Crystal and molecular structure of a collagen-like peptide at 1.9 A resolution

Nuclear magnetic resonance shows asymmetric loss of triple helix in peptides modeling a collagen mutation in brittle bone disease

To investigate a human folding disease, NMR studies were carried out on collagen-like peptides to define the structural consequences of a single amino acid change found in patients with osteogenesis imperfecta (OI), a disease characterized by fragile bones. One peptide included a normal collagen sequence, while a second peptide included a Gly --> Ser substitution as found in a nonlethal case of OI. Residue specific internal dynamics and conformational studies indicate that the normal collagen-like sequence forms a triple helix which is rigid along its entire length. The introduction of a Gly --> Ser substitution induces an asymmetric disruption of the uniform triple helix. While the C-terminal end of the peptide retains the triple helix, the Ser substitution site and residues N-terminal to it exhibit the mobility of a random chain. This equilibrium state indicates that a Gly substitution can terminate the C to N propagation of the triple helix and suggests that renucleation is required for folding to continue. Defective folding has been implicated in brittle bone disease, and these results begin to characterize the folding process in OI collagens. OI collagen studies may also provide insights about defective protein folding, assembly, and aggregation in other human diseases.

Symmetry laws for interaction between helical macromolecules

The power of symmetry laws is applied in many scientific areas from elementary particle physics to structural biology. The structures of many biological helices, including DNA, were resolved with the use of pertinent symmetry constraints. It was not recognized, however, that similar constraints determine cardinal features of helix-helix interactions vital for many recognition and assembly reactions in living cells. We now formulate such symmetry-determined interaction laws and apply them to explain DNA "over-winding" from 10.5 base pairs per turn in solution to 10 in hydrated fibers, counterion specificity in DNA condensation, and forces observed over the last 15 A of separation between DNA, collagen, and four-stranded guanosine helices.

Conformational study of a collagen peptide by 1H NMR spectroscopy: observation of the 14N-1H spin-spin coupling of the Arg guanidinium moiety in the triple-helix structure

CB2, a CNBr peptide of 36 residues from type I collagen alpha1(I) chain has been studied by NMR spectroscopy as a function of temperature. At low temperature, the guanidinium protons of Arg9 showed sharp 1:1:1 NMR triplets around 6.95 ppm, characteristic of 14N coupled protons (1J(NH)=52 Hz) when the quadrupolar relaxation rate is drastically reduced. These spectral characteristics and the low temperature coefficient of the 1:1:1 triplets (delta delta/delta T of -3.6 ppb/degrees C) suggest that the H atoms of the protonated guanidinium moiety of Arg9 in the triple helix are slowly exchanging with bulk water, most likely involved in hydrogen bonds. On the basis of conformational energy computations on a model segment of type I collagen (Vitagliano, L., Némethy, G., Zagari, A. and Scheraga, H.A. (1993) Biochemistry 32, 7354-7359), similar to CB2, our data could indicate that the guanidinium group of Arg9 form hydrogen bonds with a backbone carbonyl of an adjacent chain probably by using the N(epsilon) hydrogen, leaving the four N(eta) hydrogens bound to water molecules that must be in slow exchange with bulk water and that could therefore be considered structural elements of the trimeric alpha1(I) CB2 triple helix. The behaviour of Arg9 has been investigated also in terms of equilibrium between random monomer and helical trimer conformations controlled by temperature. The thermal unfolding process was found to be reversible and the melting point resulted to be 17 degrees C.

Gly-X-Y tripeptide frequencies in collagen: a context for host-guest triple-helical peptides

The collagen triple-helix consists of a repeating (Gly-X-Y)n sequence. In theory, there are more than 400 possible Gly-X-Y triplets, but analysis of sequences from fibrillar and nonfibrillar collagens shows that only a limited set of triplets are found in significant numbers, and many are never observed. The nonrandom frequency of Gly-X-Y triplets makes it practical to experimentally approach the stability of much of the collagen sequence through the study of a limited set of host-guest peptides. In these peptides, individual Gly-X-Y triplets constitute the guest, while the host consists of Gly-Pro-Hyp tripeptides. A set of host-guest peptides was designed to contain the most common nonpolar and charged triplets found in collagen. All formed stable triple-helices, with their melting temperature depending on the identity of the guest triplet. While including less than 10% of all possible triplets, the data set covers 50-60% of collagen sequences and provides a starting point for establishing a stability scale to predict the relative stability of important collagen regions, such as the matrix metalloproteinase cleavage site or binding sites.

Supercoiled protein motifs: the collagen triple-helix and the alpha-helical coiled coil

The collagen triple-helix and the alpha-helical coiled coil represent the two basic supercoiled multistranded protein motifs. Originally they were characterized in fibrous proteins, but have been found more recently in a number of other proteins containing rod-shaped domains. Coiled-coil domains are responsible for the oligomerization of proteins, as well as other specific functions, while the triple-helix domains associate to form supramolecular structures and bind a variety of ligands. Both structures were originally solved by fiber diffraction, and recent crystallographic studies on small proteins and peptide models have confirmed the structure and provided molecular details. The differences in the molecular conformations of these two motifs and the interactions stabilizing these conformations are discussed. The molecular structures of both motifs constrain the amino acid sequence to recognizable patterns, requiring the (Gly-X-Y)n repeating sequence for the collagen triple-helix and a less stringent heptad repeat requirement (h-x-x-h-x-x-x)n for the coiled-coil domains, where h represents hydrophobic residues. The features and roles of these supercoiled domains in proteins are considered when they are found adjacent in the same protein.

Sugars and polyols inhibit fibrillogenesis of type I collagen by disrupting hydrogen-bonded water bridges between the helices

To better understand the mechanism of collagen fibrillogenesis, we studied how various sugars and polyols affect the formation and stability of collagen fibers. We combined traditional fiber assembly assays with direct measurement of the interaction between collagen triple helices in fibers by osmotic stress and X-ray diffraction. We found that the effects of sugars and polyols were highly specific with respect to small structural differences between these solutes. For example, 1,2-propane diol only weakly inhibited the fiber assembly and practically did not affect the interaction between collagen helices in fibers. At the same concentration, 1,3-propane diol eliminated the attraction between collagen helices and strongly suppressed fibrillogenesis. The two diols have the same atomic composition and differ only by the position of one of their hydroxyls. Still, their ability to inhibit fiber assembly differs by more than an order of magnitude, as judged by protein solubility. We argue that this is because collagen fibrillogenesis requires formation of hydrogen-bonded water clusters bridging recognition sites on the opposing helices. The ability of various sugars and polyols to inhibit the fiber assembly and to destabilize existing fibers is determined by how efficiently these solutes can compete with water for crucial hydrogen bonds and, thus, disrupt the water bridges. The effect of a sugar or a polyol appears to be strongly dependent on the specific stereochemistry of the solute hydroxyls that defines the preferred hydrogen-bonding pattern of the solute.

Mannose-binding lectin (MBL) in health and disease

Mannose-binding lectin (MBL) is the most intensively studied human collectin. It is recognized to be a versatile macro-molecule with many of the functional characteristics of IgM, IgG and Clq. In the presence of calcium the protein can bind to a wide spectrum of oligosaccharides through multiple lectin domains. Such binding to the repeating sugar arrays on microbial surfaces may result in direct uptake by one or more collectin receptors on phagocyte surface or may trigger the activation of a pro-serine protease complex (MASP 1 and MASP 2) leading to cleavage of C4 and C2 of the classical complement pathway. Although serum levels of MBL are normally rather low (1500 micrograms/litre) there is increasing evidence that the protein plays an important role in immune defence, particularly during the phase of primary contact with a microorganism. This is suggested by the observed association of an increased incidence of infections in individuals with structural mutations in exon 1 of the MBL gene. A cluster of such mutations in codons 52, 54 and 57 lead to secondary structural abnormalities of the collagenous triple helix and a failure to form biologically functional higher order oligomers. The codon 54 mutation has been identified in several Eurasian populations whereas the codon 57 mutation is characteristic of sub-Saharan populations. One intriguing paradox arising from the MBL genotyping studies is the observation that in many populations there are surprisingly high frequencies of either the codon 54 or codon 57 mutation, suggesting that there may be some biological advantage associated with absence of the protein. Nevertheless, various groups have reported either low serum levels of MBL or an increased frequency of the structural gene mutations in patients with suspected immunodeficiencies, those with frequent unexplained infections and those with systemic lupus erythematosus. There is also evidence that the rate of progression of AIDS in HIV positive men is faster in those with such mutations. A recently published study of a consecutive series of admissions to a paediatric unit suggests that children presenting with an infectious aetiology are significantly more likely to have a MBL mutation. Moreover, this association was independent of age. Prospective studies are underway to address the questions raised by these findings.

X-ray crystallographic determination of a collagen-like peptide with the repeating sequence (Pro-Pro-Gly)

The crystal structure of the triple-helical peptide (Pro-Pro-Gly)10 has been re-determined to obtain a more accurate description for this widely studied collagen model and to provide a comparison with the recent high-resolution crystal structure of a collagen-like peptide containing Pro-Hyp-Gly regions. This structure demonstrated that hydroxyproline participates extensively in a repetitive hydrogen-bonded assembly between the peptide and the solvent molecules. Two separate structural studies of the peptide (Pro-Pro-Gly)10 were performed with different crystallization conditions, data collection temperatures, and X-ray sources. The polymer-like structure of one triple-helical repeat of Pro-Pro-Gly has been determined to 2.0 A resolution in one case and 1.7 A resolution in the other. The solvent structures of the two peptides were independently determined specifically for validation purposes. The two structures display a reverse chain trace compared with the original structure determination. In comparison with the Hyp-containing peptide, the two Pro-Pro-Gly structures demonstrate very similar molecular conformation and analogous hydration patterns involving carbonyl groups, but have different crystal packing. This difference in crystal packing indicates that the involvement of hydroxyproline in an extended hydration network is critical for the lateral assembly and supermolecular structure of collagen.

The role of bound water in the stability of the triple-helical conformation of (Pro-Pro-Gly)10

There is significant experimental evidence for bound water in collagen and related polymers. (Pro-Pro-Gly)10 [(PPG)10] is a polymer that forms a collagen-like triple-helical structure in aqueous solution. Like collagen, (PPG)10 adopts a structure in which side chains are mostly exposed to solvent, and the backbone polar groups are limited in their ability to form hydrogen bonds with each other. (PPG)10, like collagen, also has many of its backbone polar groups in positions that inhibit complete solvation in aqueous solution; thus the necessity of bound waters for stabilization of the structure. We have constructed a model for bound waters in (PPG)10, based on an examination of the geometry and steric environment of the backbone polar groups. As will become clear, the number of bound waters is determined by the geometry of the backbone carbonyl groups and the steric crowding surrounding them. In this model, each water forms one hydrogen bond with each of two backbone carbonyls from a glycine and a proline in different monomer chains, thus bridging the two carbonyls. The carbonyls in question are quite sterically crowded by neighboring (PPG)10 atoms and would not be likely to experience complete solvation by bulk solvent in aqueous solution. The bound waters are therefore likely to be present even in solution, since otherwise the unsatisfied hydrogen-bonding potential of the carbonyls would destabilize the structure. Other carbonyls also are sterically crowded and possibly prevented from experiencing full solvation, but are not in a favorable geometry for such bridging hydrogen bonds. The intra- and interchain interactions found in a previous computational study of (PPG)10 without bound waters are not disrupted by the addition of waters.

Crystal structure and mapping by site-directed mutagenesis of the collagen-binding epitope of an activated form of BM-40/SPARC/osteonectin

The extracellular calcium-binding domain (positions 138-286) of the matrix protein BM-40 possesses a binding epitope of moderate affinity for several collagen types. This epitope was predicted to reside in helix alphaA and to be partially masked by helix alphaC. Here we show that deletion of helix alphaC produces a 10-fold increase in collagen affinity similar to that seen after proteolytic cleavage of this helix. The predicted removal of the steric constraint was clearly demonstrated by the crystal structure of the mutant at 2.8 A resolution. This constitutively activated mutant was used to map the collagen-binding site following alanine mutagenesis at 13 positions. Five residues were crucial for binding, R149 and N156 in helix alphaA, and L242, M245 and E246 in a loop region connecting the two EF hands of BM-40. These residues are spatially close and form a flat ring of 15 A diameter which matches the diameter of a triple-helical collagen domain. The mutations showed similar effects on binding to collagens I and IV, indicating nearly identical binding sites on both collagens. Selected mutations in the non-activated mutant DeltaI also reduced collagen binding, consistent with the same location of the epitope but in a more cryptic form in intact BM-40.

Observation of geometric structure of collagen molecules by atomic force microscopy

Atomic force microscopy was used to study the geometric structure of collagen fibrils and molecules of rat calcanean tendon tissues. The authors found that the diameter of the fibrils ranged from 124 to 170 nm, and their geometric form suggested a helical winding with spectral period from 59.4 to 61.7 nm, close to the band dimensions reported by electron microscopy. At high magnification, the surface of these bands revealed images that probably correspond to the almost crystalline array of collagen molecules, with the triple helix structure almost visible. The typical helix width is 1.43 nm, with main periods of 1.15 and 8.03 nm, very close to the dimensions reported by X-ray diffraction.

Identification of the oim mutation by dye terminator chemistry combined with automated direct DNA sequencing

The homozygous oim/oim mouse, a model of moderate-to-severe human osteogenesis imperfecta, contains a G-nucleotide deletion in the Cola-2 gene (the murine pro alpha(I) collagen gene) that results in accumulation of alpha1(I) homotrimer collagen. Although these mice have a distinctive phenotype that includes multiple fractures and deformities, genotyping is necessary to distinguish them from their wildtype (+/+) and heterozygote (oim/+) littermates. In this study, the dye primer and dye terminator chemistry methods, in combination with automated direct DNA sequencing, were compared for accuracy and ease in genotyping. A total of 82 mice from 14 litters were bred and genotyped; this resulted in 18 +/+, 35 oim/+, and 29 oim/oim mice. The dye primer and dye terminator chemistry methods worked equally well for identification of the deletion mutation and thus the genotype of all of the mice. However, the dye terminator method was found to be superior on the basis of the reduced amount of sample handling and reduced quantity of reagent required.

Refined atomic model of the four-layer aggregate of the tobacco mosaic virus coat protein at 2.4-A resolution

Previous x-ray studies (2.8-A resolution) on crystals of tobacco mosaic virus coat protein grown from solutions containing high salt have characterized the structure of the protein aggregate as a dimer of a bilayered cylindrical disk formed by 34 chemically identical subunits. We have determined the crystal structure of the disk aggregate at 2.4-A resolution using x-ray diffraction from crystals maintained at cryogenic temperatures. Two regions of interest have been extensively refined. First, residues of the low-radius loop region, which were not modeled previously, have been traced completely in our electron density maps. Similar to the structure observed in the virus, the right radial helix in each protomer ends around residue 87, after which the protein chain forms an extended chain that extends to the left radial helix. The left radial helix appears as a long alpha-helix with high temperature factors for the main-chain atoms in the inner portion. The side-chain atoms in this region (residues 90-110) are not visible in the electron density maps and are assumed to be disordered. Second, interactions between subunits in the symmetry-related central A pair have been determined. No direct protein-protein interactions are observed in the major overlap region between these subunits; all interactions are mediated by two layers of ordered solvent molecules. The current structure emphasizes the importance of water in biological macromolecular assemblies.

Positional preferences of ionizable residues in Gly-X-Y triplets of the collagen triple-helix

Collagens contain a high amount of charged residues involved in triple-helix stability, fibril formation, and ligand binding. The contribution of charged residues to stability was analyzed utilizing a host-guest peptide system with a single Gly-X-Y triplet embedded within Ac(Gly-Pro-Hyp)3-Gly-X-Y-(Gly-Pro-Hyp)4-Gly-Gly-NH2. The ionizable residues Arg, Lys, Glu, and Asp were incorporated into the X position of Gly-X-Hyp; in the Y position of Gly-Pro-Y; or as pairs of oppositely charged residues occupying X and Y positions. The Gly-X-Hyp peptides had similar thermal stabilities, only marginally less stable than Gly-Pro-Hyp, whereas Gly-Pro-Y peptides showed a wide thermal stability range (Tm = 30-45 degrees C). The stability of peptides with oppositely charged residues in the X and Y positions appears to reflect simple additivity of the individual residues, except when X is occupied by a basic residue and Y = Asp. The side chains of Glu, Lys, and Arg have the potential to form hydrogen bonds with available peptide backbone carbonyl groups within the triple-helix, whereas the shorter Asp side chain does not. This may relate to the unique involvement of Asp residues in energetically favorable ion pair formation. These studies clarify the dependence of triple-helix stability on the identity, position, and ionization state of charged residues.

Construction of biologically active protein molecular architecture using self-assembling peptide-amphiphiles

The peptide-amphiphiles described here provide a simple approach for building stable protein structural motifs using peptide head groups. One of the most intriguing features of this system is the possible formation of stable lipid films on solid substrates, or the use of the novel amphiphiles in bilayer membrane systems, where the lipid tail serves not only as a peptide structure-inducing agent but also as an anchor of the functional head group in the lipid assembly. The peptide-amphiphile system potentially offers great versatility with regard to head and tail group composition and overall geometries and macromolecular structures. For building materials with molecular and cellular recognition capacity, it is essential to have a wide repertoire of tools to produce characteristic supersecondary structures at surfaces and interfaces.

Gly-Pro-Arg confers stability similar to Gly-Pro-Hyp in the collagen triple-helix of host-guest peptides

A set of host-guest peptides of the form Ac(Gly-Pro-Hyp)3-Gly-X-Y-(Gly-Pro-Hyp)4-Gly-Gly-NH2 has been designed to evaluate the propensity of different Gly-X-Y triplets for the triple-helix conformation (Shah, N. K., Ramshaw, J. A. M., Kirkpatrick, A., Shah, C., and Brodsky, B. (1996) Biochemistry 35, 10262-10268). All Gly-X-Y guest triplets led to a decrease in melting temperature from the host (Gly-Pro-Hyp)8 peptide except for Gly-Pro-Arg. In this Gly-Pro-Hyp-rich environment, Gly-Pro-Arg was found to be as stabilizing as Gly-Pro-Hyp. Decreased stability of host-guest peptides containing Gly-Pro-Lys, Gly-Pro-homo-Arg, and Gly-Arg-Hyp compared with Gly-Pro-Arg indicated a stabilization that is optimal for Arg and specific to the Y-position. Arg was found to have a similar stabilizing effect when residues other than Pro are in the X-position. Both Arg and Hyp stabilize the triple-helix preferentially in the Y-position in a stereospecific manner and occupy largely Y-positions in collagen. However, contiguous Gly-Pro-Hyp units are highly stable and promote triple-helix folding, whereas incorporation of multiple Gly-Pro-Arg triplets was destabilizing and folded slowly due to charge repulsion. In collagen, Gly-Pro-Arg may contribute maximally to local triple-helix stability while also having the potential for electrostatic interactions in fibril formation and binding.

Crystal structure of the I domain from integrin alpha2beta1

We have determined the high resolution crystal structure of the I domain from the alpha-subunit of the integrin alpha2beta1, a cell surface adhesion receptor for collagen and the human pathogen echovirus-1. The domain, as expected, adopts the dinucleotide-binding fold, and contains a metal ion-dependent adhesion site motif with bound Mg2+ at the top of the beta-sheet. Comparison with the crystal structures of the leukocyte integrin I domains reveals a new helix (the C-helix) protruding from the metal ion-dependent adhesion site face of the domain which creates a groove centered on the magnesium ion. Modeling of a collagen triple helix into the groove suggests that a glutamic acid side chain from collagen can coordinate the metal ion, and that the C-helix insert is a major determinant of binding specificity. The binding site for echovirus-1 maps to a distinct surface of the alpha2-I domain (one edge of the beta-sheet), consistent with data showing that virus and collagen binding occur by different mechanisms. Comparison with the homologous von Willebrand factor A3 domain, which also binds collagen, suggests that the two domains bind collagen in different ways.

Raman spectral evidence for hydration forces between collagen triple helices

Hydration forces are thought to result from the energetic cost of water rearrangement near macromolecular surfaces. Raman spectra, collected on the same collagen samples on which these forces were measured, reveal a continuous change in water hydrogen-bonding structure as a function of separation between collagen triple helices. The varying spectral parameters track the force-distance curve. The energetic cost of water "restructuring," estimated from the spectra, is consistent with the measured energy of intermolecular interaction. These correlations support the idea that the change in water structure underlies the exponentially varying forces seen in this system at least over the 13-18-A range of interaxial separations.

Structure of the collagen-binding domain from a Staphylococcus aureus adhesin

The crystal structure of the recombinant 19,000 M(r) binding domain from the Staphylococcus aureus collagen adhesin has been determined at 2 A resolution. The domain fold is a jelly-roll, composed of two antiparallel beta-sheets and two short alpha-helices. Triple-helical collagen model probes were used in a systematic docking search to identify the collagen-binding site. A groove on beta-sheet I exhibited the best surface complementarity to the collagen probes. This site partially overlaps with the peptide sequence previously shown to be critical for collagen binding. Recombinant proteins containing single amino acid mutations designed to disrupt the surface of the putative binding site exhibited significantly lower affinities for collagen. Here we present a structural perspective for the mode of collagen binding by a bacterial surface protein.

Spontaneous rupture of liver in a patient with Ehlers Danlos disease type IV

Ehlers-Danlos syndrome (EDS) type IV is an autosomal dominant connective tissue disease caused by mutations in the type III collagen gene resulting in extreme tissue fragility. Affected individuals are at risk of dramatic and often fatal complications, mostly spontaneous arterial, uterine, or colonic ruptures. Phenotypic expression of EDS type IV is variable and clinical signs are generally quite subtle, thus making a prompt diagnosis difficult. The case of a 33-year-old woman is described who presented with a wide range of clinical features and sequelae that eventually led to the diagnosis of EDS type IV. She presented with spontaneous liver rupture, renal infarction, and pneumothorax, all representing rare complications of EDS type IV. Prior history revealed a uterine rupture in advanced pregnancy associated with ischemic necrosis of the descending and sigmoid colon. EDS type IV should be suspected in young individuals who present with such unusual complications. Early diagnosis is essential if severe or even lethal complications are to be avoided in the diagnostic and therapeutic management of such patients.

Collagen-based structures containing the peptoid residue N-isobutylglycine (Nleu): synthesis and biophysical studies of Gly-Nleu-Pro sequences by circular dichroism and optical rotation

Single-chain peptide-peptoid structures, Ac-(Gly-Nleu-Pro)n-NH2 (n = 3, 6, and 10) and (Gly-Nleu-Pro)n-NH2 (n = 1 and 9), and template-assembled collagen analogs, KTA-[Gly-(Gly-Nleu-Pro)n-NH2]3 (n = 3 and 6; KTA represents cis,cis-1,3,5-trimethylcyclohexane-1,3, 5-tricarboxylic acid, also known as the Kemp triacid; Nleu denotes N-isobutylglycine), were prepared by solid-phase peptide synthesis methods. Biophysical studies using circular dichroism (CD) and optical rotation measurements show that these collagen analogs form triple-helical conformations when the chain is longer than a critical length. Unlike collagen-based structures composed of Gly-Pro-Hyp and Gly-Pro-Nleu sequences, results reveal that the presence of a positive CD peak between 220 and 225 nm is indicative of triple-helical conformations for these collagen-based structures composed of Gly-Nleu-Pro sequences. Results also indicate that the Gly-Nleu-Pro sequence possesses a higher triple-helical propensity than the Gly-Pro-Nleu sequence as demonstrated by the higher melting temperatures, the faster triple-helix folding, and the lower minimum concentration necessary to detect triple-helicity for the single-chain structures. Therefore, we conclude that the Nleu residue in the second position of the trimeric repeat is more effective in inducing triple-helix formation than Pro in the same position.

Asymmetry adjacent to the collagen-like domain in rat liver mannose-binding protein

Rat liver mannose-binding protein (MBP-C) is the smallest known member of the collectin family of animal lectins, many of which are involved in defence against microbial pathogens. It consists of an N-terminal collagen-like domain linked to C-terminal carbohydrate-recognition domains. MBP-C, overproduced in Chinese-hamster ovary cells, is post-translationally modified and processed in a manner similar to the native lectin. Analytical ultracentrifugation experiments indicate that MBP-C is trimeric, with a weight-averaged molecular mass of approx. 77 kDa. The rate of sedimentation of MBP-C and its mobility on gel filtration suggest a highly elongated molecule. Anomalous behaviour on gel filtration due to this extended conformation may explain previous suggestions that MBP-C forms a higher oligomer. The polypeptide chains of the MBP-C trimer are linked by disulphide bonds between two cysteine residues at the N-terminal junction of the collagen-like domain. Analysis of an N-terminal tryptic fragment reveals that the disulphide bonding in MBP-C is heterogeneous and asymmetrical. These results indicate that assembly of MBP-C oligomers probably proceeds in a C- to N-terminal direction: trimerization at the C-terminus is followed by assembly of the collagenous domain and finally formation of N-terminal disulphide bonds. The relatively simple organization of MBP-C provides a template for understanding larger, more complex collectins.

Human factor IX binds to specific sites on the collagenous domain of collagen IV

The primary region of factor IX that mediates binding to bovine aortic endothelial cells resides in residues 3-11 of the N-terminal region known as the Gla domain. Recently, it was proposed that the observed binding to endothelial cells is actually a measure of the interaction between factor IX and collagen IV (Cheung, W. F., van den Born, J., Kuhn, K., Kjellen, L., Hudson, B. G., and Stafford, D. W. (1996) Proc. Natl. Acad. Sci. U. S. A. 93, 11068-11073). To confirm that factor IX binds to collagen IV and to examine the specificity of this interaction, we used scanning force microscopy to examine factor IX binding to collagen IV. We imaged collagen IV in the presence and the absence of factor IX and observed specific interactions between factor IX and collagen IV. Our results demonstrate that factor IX binds to collagen IV at specific sites in the collagenous domain approximately 98 and approximately 50 nm from the C-terminal pepsin-cleaved end.

Stability of type I collagen CNBr peptide trimers

As indices of triple helix stability of type I collagen CNBr peptide homotrimers, deltaG degrees for monomer-trimer transitions and melting temperatures were obtained from circular dichroism measurements at increasing temperatures. The data were compared with the stability of the parent native molecule. Peptides were found to have a lower stability than the whole collagen molecule. The general implication is that the coordinated water molecules play a key role in determining collagen triple helical stability and high cooperativity at melting. Other factors (monomer stability, ionic and hydrophobic factors, variations of composition, specific sequences) could also contribute towards peptide stability; these factors could explain the data obtained in the case of peptide alpha1(I) CB3.

Amino acid sequence environment modulates the disruption by osteogenesis imperfecta glycine substitutions in collagen-like peptides

Ostoegenesis imperfecta (OI) or "brittle bone" disease is associated with mutations in the genes for type I collagen chains and produces variable phenotypes, ranging from lethal cases at birth to mild cases with increased bone fractures. The most common OI mutations are single base substitutions leading to replacement of Gly by another residue, breaking the typical (Gly-X-Y)n repeating sequence pattern of the collagen triple-helix. Triple-helical peptides were designed to focus on residues 892-921 of the alpha1 chain of type I collagen, where two OI Gly-->Ser mutations are found in close proximity, a mild mutation at site 901 and a lethal mutation at site 913. Peptides were designed to include amino acid sequences around these mutation sites, and were synthesized with the normal sequence or with the Gly-->Ser mutated sequence. The peptide including the normal sequence residues 892-909 with four Gly-Pro-Hyp triplets at the C-terminus formed a stable triple-helix, and introduction of a Ser residue for Gly at the 901 mutation site led to a 50% loss of triple-helix content and a decrease in thermal stability, with little effect on folding. A peptide including residues 904-921 again formed a stable triple-helix, but the introduction of the Gly-->Ser substitution at site 913 led to a much greater decrease in thermal stability. These studies demonstrate the impact of local sequences flanking the Gly substitution on structural consequences and support the concept of variability and regional effects along the collagen molecule.

Gly-Gly-containing triplets of low stability adjacent to a type III collagen epitope

Collagens, in addition to their structural role in the extracellular matrix, possess a number of functional binding domains. In this study, the binding to collagen of a monoclonal antibody is used as a model to define the molecular features involved in triple-helix interactions with other proteins. Here we report the thermal stability of an overlapping set of triple-helical peptides that includes the epitope recognized by a monoclonal antibody to type III collagen. Although the sequences of these peptides are very closely related, by a translation of a single triplet along the collagen chain, substantial variations in the melting temperatures were observed. These variations in thermal stability could not be readily explained by differences in imino acid content, or in numbers of charged or hydrophobic residues. The results indicate that Gly-Gly-Y triplets, which are adjacent to the epitope, have a strong influence in reducing the thermal stability of triple-helical peptides. Further studies, which were carried out on a set of "host-guest" triple-helical peptides containing different Gly-Gly-Y guest triplets, confirm the destabilizing effect of such tripeptides. The presence of Gly-Gly-Y triplets may play an important role in specific functions of type III collagen by modulating the local triple-helical structure or dynamics.

Crystal structure of an ecotin-collagenase complex suggests a model for recognition and cleavage of the collagen triple helix

The crystal structure of fiddler crab collagenase complexed with the dimeric serine protease inhibitor ecotin at 2.5 A resolution reveals an extended cleft providing binding sites for at least 11 contiguous substrate residues. Comparison of the positions of nine intermolecular main chain hydrogen bonding interactions in the cleft, with the known sequences at the cleavage site of type I collagen, suggests that the protease binding loop of ecotin adopts a conformation mimicking that of the cleaved strand of collagen. A well-defined groove extending across the binding surface of the enzyme readily accommodates the two other polypeptide chains of the triple-helical substrate. These observations permit construction of a detailed molecular model for collagen recognition and cleavage by this invertebrate serine protease. Ecotin undergoes a pronounced internal structural rearrangement which permits binding in the observed conformation. The capacity for such rearrangement appears to be a key determinant of its ability to inhibit a wide range of serine proteases.

The collagen triple-helix structure

Recent advances, principally through the study of peptide models, have led to an enhanced understanding of the structure and function of the collagen triple helix. In particular, the first crystal structure has clearly shown the highly ordered hydration network critical for stabilizing both the molecular conformation and the interactions between triple helices. The sequence dependent nature of the conformational features is also under active investigation by NMR and other techniques. The triple-helix motif has now been identified in proteins other than collagens, and it has been established as being important in many specific biological interactions as well as being a structural element. The nature of recognition and the degree of specificity for interactions involving triple helices may differ from globular proteins. Triple-helix binding domains consist of linear sequences along the helix, making them amenable to characterization by simple model peptides. The application of structural techniques to such model peptides can serve to clarify the interactions involved in triple-helix recognition and binding and can help explain the varying impact of different structural alterations found in mutant collagens in diseased states.

Real-time NMR investigations of triple-helix folding and collagen folding diseases

Folding of the collagen triple helix provides an opportunity to look at multichain molecular assembly. This triple helix also offers unique advantages for the study of folding because the process is very slow compared to globular proteins, and the kinetics of folding can be obtained in real time by NMR. Studies on triple-helical peptides illustrate the ability to observe kinetic folding intermediates directly and the ability to propose detailed mechanisms of folding through the use of real-time NMR methods. Defective collagen folding has been implicated in various connective tissue diseases and the capacity of NMR to look at the folding of specific sites provides a tool for obtaining information about altered folding mechanisms. Comparison of folding in peptides that model normal and diseased collagens could shed light on the molecular perturbation and the etiology of disease.

Solvent hydrogen-bond network in protein self-assembly: solvation of collagen triple helices in nonaqueous solvents

Forces between type I collagen triple helices are studied in solvents of varying hydrogen-bonding ability. The swelling of collagen fibers in reconstituted films is controlled by the concentration of soluble polymers that are excluded from the fibers and that compete osmotically with collagen for available solvent. The interaxial spacing between the triple helices as a function of the polymer concentration is measured by x-ray diffraction. Exponential-like changes in the spacing with increasing osmotic stress, qualitatively similar to the forces previously found in aqueous solution, are also seen in formamide and ethylene glycol. These are solvents that, like water, are capable of forming three-dimensional hydrogen-bond networks. In solvents that either cannot form a network or have a greatly impaired ability to form a hydrogen-bonded network, strikingly different behavior is observed. A hard-wall repulsion is seen with collagen solvated by ethanol, 2-propanol, and N,N-dimethylformamide. The spacing between helices hardly changes with increasing polymer concentration until the stress exceeds some threshold where removal of the solvent becomes energetically favorable. No solvation of collagen is observed in dimethoxyethane. In solvents with an intermediate ability to form hydrogen-bonded networks, methanol, 2-methoxyethanol, or N-methylformamide, the change in spacing with polymer concentration is intermediate between exponential-like and hard-wall. These results provide direct evidence that the exponential repulsion observed between collagen helices at 0-8-A surface separations in water is due to the energetic cost associated with perturbing the hydrogen-bonded network of solvent molecules between the collagen surfaces.

Aspartate residue 7 in amyloid beta-protein is critical for classical complement pathway activation: implications for Alzheimer's disease pathogenesis

Fibrillar amyloid beta-protein has been implicated in the pathogenesis of Alzheimer's disease because of its neurotoxicity and its ability to activate complement. Reactive microglia, astrocytes and complement (C') components (reviewed in ref. 6) are associated with senile plaques, the fibrillar, beta-sheet assemblies of amyloid beta-peptide found predominantly in brain from individuals with AD (ref. 7). These indications of inflammatory events are not prevalent in the nonfibrillar "diffuse" plaques often seen in age-matched control cases without dementia. Clinical studies over the past several years have correlated the use of anti-inflammatory drugs with a decrease in the incidence and progression of AD dementia and/or dysfunction, supporting a role for gliosis and inflammation in AD pathogenesis (reviewed in ref. 6). C5a, a product of C' activation, is chemotactic for microglia. Thus, complement activation provides a specific mechanism for recruiting reactive glial cells to the site of the fibrillar amyloid beta-protein plaque, which could lead to inflammatory events, neuronal dysfunction and degeneration. With the use of truncated amyloid beta-peptides, the region of amyloid beta-protein limited by residues 4 and 11 has been identified as critical in the interaction between amyloid beta-protein and C1q, the recognition component of the classical complement pathway (CCP), which results in the activation of C'. Furthermore, substitution of an isoaspartic acid for aspartic acid at amyloid beta-protein residue 7 resulted in the complete elimination of CCP-activating activity. A molecular model of this interaction has been generated that should be useful in the design of candidate therapeutic inhibitors of CCP activation by amyloid beta-protein.

Design and synthesis of heterotrimeric collagen peptides with a built-in cystine-knot. Models for collagen catabolism by matrix-metalloproteases

A heterotrimeric collagen peptide was designed and synthesized which contains the collagenase cleavage site (P4-P'9/10) of type I collagen linked to a C-terminal cystine-knot, and N-terminally extended with (Gly-Pro-Hyp)5 triplets for stabilization of the triple-helical conformation. By employing a newly developed regioselective cysteine pairing strategy based exclusively on thiol disulfide exchange reactions, we succeeded in assembling in high yields and in a reproducible manner the triple-stranded cystine peptide. While the single chains showed no tendency to self-association into triple helices, the heterotrimer (alpha1 alpha2 alpha1') was found to exhibit a typical collagen-like CD spectrum at room temperature and a melting temperature (Tm) of 33 degrees C. This triple-helical collagen-like peptide is cleaved by the full-length human neutrophil collagenase (MMP-8) at a single locus fully confirming the correct raster of the heterotrimer. Its digestion proceeds at rates markedly higher than that of a single alpha1' chain. In contrast, opposite digestion rates were measured with the catalytic Phe79-MMP-8 domain of HNC. Moreover, the full-length enzyme exhibits Km values of 5 microM and 1 mM for the heterotrimer and the single alpha1' chain, respectively, which compare well with those reported for collagen type I (approximately 1 microM), gelatine (approximately 10 microM) and for octapeptides of the cleavage sequence (> or = 1 mM). The high affinity of the MMP-8 for the triple-helical heterotrimer and the fast digestion of this collagenous peptide confirm the decisive role of the hemopexin domain in recognition and possibly, partial unfolding of collagen.

Involvement of prolyl 4-hydroxylase in the assembly of trimeric minicollagen XII. Study in a baculovirus expression system

We have shown previously that hydroxylation played a critical role in the trimer assembly and disulfide bonding of the three constituent alpha chains of a minicollagen composed of the extreme C-terminal collagenous (COL1) and noncollagenous (NC1) domains of type XII collagen in HeLa cells (Mazzorana, M., Gruffat, H., Sergeant, A., and van der Rest, M. (1993) J. Biol. Chem. 268, 3029-3032). We have further characterized the involvement of prolyl 4-hydroxylase in the assembly of the three alpha chains to form trimeric disulfide-bonded type XII minicollagen in an insect cell expression system. For this purpose, type XII minicollagen was produced in insect cells from baculovirus vectors, alone or together with wild-type human prolyl 4-hydroxylase or with the human enzyme mutated in the catalytic site of its alpha or beta subunits or with the individual alpha or beta subunits. When type XII minicollagen was produced alone, negligible amounts of disulfide-bonded trimers were found to be produced by the cells. However, coproduction of the collagen with the two subunits of the wild-type human enzyme dramatically increased the amount of disulfide-bonded trimeric type XII minicollagen molecules. In contrast, coproduction of the collagen with alpha subunits that had a mutation completely inactivating the human enzyme failed to enhance the trimer assembly. These results directly show that an active prolyl 4-hydroxylase is required for the assembly of disulfide-bonded trimers of type XII minicollagen.

Variable clinical expression in a family with OI type IV due to deletion of three base pairs in COL1A1

We have studied a family with autosomal dominant osteogenesis imperfecta (OI) type IV. Electrophoresis of collagen produced by cultured fibroblasts revealed a slower migrating population of collagen I. Cyanogen bromide peptide mapping localised the structural defect to the area of the alpha 1(1)CB3 peptide. Subsequent sequencing revealed a deletion of nucleotides 1964-1966 in exon 27 of COL1A1. By means of restriction enzyme analysis, the deletion could be detected in all affected family members. This in-frame deletion resulted in the removal of alanine-438 and Glu437Asp substitution in the pro alpha 1(I) collagen chain. Clinical variation was considerable among affected family members. The most consistent clinical features were reduced height and extraosseous manifestations of OI.

Structures of class A macrophage scavenger receptors. Electron microscopic study of flexible, multidomain, fibrous proteins and determination of the disulfide bond pattern of the scavenger receptor cysteine-rich domain

Structures of secreted forms of the human type I and II class A macrophage scavenger receptors were studied using biochemical and biophysical methods. Proteolytic analysis was used to determine the intramolecular disulfide bonds in the type I-specific scavenger receptor cysteine-rich (SRCR) domain: Cys2-Cys7, Cys3-Cys8, and Cys5-Cys6. This pattern is likely to be shared by the highly homologous domains in the many other members of the SRCR domain superfamily. Electron microscopy using rotary shadowing and negative staining showed that the type I and II receptors are extended molecules whose contour lengths are approximately 440 A. They comprised two adjacent fibrous segments, an alpha-helical coiled-coil ( approximately 230 A, including a contribution from the N-terminal spacer domain) and a collagenous triple helix ( approximately 210 A). The type I molecules also contained a C-terminal globular structure ( approximately 58 x 76 A) composed of three SRCR domains. The fibrous domains were joined by an extremely flexible hinge. The angle between these domains varied from 0 to 180 degrees and depended on the conditions of sample preparation. Unexpectedly, at physiologic pH, the prevalent angle seen using rotary shadowing was 0 degrees , resulting in a structure that is significantly more compact than previously suggested. The apparent juxtaposition of the fibrous domains at neutral pH provides a framework for future structure-function studies of these unusual multiligand receptors.

Thermal stability of calf skin collagen type I in salt solutions

The thermal stability of acid-soluble collagen type I from calf skin in salt solutions is studied by high-sensitivity differential scanning calorimetry. Three concentration ranges have been clearly distinguished in the dependence of collagen thermal stability on ion concentration. At concentrations below 20 mM, all studied salts reduce the temperature of collagen denaturation with a factor of about 0.2 degree C per 1 mM. This effect is attributed to screening of electrostatic interactions leading to collagen stabilisation. At higher concentrations, roughly in the range 20-500 mM, the different salts either slightly stabilise or further destabilise the collagen molecule in salt-specific way that correlates with their position in the lyotropic series. The effect of anions is dominating and follows the order H2PO4- > or = SO4(2-) > Cl- > SCN-, with sign inversion at about SO4(2-). This effect, generally known as the Hofmeister effect, is associated with indirect protein-salt interactions exerted via competition for water molecules between ions and the protein surface. At still higher salt concentrations (onset concentrations between 200 and 800 mM for the different salts), the temperature of collagen denaturation and solution opacity markedly increase for all studied salts due to protein salting out and aggregation. The ability of salts to salt out collagen also correlates with their position in the lyotropic series and increases for chaotropic ions. The SO4(2-) anions interact specifically with collagen - they induce splitting of the protein denaturation peak into two components in the range 100-150 mM Na2SO4 and 300-750 mM Li2SO4. The variations of the collagen denaturation enthalpy at low and intermediate salt concentrations are consistent with a weak linear increase of the enthalpy with denaturation temperature. Its derivative, d(delta H)/dT, is approximately equal to the independently measured difference in the heat capacities of the denatured and native states, delta Cp = Cp(D) - Cp(N) approximately 0.1 cal.g-1 K-1.