Reductive cleavage and reformation of the interchain and intrachain disulfide bonds in the globular hexameric domain NC1 involved in network assembly of basement membrane collagen (type IV)

Structural and Functional Plasticity of Collagen Fibrils

Collagen is a major component of the subendothelial matrix and participates in bleeding arrest by activating and aggregating platelets at the site of vascular injury. The most common type I collagen exists in both soluble and fibrillar forms, but structural exchangeability between the two forms is currently unknown. Using atomic force microscopy, we show that type I collagen switches between soluble and fibrillar forms in a pH-dependent and ion-independent manner. Fibrillar collagen is rope like with characteristic "D-bands." The collagen fibrils can be disrupted with 0.1 M acetic acid and will reform when the pH is adjusted to 7.4. This structural plasticity leads to drastically different activities, with fibrillar collagen being significantly more active for platelets under static and flow conditions. More important, by probing with noncontact hopping probe ion-conductance microscopy, we find that platelets adherent to fibrillar collagen present primarily as high-density bubble shapes that have undergone rapid microvesiculation.

The Ancient Immunoglobulin Domains of Peroxidasin Are Required to Form Sulfilimine Cross-links in Collagen IV

The collagen IV sulfilimine cross-link and its catalyzing enzyme, peroxidasin, represent a dyad critical for tissue development, which is conserved throughout the animal kingdom. Peroxidasin forms novel sulfilimine bonds between opposing methionine and hydroxylysine residues to structurally reinforce the collagen IV scaffold, a function critical for basement membrane and tissue integrity. However, the molecular mechanism underlying cross-link formation remains unclear. In this work, we demonstrate that the catalytic domain of peroxidasin and its immunoglobulin (Ig) domains are required for efficient sulfilimine bond formation. Thus, these molecular features underlie the evolutionarily conserved function of peroxidasin in tissue development and integrity and distinguish peroxidasin from other peroxidases, such as myeloperoxidase (MPO) and eosinophil peroxidase (EPO).

Peroxidasin forms sulfilimine chemical bonds using hypohalous acids in tissue genesis

Collagen IV comprises the predominant protein network of basement membranes, a specialized extracellular matrix, which underlie epithelia and endothelia. These networks assemble through oligomerization and covalent crosslinking to endow mechanical strength and shape cell behavior through interactions with cell-surface receptors. A recently discovered sulfilimine (S=N) bond between a methionine sulfur and hydroxylysine nitrogen reinforces the collagen IV network. We demonstrate that peroxidasin, an enzyme found in basement membranes, catalyzes formation of the sulfilimine bond. Drosophila peroxidasin mutants have disorganized collagen IV networks and torn visceral muscle basement membranes, pointing to a critical role for the enzyme in tissue biogenesis. Peroxidasin generates hypohalous acids as reaction intermediates, suggesting a paradoxically anabolic role for these usually destructive oxidants. This work highlights sulfilimine bond formation as what is to our knowledge the first known physiologic function for peroxidasin, a role for hypohalous oxidants in tissue biogenesis, and a possible role for peroxidasin in inflammatory diseases.

Increased Goodpasture antigen-binding protein expression induces type IV collagen disorganization and deposit of immunoglobulin A in glomerular basement membrane

Increased expression of Goodpasture antigen-binding protein (GPBP), a protein that binds and phosphorylates basement membrane collagen, has been associated with immune complex-mediated pathogenesis. However, recent reports have questioned this biological function and proposed that GPBP serves as a cytosolic ceramide transporter (CERT(L)). Thus, the role of GPBP in vivo remains unknown. New Zealand White (NZW) mice are considered healthy animals although they convey a genetic predisposition for immune complex-mediated glomerulonephritis. Here we show that NZW mice developed age-dependent lupus-prone autoimmune response and immune complex-mediated glomerulonephritis characterized by elevated GPBP, glomerular basement membrane (GBM) collagen disorganization and expansion, and deposits of IgA on disrupted GBM. Transgenic overexpression of human GPBP (hGPBP) in non-lupus-prone mice triggered similar glomerular abnormalities including deposits of IgA on a capillary GBM that underwent dissociation, in the absence of an evident autoimmune response. We provide in vivo evidence that GPBP regulates GBM collagen organization and its elevated expression causes dissociation and subsequent accumulation of IgA on the GBM. Finally, we describe a previously unrecognized pathogenic mechanism that may be relevant in human primary immune complex-mediated glomerulonephritis.

The central role of vascular extracellular matrix and basement membrane remodeling in metabolic syndrome and type 2 diabetes: the matrix preloaded

The vascular endothelial basement membrane and extra cellular matrix is a compilation of different macromolecules organized by physical entanglements, opposing ionic charges, chemical covalent bonding, and cross-linking into a biomechanically active polymer. These matrices provide a gel-like form and scaffolding structure with regional tensile strength provided by collagens, elasticity by elastins, adhesiveness by structural glycoproteins, compressibility by proteoglycans – hyaluronans, and communicability by a family of integrins, which exchanges information between cells and between cells and the extracellular matrix of vascular tissues.

Each component of the extracellular matrix and specifically the capillary basement membrane possesses unique structural properties and interactions with one another, which determine the separate and combined roles in the multiple diabetic complications or diabetic opathies.

Metabolic syndrome, prediabetes, type 2 diabetes mellitus, and their parallel companion (atheroscleropathy) are associated with multiple metabolic toxicities and chronic injurious stimuli. The adaptable quality of a matrix or form genetically preloaded with the necessary information to communicate and respond to an ever-changing environment, which supports the interstitium, capillary and arterial vessel wall is individually examined.

Identification of S-hydroxylysyl-methionine as the covalent cross-link of the noncollagenous (NC1) hexamer of the alpha1alpha1alpha2 collagen IV network: a role for the post-translational modification of lysine 211 to hydroxylysine 211 in hexamer assembly

Collagen IV networks are present in all metazoans as components of basement membranes that underlie epithelia. They are assembled by the oligomerization of triple-helical protomers, composed of three alpha-chains. The trimeric noncollagenous domains (NC1) of each protomer interact forming a hexamer structure. Upon exposure to acidic pH or denaturants, the hexamer dissociates into monomer and dimer subunits, the latter reflect distinct interactions that reinforce/cross-link the quaternary structure of hexamer. Recently, the cross-link site of the alpha1alpha1alpha2 network was identified, on the basis of x-ray crystal structures at 1.9-A resolution, in which the side chains of Met93 and Lys211 were proposed to be connected by a novel thioether bond (Than, M. E., Henrich, S., Huber, R., Ries, A., Mann, K., Kuhn, K., Timpl, R., Bourenkov, G. P., Bartunik, H. D., and Bode, W. (2002) Proc. Natl. Acad. Sci. U. S. A. 99, 6607-6612); however, at the higher resolution of 1.5 A, we found no evidence for this cross-link (Vanacore, R. M., Shanmugasundararaj, S., Friedman, D. B., Bondar, O., Hudson, B. G., and Sundaramoorthy, M. (2004) J. Biol. Chem. 279, 44723-44730). Given this discrepancy in crystallographic findings, we sought chemical evidence for the location and nature of the reinforcement/cross-link site. Trypsin digestion of monomer and dimer subunits excised a approximately 5,000-Da complex that distinguished dimers from monomers; the complex was characterized by mass spectrometry, Edman degradation, and amino acid composition analyses. The tryptic complex, composed of two peptides of 44 residues derived from two alpha1 NC1 monomers, contained Met93 and Lys211 post-translationally modified to hydroxylysine (Hyl211). Truncation of the tryptic complex with post-proline endopeptidase reduced its size to 14 residues to facilitate characterization by tandem mass spectrometry, which revealed a covalent linkage between Met93 and Hyl211. The novel cross-link, termed S-hydroxylysyl-methionine, reflects at least two post-translational events in its formation: the hydroxylation of Lys211 to Hyl211 within the NC1 domain during the biosynthesis of alpha-chains and the connection of Hyl211 to Met93 between the trimeric NC1 domains of two adjoining triple-helical protomers, reinforcing the stability of collagen IV networks.

The alpha1.alpha2 network of collagen IV. Reinforced stabilization of the noncollagenous domain-1 by noncovalent forces and the absence of Met-Lys cross-links

Collagen IV networks are present in all metazoa and underlie epithelia as a component of basement membranes. The networks are essential for tissue function and are defective in disease. They are assembled by the oligomerization of triple-helical protomers that are linked end-to-end. At the C terminus, two protomers are linked head-to-head by interactions of their trimeric noncollagenous domains, forming a hexamer structure. This linkage in the alpha1.alpha2 network is stabilized by a putative covalent Met-Lys cross-link between the trimer-trimer interface (Than, M. E., Henrich, S., Huber, R., Ries, A., Mann, K., Kuhn, K., Timpl, R., Bourenkov, G. P., Bartunik, H. D., and Bode, W. (2002) Proc. Natl. Acad. Sci. U. S. A. 99, 6607-6612) forming a nonreducible dimer that connects the hexamer. In the present study, this cross-link was further investigated by: (a) comparing the 1.5-A resolution crystal structures of the alpha1.alpha2 hexamers from bovine placenta and lens capsule basement membranes, (b) mass spectrometric analysis of monomer and nonreducible dimer subunits of placenta basement membrane hexamers, and (c) hexamer dissociation/re-association studies. The findings rule out the novel Met-Lys cross-link, as well as other covalent cross-links, but establish that the nonreducible dimer is an inherent structural feature of a subpopulation of hexamers. The dimers reflect the reinforced stabilization, by noncovalent forces, of the connection between two adjoining protomers of a network. The reinforcement extends to other types of collagen IV networks, and it underlies the cryptic nature of a B-cell epitope of the alpha3.alpha4.alpha5 hexamer, implicating the stabilization event in the etiology and pathogenesis of Goodpasture autoimmune disease.

Quaternary organization of the goodpasture autoantigen, the alpha 3(IV) collagen chain. Sequestration of two cryptic autoepitopes by intrapromoter interactions with the alpha4 and alpha5 NC1 domains

Goodpasture's (GP) disease is caused by autoantibodies that target the alpha3(IV) collagen chain in the glomerular basement membrane (GBM). Goodpasture autoantibodies bind two conformational epitopes (E(A) and E(B)) located within the non-collagenous (NC1) domain of this chain, which are sequestered within the NC1 hexamer of the type IV collagen network containing the alpha3(IV), alpha4(IV), and alpha5(IV) chains. In this study, the quaternary organization of these chains and the molecular basis for the sequestration of the epitopes were investigated. This was accomplished by physicochemical and immunochemical characterization of the NC1 hexamers using chain-specific antibodies. The hexamers were found to have a molecular composition of (alpha3)(2)(alpha4)(2)(alpha5)(2) and to contain cross-linked alpha3-alpha5 heterodimers and alpha4-alpha4 homodimers. Together with association studies of individual NC1 domains, these findings indicate that the alpha3, alpha4, and alpha5 chains occur together in the same triple-helical protomer. In the GBM, this protomer dimerizes through NC1-NC1 domain interactions such that the alpha3, alpha4, and alpha5 chains of one protomer connect with the alpha5, alpha4, and alpha3 chains of the opposite protomer, respectively. The immunodominant Goodpasture autoepitope, located within the E(A) region, is sequestered within the alpha3alpha4alpha5 protomer near the triple-helical junction, at the interface between the alpha3NC1 and alpha5NC1 domains, whereas the E(B) epitope is sequestered at the interface between the alpha3NC1 and alpha4NC1 domains. The results also reveal the network distribution of the six chains of collagen IV in the renal glomerulus and provide a molecular explanation for the absence of the alpha3, alpha4, alpha5, and alpha6 chains in Alport syndrome.

Sulphur: the vulnerable factor X in atherosclerosis

In the light of Erwin Schrödinger's remark, that all science is in constant interaction with culture and human thoughts, I want to propose a hypothesis regarding the etiology of atherosclerosis, that challenges the lipid theory. In our Western culture fat is associated with bad habits, bad character and illness, thus it was not surprising that hyperlipidemia was proposed to be the etiological cause of atherosclerosis and early death in coronary heart disease. This made us look in the wrong direction and enormous amounts of money and personal prestige has been invested in the lipid theory. I believe instead that the answer to the question of what is behind the atherosclerotic process, is hidden in the structure of the extracellular matrix. Our defenses, developed by evolution, against harmful protein modulation are today overcome by new substances in food and environment. The result of this is an injury to the basal membrane in the wall of the blood vessel and the atherosclerotic process starts.

The 1.9-A crystal structure of the noncollagenous (NC1) domain of human placenta collagen IV shows stabilization via a novel type of covalent Met-Lys cross-link

Triple-helical collagen IV protomers associate through their N- and C-termini forming a three-dimensional network, which provides basement membranes with an anchoring scaffold and mechanical strength. The noncollagenous (NC1) domain of the C-terminal junction between two adjacent collagen IV protomers from human placenta was crystallized and its 1.9-A structure was solved by multiple anomalous diffraction (MAD) phasing. This hexameric NC1 particle is composed of two trimeric caps, which interact through a large planar interface. Each cap is formed by two alpha 1 fragments and one alpha 2 fragment with a similar previously uncharacterized fold, segmentally arranged around an axial tunnel. Each monomer chain folds into two structurally very similar subdomains, which each contain a finger-like hairpin loop that inserts into a six-stranded beta-sheet of the neighboring subdomain of the same or the adjacent chain. Thus each trimer forms a quite regular, but nonclassical, sixfold propeller. The trimer-trimer interaction is further stabilized by a previously uncharacterized type of covalent cross-link between the side chains of a Met and a Lys residue of the alpha 1 and alpha 2 chains from opposite trimers, explaining previous findings of nonreducible cross-links in NC1. This structure provides insights into NC1-related diseases such as Goodpasture and Alport syndromes.

A switch in disulfide linkage during minicollagen assembly in hydra nematocysts or how to assemble a 150-bar-resistant structure

Hydra minicollagen, the shortest collagen known, is an important component of the nematocyst wall, which has a very high tensile strength. It has an unusual structure, with small and closely related Cys-rich domains at both ends of its chains. Three chains are trimerized by a central collagenous domain. Polyhydroxyproline helices connect the Cys-rich domains with the collagenous domain. The minicollagen precursor contains three internal disulfide bridges in each Cys-rich domain and no disulfide bridges between chains of the same trimeric molecule or between different molecules. Biochemical and structural evidence as well as confocal immunofluorescence microscopy points to disulfide-mediated assembly during maturation of nematocysts.

Comparative analysis of the noncollagenous NC1 domain of type IV collagen: identification of structural features important for assembly, function, and pathogenesis

Type IV collagen alpha1-alpha6 chains have important roles in the assembly of basement membranes and are implicated in the pathogenesis of Goodpasture syndrome, an autoimmune disorder, and Alport syndrome, a hereditary renal disease. We report comparative sequence analyses and structural predictions of the noncollagenous C-terminal globular NC1 domain (28 sequences). The inferred tree verified that type IV collagen sequences fall into two groups, alpha1-like and alpha2-like, and suggested that vertebrate alpha3/alpha4 sequences evolved before alpha1/alpha2 and alpha5/alpha6. About one fifth of NC1 residues were identified to confer either the alpha1 or alpha2 group-specificity. These residues accumulate opposite charge in subdomain B of alpha1 (positive) and alpha2 (negative) sequences and may play a role in the stoichiometric chain selection upon type IV collagen assembly. Neural network secondary structure prediction on multiple aligned sequences revealed a subdomain core structure consisting of six hydrophobic beta-strands and one short alpha-helix with a significant hydrophobic moment. The existence of opposite charges in the alpha-helices may carry implications for intersubdomain interactions. The results provide a rationale for defining the epitope that binds Goodpasture autoantibodies and a framework for understanding how certain NC1 mutations may lead to Alport syndrome. A search algorithm, based entirely on amino acid properties, yielded a possible similarity of NC1 to tissue inhibitor of metalloproteinases (TIMP) and prompted an investigation of a possible functional relationship. The results indicate that NC1 preparations decrease the activity of matrix metalloproteinases 2 and 3 (MMP-2, MMP-3) toward a peptide substrate, though not to [14C]-gelatin. We suggest that an ancestral NC1 may have been incorporated into type IV collagen as an evolutionarily mobile domain carrying proteinase inhibitor function.

Characterization of thioredoxins by sodium dodecyl sulfate-slab gel electrophoresis and high performance capillary electrophoresis

Disulfide containing proteins--thioredoxins from E. coli and pig heart mitochondria--were characterized by sodium dodecyl sulfate (SDS)-electrophoresis and high performance capillary electrophoresis (HPCE). Following the mitochondrial thioredoxin samples at different stages of purification, we found that their electrophoretic patterns vary, dependent on the redox condition of isolation, preparation of the samples for SDS-electrophoresis, and sample storage. All these factors influenced the relative intensities of several protein bands with thioredoxin-like mobility, whereas the sample storage also resulted in the appearance of SDS- and dithiothreitol (DTT)-resistant high molecular mass forms, probably thioredoxin dimers. The multiple forms of the thioredoxin from pig heart mitochondria in SDS-electrophoresis might be dependent on the oxidation state of the protein cysteine residues. A commercial preparation of the thioredoxin from E. coli did not exhibit any changes in mobility in SDS gels whether the sample was prepared with or without DTT. After the final purification step no correlation was found between mitochondrial thioredoxin activity, determined in the insulin assay, and its purity in SDS-electrophoresis. A correlation was, however, found when analyzing the thioredoxin by HPCE. The latter approach demonstrated the heterogeneity of the thioredoxin samples homogeneous on SDS electrophoresis, only one of the several HPCE peaks being active in the insulin assay. Also, thioredoxin from E. coli, homogeneous on SDS-electrophoresis, was found heterogeneous on HPCE. The peak corresponding to the insulin-dependent thioredoxin activity was split into two by DTT treatment, suggesting that redox transformations of thioredoxin could be followed by HPCE.

Basement-membrane stromal relationships: interactions between collagen fibrils and the lamina densa

Collagens, the most abundant molecules in the extracellular space, predominantly form either fibrillar or sheet-like structures-the two major supramolecular conformations that maintain tissue integrity. In connective tissues, other than cartilage, collagen fibrils are mainly composed of collagens I, III, and V at different molecular ratios, exhibiting a D-periodic banding pattern, with diameters ranging from 30 to 150 nm, that can form a coarse network in the extracellular matrix in comparison with a fine meshwork of lamina densa. The lamina densa represents a stable sheet-like meshwork composed of collagen IV, laminin, nidogen, and perlecan compartmentalizing tissue from one another. We hypothesize that the interactions between collagen fibrils and the lamina densa are crucial for maintaining tissue-tissue interactions. A detailed analysis of these interactions forms the basis of this review article. Here, we demonstrate that there is a direct connection between collagen fibrils and the lamina densa and propose that collagen V may play a crucial role in this connection. Collagen V might also be involved in regulation of collagen fibril diameter and anchoring of epithelia to underlying connective tissues.

Different immunologic properties of the globular NC1 domain of collagen type IV isolated from various human basement membranes

The C-terminal globular domain NC1 of collagen IV, which carries the epitopes recognized by anti-GBM antibodies in Goodpasture's syndrome, was isolated from human basement membranes (BM) of glomeruli (GBM-NC1), tubules (TBM-NC1), lung (ABM-NC1), placenta (PBM-NC1), and small intestine (IBM-NC1). All NC1 hexamers were of globular size on electron microscopy. On SDS-PAGE, the hexamers dissociated into monomeric and dimer-sized subunits of similar molecular weights. The following monomer:dimer relationships were identified: GBM-NC1, and PBM-NC1 = 1:3; ABM-NC1 = 1:4; and IBM-NC1 = 1:32. On immunoblot, all dimers of the various NC1 globules showed binding of anti-GBM antibodies. However, monomers stained differently, with three monomers demonstrable in GBM-NC1 and no monomer staining in PBM-NC1. In addition, studies with monoclonal antibodies showed that the C-terminus of the alpha 1(IV) collagen chain was demonstrable in all different NC1 hexamers. In contrast, the alpha 3(IV) chain, to which Goodpasture sera preferentially bind, showed a restricted distribution. One monomer and dimers were demonstrable in GBM-NC1 and ABM-NC1, only a weak dimer staining was seen in TBM-NC1, while no evidence for alpha 3(IV) was found in IBM-NC1 and PBM-NC1. Dissociation by 6 M guanidine-HC1 or treatment by acid increases the apparent number of accessible epitopes for anti-GBM antibodies. In addition, dose-response curves, which were obtained by incubation of increasing concentrations of NC1 with anti-GBM antibody positive sera, indicated that for GBM-NC1 and ABM-NC1 the lowest NC1 protein concentrations were necessary to bind 50% of the antibodies.(ABSTRACT TRUNCATED AT 250 WORDS)

Basement membrane proteins: structure, assembly, and cellular interactions

Basement membranes are thin layers of a specialized extracellular matrix that form the supporting structure on which epithelial and endothelial cells grow, and that surround muscle and fat cells and the Schwann cells of peripheral nerves. One common denominator is that they are always in close apposition to cells, and it has been well demonstrated that basement membranes do not only provide a mechanical support and divide tissues into compartments, but also influence cellular behavior. The major molecular constituents of basement membranes are collagen IV, laminin-entactin/nidogen complexes, and proteoglycans. Collagen IV provides a scaffold for the other structural macromolecules by forming a network via interactions between specialized N- and C-terminal domains. Laminin-entactin/nidogen complexes self-associate into less-ordered aggregates. These two molecular assemblies appear to be interconnected, presumably via binding sites on the entactin/nidogen molecule. In addition, proteoglycans are anchored into the membrane by an unknown mechanism, providing clusters of negatively charged groups. Specialization of different basement membranes is achieved through the presence of tissue-specific isoforms of laminin and collagen IV and of particular proteoglycan populations, by differences in assembly between different membranes, and by the presence of accessory proteins in some specialized basement membranes. Many cellular responses to basement membrane proteins are mediated by members of the integrin class of transmembrane receptors. On the intracellular side some of these signals are transmitted to the cytoskeleton, and result in an influence on cellular behavior with respect to adhesion, shape, migration, proliferation, and differentiation. Phosphorylation of integrins plays a role in modulating their activity, and they may therefore be a part of a more complex signaling system.

Short and long range order in basement membrane type IV collagen revealed by enzymic and chemical extraction

Oriented bovine lens capsules give X-ray diffraction patterns suggesting a considerable degree of order in the collagenous components, predominantly type IV collagen. Here we report the effects of preliminary treatment of lens capsules before orientation. Extraction with 4 M guanidinium hydrochloride or with heparinase/hyaluronidase reveals the same collagenous diffraction patterns previously seen after extraction with 1 M NaCl. There is a four-point pattern of d-spacing 3.9 nm, indicating liquid crystal cybotactic nematic organization, along with sharp streaked meridional reflections which index as orders of 21 nm. This suggests that the removal of basement membrane proteoglycans results in a reduction in diffuse scatter and clarification of the pattern. Extraction of the lens capsules with trypsin or dithiothreitol greatly reduces the intensity of the four-point pattern while leaving the meridional pattern unaffected. This strengthens the evidence that the 21 nm period has its origins in the collagen IV helix. Reduction in the four-point pattern could arise if disruption of non-helical NC1 domains or 7S overlap regions allows slippage of the collagen molecules on orientation, weakening the proposed 1 nm intermolecular stagger. Ultra-low angle diffraction patterns of extracted lens capsules show meridional reflections which index as a long-range axial repeat of approximately 95 nm. This is consistent with a model of microfibrils of type IV collagen in which the NC1 domains bind to the collagen helix at approximately 100 nm intervals, as has been previously suggested.

Properties of bovine nephritogenic antigen that induces anti-GBM nephritis in rats and its similarity to the Goodpasture antigen

The nephritogenic antigen that induces antiglomerular basement membrane antibody-induced glomerulonephritis (anti-GBM nephritis) in rats was isolated from collagenase-solubilized bovine renal basement membranes. Purification was achieved using antibody-coupled affinity columns which were originally used for the purification of trypsin-solubilized nephritogenic antigen (Sado et al. 1984a). The nephritogenic antigen was a heteropolymer composed of P2 (Mr 28 kDa) and P3 (Mr 30 kDa) polypeptides as monomers and their dimers in sodium-dodecyl-sulfate (SDS) polyacrylamide gel electrophoresis. The P3 polypeptide was considered to be the nephritogenic epitope, since a fraction composed of the P2 polypeptide alone was not nephritogenic. The properties of the nephritogenic epitope were the same as those of the Goodpasture epitope (M2*), which is a noncollagenous domain of the alpha 3 chain of type IV collagen (Butkowski et al. 1985; Saus et al. 1988), indicating that the nephritogenic antigen is the same as the Goodpasture antigen.

Immunogold studies of monomeric elements from the globular domain (NC1) of type IV collagen in renal basement membranes during experimental diabetes in the rat

The protein A-gold immunocytochemical technique was applied to reveal the monomeric elements M1, M2* and M3 from the non-collagenous globular domain (NC1) of type IV collagen over various renal basement membranes from control and long-term streptozotocin-induced diabetic rats. This study includes the basement membranes of the proximal tubule, the Bowman's capsule and the glomerulus as well as the extracellular matrix of the mesangium. The labellings obtained were confined to basement membrane material. The quantitative analysis demonstrated changes in labelling intensities and distribution between tissues from normal and diabetic animals. Increased labelling intensities were observed for M1 and M2* monomers in all the basement membranes studied except for the mesangial matrix which remained unchanged. In addition, the labelling for M1 monomers, present on the endothelial side of the glomerular basement membrane of control animals, was found to be distributed throughout the entire thickness of the basement membrane of diabetic animals. In contrast, neither the intensity of the labelling, nor the distribution of M3 monomers were altered in diabetic animals. Since M1 monomers are markers of the alpha 1(IV) and alpha 2(IV) chains of type IV collagen while M2* and M3 mark alpha 3(IV) and alpha 4(IV) chains respectively, the present results demonstrate changes in the nature of the collagenous elements of basement membranes during diabetes. Furthermore, the results indicate that the alpha 3(IV) and the alpha 4(IV) chains are not necessarily present in the same molecule. The modifications of the collagenous elements of the basement membranes during diabetes must alter the structural characteristics of these matrices which in turn might influence their functional properties.

Collagens as multidomain proteins

The number of proteins known to contain collagen-like triple helical domains is rapidly increasing. The functions of these domains are to provide molecular rods that separate spatially non-triple helical domains with varied properties and structures and to permit lateral interactions between molecules. Two-thirds of the amino acids of the triple helical domains have their side-chains at the surface of the protein. The triple helix is also a structure that is easily predictable from the primary structure. The structure of several recently discovered collagens are discussed in terms of domains and functions. The triple helical domains have sizes varying from 33 to over 1,000 amino acid residues. The longest uninterrupted triple helices are involved in the formation of the classical quarter-staggered fibrils. Other triple helical domains permit varied molecular aggregates. A very broad spectrum of non-triple helical or globular domains are interspersed by triple helices. Only those located at the extremities of the molecules are large in size, sometimes several hundred kDa, while the domains separating 2 triple helices are small (less than 50 amino acids) and provide the molecules with hinges, proteolytic cleavage sites or other specialized functions like a glycosaminoglycan attachment site. If the assembly of the 3 chains required for the triple helix formation can be controlled in vitro, collagen-like molecules offer an as yet unexploited potential for protein engineering.

Crystals of the NC1 domain of human type IV collagen

Crystals of the non-collagenous C-terminal region (NC1) of type IV collagen have been obtained from human placenta. These crystals diffract to 2.0 A, and belong to space group P22(1)2(1), with cell dimensions a = 81 A, b = 158 A, c = 138 A, alpha = beta = gamma = 90 degrees. The crystals contain one hexamer in the asymmetric unit; they are very stable with respect to X-rays.

Detection by chemical cross-linking of bovine brain synapsin I self-association

Synapsin I is believed to play an important role in the regulation of neurotransmitter release, since it is able to bind to synaptic vesicles, to the cytoskeleton and to membrane proteins; in addition, it bundles F-actin and microtubules. These properties, which are controlled by phosphorylation, could be explained if synapsin has different and multiple binding sites or if synapsin I is able to form polymers by self-association. In this study we present experimental evidence that synapsin I at low concentration forms self-associated dimers, as revealed after mild treatments with cross-linking agents. We have especially studied here the effects of copper/o-phenanthroline, a zero-length cross-linking agent which forms covalent links by oxidative formation of S-S bridges between adjacent cysteines. The time course and concentration-dependence of synapsin-dimer formation are studied; interestingly, these experiments could suggest a different behaviour of the two polypeptides. Limited proteolysis of phosphorylated synapsin I by V8 protease, alpha-chymotrypsin or collagenase, performed on the isolated dimer and monomer, allows us to localize tentatively in the central hydrophobic core of the molecule the cysteine residues the oxidation of which by copper/o-phenanthroline gives rise to synapsin dimers.

Structure and biological activity of basement membrane proteins

Collagen type IV, laminin, heparan sulfate proteoglycans, nidogen (entactin) and BM-40 (osteonectin, SPARC) represent major structural proteins of basement membranes. They are well-characterized in their domain structures, amino acid sequences and potentials for molecular interactions. Such interactions include self-assembly processes and heterotypic binding between individual constituents, as well as binding of calcium (laminin, BM-40) and are likely to be used for basement membrane assembly. Laminin, collagen IV and nidogen also possess several cell-binding sites which interact with distinct cellular receptors. Some evidence exists that those interactions are involved in the control of cell behaviour. These observations have provided a more defined understanding of basement membrane function and the definition of new research goals in the future.

Folding of collagen IV

Collagen IV dimers of two collagen IV molecules connected by their C-terminal globular NC1 domains were isolated by limited digestion with bacterial collagenase from mouse Engelbreth-Holm-Swarm (EHS) sarcoma tissue. The collagenous domains were only 300 nm long as compared to 400 nm of intact collagen IV but the disulfide bonds in the N-terminal region of the major triple helix were retained. Unfolding of the collagenous domains as monitored by circular dichroism occurred in a temperature range of 30 to 44 degrees C with a midpoint at 37 degrees C. The transition is significantly broader than that of the continuous triple helices in collagens I, II and III, a feature which can be explained by the frequent non-collagenous interruptions in the triple-helical domain of collagen IV. Refolding at 25 degrees C following complete unfolding at 50 degrees C was monitored by circular dichroism, selective proteolytic digestion of non-refolded segments and by a newly developed method in which the recovered triple-helical segments were visualized by electron microscopy. Triple-helix formation was found to proceed in a zipper-like fashion from the C-terminal NC1 domains towards the N-terminus, indicating that this domain is essential for nucleations. For collagen IV dimers with intact NC1 domains the rate of triple-helix growth was of comparable magnitude to that of collagen III, demonstrating that the non-collagenous interruptions do not slow down the refolding process where the rate-limiting step is the cis-trans isomerization of proline peptide bonds. Refolding was near to 100% and the refolding products were similar to the starting material as judged by thermal stability and electron microscopic appearance. Removal of the NC1 domains by pepsin or dissociation of their hexametric structures by acetic acid led to a loss of the refolding ability. Instead products with randomly dispersed short triple-helical segments were formed in a slow reaction. In no case, even when the disulfide bonds in the N-terminal region of the triple-helical domain were intact, was refolding from the N- towards the C-terminus observed. Taken together with results in other collagens, this suggests that C to N directionality might be an intrinsic property of triple-helix folding.