111; Sai, Tanaka, Kosher & Tanzer (1986) Proc. Natl. Acad. Sci. 83, 5081-5085; Oldberg, Antonsson & Heinegård (1987) Biochem. J. 243, 255-259].

Domain structure of cartilage proteoglycans revealed by rotary shadowing of intact and fragmented molecules

The rotary-shadowing technique for molecular electron microscopy was used to study cartilage proteoglycan structure. The high resolution of the method allowed demonstration of two distinct globular domains as well as a more strand-like portion in the core protein of large aggregating proteoglycans. Studies of proteoglycan aggregates and fragments showed that the globular domains represent the part of the proteoglycans that binds to the hyaluronic acid, i.e. the hyaluronic acid-binding region juxtapositioned to the keratan sulphate-attachment region. The strand-like portion represents the chondroitin sulphate-attachment region. Low-Mr proteoglycans from cartilage could be seen as a globule connected to one or two side-chain filaments of chondroitin sulphate.

Structure and interactions of heparan sulfate proteoglycans from a mouse tumor basement membrane

Various forms of heparan sulfate proteoglycan were solubilized from the mouse Engelbreth-Holm-Swarm (EHS) sarcoma by extraction with 0.5 M NaCl, collagenase digestion and extraction with 4 M guanidine. They could be separated into high (greater than or equal to 1.65 g/ml) and low (1.38 g/ml) buoyant density variants. The high-density form from the NaCl extract and collagenase digest had Mr = 130000 and So20,W = 4.5 S and contained 4-10% protein, indicating Mr = 5 000-12 000 for the protein core. This proteoglycan exhibited polydispersity as shown by rotary shadowing electron microscopy and ultracentrifugation. An average molecule consisted of four heparan sulfate chains (Mr = 29 000) each with a length of 32 +/- 10 nm. The low-density form (Mr about 400 000) could not be completely purified and contained about 50% protein. As shown by radioimmunoassay, the various proteoglycans shared similar protein cores. Labeling of the tumor in vivo or in vitro demonstrated preferential incorporation of radioactive sulfate in the high-density form. The high-density proteoglycan interacted in affinity chromatography by virtue of its heparan sulfate chains with laminin, fibronectin, the globular domain NC1 and the triple helix of collagen IV. These interactions were abolished at moderate concentrations of NaCl (0.1-0.2 M) and in the presence of heparin, chondroitin sulfate or dextran sulfate. Interactions with the globule NC1 could also be demonstrated by velocity band centrifugation in sucrose gradients and a binding constant of about 10(6) M-1 was derived.

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.

Subunit structure and assembly of the globular domain of basement-membrane collagen type IV

The globular domain of collagen IV was solubilized by collagenase digestion from a mouse tumor, human placenta and bovine aorta and was purified by chromatographic methods. The materials show a unique, mainly non-collagenous amino acid composition and contain small amounts of glucosamine and galactosamine. The globular structures with Mr = 170 000 appear as a hexameric assembly originating from two collagen IV molecules. Subunits of this assembly are two different dimers Da and Db (Mr about 56 000) and monomers (Mr = 28 000). Their N-terminal amino acid sequences start with short triple-helical sequences, which overlap with the C-terminal triple helix of the alpha 1(IV) and alpha 2(IV) chain, demonstrating that the globule originates from the C terminus of collagen IV. Dimers arise from monomers by disulfide cross-linking (form Db) and/or formation of non-reducible cross-links (form Da). Reduction under non-denaturing conditions causes partial dissociation of the globule and of collagen IV dimers, indicating that reducible cross-links are formed between monomers of two different collagen IV molecules. Dissociation of the hexamer into the subunits can be achieved with 8 M urea, sodium dodecyl sulfate or in the pH range 2.5-4. The latter indicates that carboxyl groups are essential for association. Mixtures of the subunits (monomers and dimers) or purified dimers reassemble in neutral buffer into hexamers as shown by ultracentrifugation and electron microscopy. Reconstituted hexamers, however, dissociate in a much broader pH range than the native globules. Circular dichroic spectra indicate that the structure is more completely refolded from acid-treated than from urea-treated material. These data suggest that globules originating from monomers (as existing in single collagen IV molecules) are stabilized by the adjacent triple helix. Covalent cross-link formation stabilizes the globular structure and allows reconstitution in stoichiometric proportions.

Isolation and characterization of a precursor form of M collagen from embryonic chicken cartilage

A disulfide-cross-linked collagen has been extracted with neutral salt solutions from organ cultures of embryonic chick sternal cartilage. This collagen, which we term pM collagen, is presumed to be the native extracellular precursor molecule to disulfide-cross-linked collagen fragments recently described. Cleavage of pM collagen under native conditions with pepsin gives rise to the collagen fragments M1 and M2, which had also been isolated from pepsin extracts of chick hyaline cartilage [K. von der Mark, M. van Menxel & H. Wiedemann (1982) Eur. J. Biochem. 124, 57-62]. Native pM collagen was purified by DEAE-cellulose chromatography and agarose gel filtration. On agarose and following polyacrylamide gel electrophoresis, the unreduced molecule migrates with an apparent Mr of 300 000. Reduction of disulfide bridges produces two subunits with Mr 80 000 (pMa) and 60 000 (pMb) when compared with collagen standards. Cyanogen bromide cleavage of pMa and pMb, excised from dodecyl sulfate gels, resulted in different peptide maps, indicating that both components are genetically distinct polypeptide chains. The occasional appearance of the unreduced pM collagen as a doublet band on dodecyl sulfate gels and the observation that pMa and pMb occur in non-stoichiometric ratios suggests that pMa and pMb form separate native molecules, although their incorporation into a single pM molecule cannot be excluded. Native pM collagen was completely digested with bacterial collagenase, and contained hydroxyproline and proline in a ratio of 1.15:1, indicating the absence of significant non-collagenous domains. Thus it represents, despite several pepsinlabile sites, more likely a largely triplehelical, processed form of collagen rather than a procollagen-like molecule containing globular domains. Processing of pM collagen to M1 and M2 fragments or other intermediate forms was not observed in cartilage organ culture or in chondrocyte cell cultures within 18 h.

The use of monoclonal antibodies to fragments of chicken type IV collagen in structural and localization studies

In previous experiments, three pepsin-resistant fragments of type IV collagen were isolated from chicken gizzards and designated 7S, F3, and (F1)2F2 (Mayne, R., and Zettergren, J. G. (1980) Biochemistry 19, 4065-4072). In the present experiments, a series of monoclonal antibodies to type IV collagen were prepared, each one of which recognized an epitope present in only one of the three fragments. A high molecular weight fraction of type IV collagen (designated 7S + arms (215 nm)) was isolated after agarose gel filtration and characterized by electron microscopy after rotary shadowing and by gel electrophoresis. Analysis of 7S + arms (215 nm) by inhibition enzyme-linked immunosorbent assay demonstrated the presence of the epitopes for 7S and F3 but not for (F1)2F2. This result, therefore, provides additional evidence that the order of the pepsin-resistant fragments of chicken type IV collagen is 7S-F3-(F1)2F2.

Electron-microscopical approach to a structural model of intima collagen

Intima collagen was studied by electron microscopy (rotary shadowing and negative staining) and by analytical ultracentrifugation. It was found that the monomeric unit (Mr 170 000) consists of a 105 nm-long triple helix terminated by a small globular domain (Mr about 30 000) at one end and a large globular domain (Mr about 40 000) at the other end. The monomer was produced by selective reduction of interchain disulphide bridges. Before reduction, dimers, tetramers and larger filamentous structures were found. Dimers are lateral staggered aggregates of two monomers aligned in an anti-parallel fashion. This gives rise to an inner 75 nm-long region of two slightly intertwisted triple helices flanked by the large globular domains. The outer triple-helical segments (length 30 nm) with the small globular domains at their ends emerge at both sides of this structure. Interchain disulphide bridges are probably located in the vicinity of the large domains. Only the outer segments could be degraded by bacterial collagenase. In tetramers the outer segments of two dimers are covalently linked, forming a scissors-like structure. In the fibrous forms several tetramers are assembled end-to-end with an overlap between the outer segments. The molecular masses and sedimentation coefficients were calculated for these various forms from the electron-microscopically observed dimensions and agreed with results obtained by ultracentrifugation. The unique structure of intima collagen suggests that it originates from a microfibrillar component and that it can be considered a unique collagenous protein, for which we propose the designation type VI collagen.