Unfolding intermediates in the triple helix to coil transition of bovine type XI collagen and human type V collagens alpha 1(2) alpha 2 and alpha 1 alpha 2 alpha 3

Isolation and Characterisation of Major and Minor Collagens from Hyaline Cartilage of Hoki (Macruronus novaezelandiae)

The composition and properties of collagen in teleost (bony fish) cartilage have never been studied. In this study, we aimed to identify and characterise all collagen species in the nasal cartilage of hoki (Macruronus novaezelandiae). Four native collagen species were extracted using two techniques, and isolated with differential salt precipitation. We were able to assign the identity of three of these collagen species on the basis of solubility, SDS-PAGE and amino acid analyses. We found that hoki cartilage contains the major collagen, type II, and the minor collagens, type IX and type XI, which are homologous to those found in mammal and chicken cartilage. Using these extraction protocols, we also isolated a full-length type IX collagen from cartilage for the first time. In addition, we detected a 90 kDa, highly glycosylated collagen that has not been identified in any other species. For each isolate, structural and biochemical characterisations were performed using circular dichroism and Fourier transform infrared spectroscopy analyses, and the thermal denaturation properties were determined. Our results showed that the properties of hoki cartilage-derived collagens are similar to those of collagens in mammalian cartilage, indicating that teleost cartilage could provide biological ingredients for the development of biomaterials to treat cartilage-related illnesses.

Engineering structure and function using thermoresponsive biopolymers

Self-assembly enables exquisite control at the smallest scale and generates order among macromolecular-building blocks that remain too small to be manipulated individually. Environmental cues, such as heating, can trigger the organization of these materials from individual molecules to multipartixcle assemblies with a variety of compositions and functions. Synthetic as well as biological polymers have been engineered for these purposes; however, biological strategies can offer unparalleled control over the composition of these macromolecular-building blocks. Biologic polymers are macromolecules composed of monomeric units that can be precisely tailored at the genetic level; furthermore, they can often utilize endogenous biodegradation pathways, which may enhance their potential clinical applications. DNA (nucleotides), polysaccharides (carbohydrates), and proteins (amino acids) have all been engineered to self-assemble into nanostructures in response to a change in temperature. This focus article reviews the growing body of literature exploring temperature-dependent nano-assembly of these biological macromolecules, summarizes some of their physical properties, and discusses future directions.

Fragility of reconstituted type V collagen fibrils with the chain composition of α1(V)α2(V)α3(V) respective of the D-periodic banding pattern

The triple-helical domains of two subtypes of type V collagen were prepared from human placenta, one with the chain composition of [α1(V)](2)α2(V) (Vp112) and the other with the chain composition of α1(V)α2(V)α3(V) (Vp123) with limited pepsin treatment. In order to characterize the triple-helical domain of the type Vp123 collagen molecule, the reconstituted aggregate structure formed from the pepsin-treated collagen was compared by using transmission electron microscopy. The diameter of the fibrils reconstituted from types pepsin-treated type Vp123 collagen and type Vp112 collagen was highly uniform and less than the D-periodicity at all the temperatures examined, suggesting that the major triple-helical domain of both subtypes has a potency to limit their lateral growth. Both fibrils were approximately 45 nm in width and showed the D-periodic banding pattern along their axes at 34°C. In contrast to type Vp112, the reconstituted type Vp123 fibrils showed no banding pattern along their axes when they were reconstituted at 37°C. The banded fibrils once reconstituted from type Vp123 at 34°C tend to lose their characteristic pattern within 60 min when they were incubated at 37°C. One explanation is that a slightly higher content of hydrophobic residues of type Vp123 collagen than those of type V112p collagen augmented the intermolecular interaction that disturbs the D-periodicity governed essentially by electrostatic interactions. Taken together with recent data in Col5a3 gene-targeted mice, the results suggest that type V123 collagen exists not only as a periodic banded fibril but also as nonfibrillar meshwork structures.

Col V siRNA engineered tenocytes for tendon tissue engineering

The presence of uniformly small collagen fibrils in tendon repair is believed to play a major role in suboptimal tendon healing. Collagen V is significantly elevated in healing tendons and plays an important role in fibrillogenesis. The objective of this study was to investigate the effect of a particular chain of collagen V on the fibrillogenesis of Sprague-Dawley rat tenocytes, as well as the efficacy of Col V siRNA engineered tenocytes for tendon tissue engineering. RNA interference gene therapy and a scaffold free tissue engineered tendon model were employed. The results showed that scaffold free tissue engineered tendon had tissue-specific tendon structure. Down regulation of collagen V α1 or α2 chains by siRNAs (Col5α1 siRNA, Col5α2 siRNA) had different effects on collagen I and decorin gene expressions. Col5α1 siRNA treated tenocytes had smaller collagen fibrils with abnormal morphology; while those Col5α2 siRNA treated tenocytes had the same morphology as normal tenocytes. Furthermore, it was found that tendons formed by coculture of Col5α1 siRNA treated tenocytes with normal tenocytes at a proper ratio had larger collagen fibrils and relative normal contour. Conclusively, it was demonstrated that Col V siRNA engineered tenocytes improved tendon tissue regeneration. And an optimal level of collagen V is vital in regulating collagen fibrillogenesis. This may provide a basis for future development of novel cellular- and molecular biology-based therapeutics for tendon diseases.

Mechanism of Stabilization of a Bacterial Collagen Triple Helix in the Absence of Hydroxyproline

The Streptococcus pyogenes cell-surface protein Scl2 contains a globular N-terminal domain and a collagen-like domain, (Gly-Xaa-X'aa)(79), which forms a triple helix with a thermal stability close to that seen for mammalian collagens. Hyp is a major contributor to triple-helix stability in animal collagens, but is not present in bacteria, which lack prolyl hydroxylase. To explore the basis of bacterial collagen triple-helix stability in the absence of Hyp, biophysical studies were carried out on recombinant Scl2 protein, the isolated collagen-like domain from Scl2, and a set of peptides modeling the Scl2 highly charged repetitive (Gly-Xaa-X'aa)(n) sequences. At pH 7, CD spectroscopy, dynamic light scattering, and differential scanning calorimetry of the Scl2 protein all showed a very sharp thermal transition near 36 degrees C, indicating a highly cooperative unfolding of both the globular and triple-helix domains. The collagen-like domain isolated by trypsin digestion showed a sharp transition at the same temperature, with an enthalpy of 12.5 kJ/mol of tripeptide. At low pH, Scl2 and its isolated collagen-like domain showed substantial destabilization from the neutral pH value, with two thermal transitions at 24 and 27 degrees C. A similar destabilization at low pH was seen for Scl2 charged model peptides, and the degree of destabilization was consistent with the strong pH dependence arising from the GKD tripeptide unit. The Scl2 protein contained twice as much charge as human fibril-forming collagens, and the degree of electrostatic stabilization observed for Scl2 was similar to the contribution Hyp makes to the stability of mammalian collagens. The high enthalpic contribution to the stability of the Scl2 collagenous domain supports the presence of a hydration network in the absence of Hyp.

Synthetic heterotrimeric collagen peptides as mimics of cell adhesion sites of the basement membrane

Collagen type IV forms a network in the basement membrane into which other constituents of the tissue are incorporated. It also provides cell-adhesion sites that are specifically recognized by cell-surface receptors, i.e., the integrins. Different from the ubiquitous sequential RGD adhesion motif found in most of the matrix proteins, in collagen type IV, the responsible binding sites for alpha1beta1 integrin have been identified as Asp461 of the two alpha1 chains and Arg461 of the alpha2 chain. Because of the heterotrimeric character of this collagen, the spatial geometry of the binding epitope depends not only on the triple-helical fold, but decisively even on the stagger of the chains. To investigate the effects of chain registration on the conformational properties and binding affinities of this adhesion epitope, two synthetic heterotrimeric collagen peptides consisting of the identical three chains were assembled by an artificial cystine knot in two different registers, i.e., in the most plausible alpha2alpha1alpha1' and less probable alpha1alpha2alpha1' chain alignment. A detailed conformational characterization of both trimers allowed to correlate their different binding affinities for alpha1beta1 integrin with the degree of local plasticity of the two different triple helices. Optimal local breathing of the rod-shaped collagens is apparently crucial for selective recognition by proteins interacting with these main components of the extracellular matrix.

Targeted disruption of Col11a2 produces a mild cartilage phenotype in transgenic mice: comparison with the human disorder otospondylomegaepiphyseal dysplasia (OSMED)

Transgenic mice were prepared by homologous recombination with a Col11a2 targeting gene in which an inverted neomycin-resistant gene was inserted between restriction sites in exons 27 and 28. The targeted allele was transcribed in shortened mRNAs, which could be detected by Northern blotting. However, translation of the full-length Col11a2 chain was unable to occur because of the presence of premature termination codons within the inverted neomycin-resistant gene. Analysis of pepsin-resistant collagen chains from rib cartilage of homozygous mice demonstrated the lack of synthesis of intact alpha2(XI) chains. However, pepsin-resistant collagen chains of either alpha1(XI) or alpha1(V) were still detected on sodium dodecyl sulfate polyacrylamide gel electrophoresis. Therefore, alpha2(XI) chains are not essential for the assembly of some molecular forms of triple-helical type V/XI collagen. The phenotype was milder than in the cho/cho mouse in which, as the result of mutation, translation of the full-length alpha1(XI) chain fails to occur and the mice die at birth (Li et al., 1995). Homozygous mice without expression of an alpha2(XI) chain had a smaller body size, receding snouts, and deafness. Nasal bones in the homozygous transgenic mice were specifically shorter and dimpled on their external surfaces. Chondrocytes in growth plates of all long bones were markedly disorganized and failed to align in columns. Analysis of growth plates from transgenic mice by in situ hybridization showed expression of alpha1(II) and alpha1(XI) but not of alpha1(I) or alpha1(V) which, in contrast, were expressed in the developing bone and in the bone collar. Expression of alpha1(X) specifically in the hypertrophic cartilage was observed in normal and transgenic mice. No obvious osteoarthritis was observed throughout the life of homozygous mice up to 1 year of age, although minor morphologic anomalies in the articular cartilages were discernible. The mild phenotype is consistent with similar mutations in the COL11A2 gene seen in patients with nonocular Stickler syndrome and some patients with otospondylomegaepiphyseal dysplasia (OSMED), as well as in patients with a nonsyndromic form of deafness called DFNA13.

Cooperative equilibrium transitions coupled with a slow annealing step explain the sharpness and hysteresis of collagen folding

Heat and guanidinium-induced denaturation curves of collagen III and its fragments were fitted by theoretical models to explain the extreme sharpness and the hysteresis between unfolding and refolding. It was shown that a recently proposed kinetic model for collagen denaturation does not account for the observed steepness, with physically reasonable values of activation energy and frequency factors in the Arrhenius equation. The extreme slope, which amounts to 0.38 per centigrade for collagen III at the midpoint of its transition, can only be explained by a highly cooperative equilibrium model. The refolding curve is shifted to lower temperatures by 6 degrees C for collagen III and reversible unfolding matching the initial profile of the native protein is observed only after long-time annealing. A simple formalism is proposed by which experimental denaturation and refolding curves are quantitatively described. The transition proceeds via many cooperative steps with slightly different equilibrium constants for unfolding and refolding. Hysteresis and annealing are caused by very slow steps, which are probably connected with a rearrangement of misfolded regions. These slow steps disappear with decreasing size of collagen fragments and hysteresis is not found for collagen model peptides.

The pro-alpha3(V) collagen chain. Complete primary structure, expression domains in adult and developing tissues, and comparison to the structures and expression domains of the other types V and XI procollagen chains

The low abundance fibrillar collagen type V is widely distributed in tissues as an alpha1(V)(2)alpha2(V) heterotrimer that helps regulate the diameters of fibrils of the abundant collagen type I. Mutations in the alpha1(V) and alpha2(V) chain genes have been identified in some cases of classical Ehlers-Danlos syndrome (EDS), in which aberrant collagen fibrils are associated with connective tissue fragility, particularly in skin and joints. Type V collagen also exists as an alpha1(V)alpha2(V)alpha3(V) heterotrimer that has remained poorly characterized chiefly due to inability to obtain the complete primary structure or nucleic acid probes for the alpha3(V) chain or its biosynthetic precursor, pro-alpha3(V). Here we provide human and mouse full-length pro-alpha3(V) sequences. Pro-alpha3(V) is shown to be closely related to the alpha1(V) precursor, pro-alpha1(V), but with marked differences in N-propeptide sequences, and collagenous domain features that provide insights into the low melting temperature of alpha1(V)alpha2(V)alpha3(V) heterotrimers, lack of heparin binding by alpha3(V) chains and the possibility that alpha1(V)alpha2(V)alpha3(V) heterotrimers are incorporated into heterotypic fibrils. In situ hybridization of mouse embryos detects alpha3(V) expression primarily in the epimysial sheaths of developing muscles and within nascent ligaments adjacent to forming bones and in joints. This distribution, and the association of alpha1(V), alpha2(V), and alpha3(V) chains in heterotrimers, suggests the human alpha3(V) gene COL5A3 as a candidate locus for at least some cases of classical EDS in which the alpha1(V) and alpha2(V) genes have been excluded, and for at least some cases of the hypermobility type of EDS, a condition marked by gross joint laxity and chronic musculoskeletal pain. COL5A3 is mapped to 19p13.2 near a polymorphic marker that should be useful in analyzing linkage with EDS and other disease phenotypes.

A 1,064 bp fragment from the promoter region of the Col11a2 gene drives lacZ expression not only in cartilage but also in osteoblasts adjacent to regions undergoing both endochondral and intramembranous ossification in mouse embryos

We isolated a 1,064 bp promoter fragment that extended from the 3'-end of the adjacent gene for retinoic X receptor-beta to beyond the most clearly defined start site of the mouse Col11a2 gene. The fragment was then joined to a beta-galactosidase gene and used to prepare transgenic mice. Three independent lines of transgenic mice were generated. The reporter beta-galactosidase gene was expressed in essentially all cartilaginous tissues in 15.5-day-old mouse embryos. In addition, the construct was expressed in osteoprogenitors within developing periosteum and in osteoblasts within mineralized bone. This pattern of expression was evident during both endochondral and intramembranous bone formation. Therefore, the results suggest that 1,064 bp promoter fragment can drive tissue-specific expression of the Col11a2 gene.

The fibronectin-like domain is required for the type V and XI collagenolytic activity of gelatinase B

Gelatinase B (matrix metalloproteinase-9) is able to degrade several extracellular matrix proteins, including gelatin, elastin, and collagen types IV, V, XI, and XIV. This enzyme contains a "fibronectin-like" domain which is composed of three tandem copies of a fibronectin type 2 homology unit inserted into its catalytic domain. We have studied the involvement of this domain in the substrate specificity of gelatinase B by expressing a mutant of the enzyme, in Escherichia coli, in which this domain has been deleted. This mutant enzyme retained its ability to cleave the peptide substrate Mca-PLGL(Dpa)AR-NH2, possessing K(m) and kcat values similar to those of the wild-type enzyme. In addition, the NH2-terminal, 14-kDa, inhibitory domain of recombinant tissue inhibitor of metalloproteinase-2 was able to inhibit the mutant and the wild-type enzymes with the same potency. The mutant's gelatinolytic activity was also retained but reduced in comparison to that of the wild-type enzyme. However, contrary to the wild-type enzyme, the mutant was not able to digest or bind fibrillar collagen types V and XI. These data indicate that the fibronectin-like domain of gelatinase B is an important determinant of the enzyme's fibrillar collagen substrate specificity. It allows the enzyme to bind to and cleave collagen types V and XI, events which are thought to be involved in several normal physiological and pathological processes such as metastasis and arthritis.

Basic fibroblast growth factor decreases type V/XI collagen expression in cultured bovine aortic smooth muscle cells

Vascular smooth muscle cells (SMCs), the major cellular constituent of an artery, synthesize the bulk of fibrillar collagens, including type V/XI, which regulates heterotypic collagen fibril assembly. Basic fibroblast growth factor (bFGF) is a heparin-binding polypeptide growth factor that has been implicated in important events during the development of atherosclerosis, such as early intimal SMC proliferation. Here we have investigated the effects of bFGF on aortic SMC expression of type V/XI collagen. Treatment of exponentially growing or serum-deprived subconfluent cultures of bovine aortic SMCs with bFGF decreased the steady-state levels of the mRNAs for collagen type V/XI, including alpha 1(V), alpha 2(V), and alpha 1(XI). The effect of bFGF was time dependent with a two- and a fourfold decrease in alpha 2(V) mRNA observed after treatment for 24 and 48 h, respectively. This decrease resulted from a drop in the rate of alpha 2(V) gene transcription; no change was observed in the stability of the alpha 2(V) mRNA. Furthermore, accumulation of collagen protein decreased upon bFGF treatment. As expected, treatment with bFGF increased the rate of proliferation of serum-deprived SMCs, as judged by DNA content in the cultures, thymidine incorporation, and steady-state mRNA levels of the S-phase-expressed histone H3.2. These results suggest that bFGF plays an important role in the regulation of collagen fibril structure, with potential implications for the development and organization of an atherosclerotic lesion.

A cDNA cassette system for the synthesis of recombinant procollagens. Variants of procollagen II lacking a D-period are secreted as triple-helical monomers

Currently there is a lack of experimental systems for defining the functional domains of the fibrillar collagens. Here we describe an experimental strategy that employs the polymerase chain reaction (PCR) to create a series of cDNA cassettes coding for seven separate domains of procollagen II. The system was used to prepare novel recombinant procollagens II from which one of the four repetitive D-periods of the triple helix was deleted. Four constructs, each lacking a different D-period, were expressed in stably transfected mammalian cells (HT-1080). Truncated procollagens of the predicted size were recovered from the medium. All were triple-helical as assayed by circular dichroism. Therefore, deletion of a complete D-period containing 234 amino acids does not destabilize the triple helix of homotrimeric collagen II as much as some naturally occurring mutations in the heterotrimeric monomer of collagen I that delete shorter sequences or that convert obligate glycine residues to residues with bulkier side chains. Moreover, the results suggest that the strategy developed here can be used to map in detail the binding sites on fibrillar collagens for other components of the extracellular matrix and for the binding, spreading and signaling of cells.

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.

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.

The mouse col11a2 gene. Some transcripts from the adjacent rxr-beta gene extend into the col11a2 gene

Type XI collagen is present in small amounts in cartilage, together with small amounts of type IX and type V collagens and large amounts of type II collagen. Here, primers based on the nucleotide sequences of partial human cDNAs and mouse genomic DNAs that were analyzed by other investigators were used to isolate a cDNA for the mouse col11a2 gene. Cosmid clones for the mouse col11a2 gene were isolated, and 12.4 kb of the nucleotide sequences were defined. Analysis of the genomic sequences identified three exons in the mouse gene that were recently shown to undergo alternative splicing (Tsumaki and Kimura, J. Biol. Chem. 270, 2372-2378, 1995; Zhidkova et al., J. Biol. Chem. 270, 94886-9493, 1995). In addition, analysis of the cosmid clones revealed that the 5' end of the mouse col11a2 gene was located head-to-tail with the mouse retinoic X receptor beta gene. RT-PCR assays demonstrated that some transcripts from the retinoic X receptor beta gene extend into the col11a2 gene. Therefore, there may be coordinate expression of the two genes.

The human COL11A2 gene structure indicates that the gene has not evolved with the genes for the major fibrillar collagens

The human COL11A2 gene was analyzed from two overlapping cosmid clones that were previously isolated in the course of searching the human major histocompatibility region (Janatipour, M., Naumov, Y., Ando, A., Sugimura, K., Okamoto, N., Tsuji, K., Abe, K., and Inoko, H. (1992) Immunogenetics 35, 272-278). Nucleotide sequencing defined over 28,000 base pairs of the gene. It was shown to contain 66 exons. As with most genes for fibrillar collagens, the first intron was among the largest, and the introns at the 5'-end of the gene were in general larger than the introns at the 3'-end. Analysis of the exons coding for the major triple helical domain indicated that the gene structure had not evolved with the genes for the major fibrillar collagens in that there were marked differences in the number of exons, the exon sizes, and codon usage. The gene was located close to the gene for the retinoic X receptor beta in a head-to-tail arrangement similar to that previously seen with the two mouse genes (P. Vandenberg and D. J. Prockop, submitted for publication). Also, there was marked interspecies homology in the intergenic sequences. The amino acid sequences and the pattern of charged amino acids in the major triple helix of the alpha 2(XI) chain suggested that the chain can be incorporated into the same molecule as alpha 1(XI) and alpha 1(V) chains but not into the same molecule as the alpha 3(XI)/alpha 1(II) chain. The structure of the carboxyl-terminal propeptide was similar to the carboxyl-terminal propeptides of the pro alpha 1(XI) chain and pro alpha chains of other fibrillar collagens, but it was shorter because of internal deletions of about 30 amino acids.

Another look at collagen V and XI molecules

The fibrillar collagens are the most abundant proteins of extracellular matrices. Among them, collagens V and XI are quantitatively minor components which participate in the formation of the fibrillar collagen network. Since these collagens were discovered, studies have demonstrated that they may play a fundamental role in the control of fibrillogenesis, probably by forming a core within the fibrils. Another characteristic of these collagens is the partial retention of their N-propeptide extensions in tissue forms, an unusual observation in comparison to the other known fibrillar collagens. The tissue locations of collagens V and XI are different, but their structural and biological properties seem to be closely related. It has been shown that their primary structures are highly conserved at both the gene and protein levels, and that these conserved features are the bases of their similar biological properties. In particular, they are both resistant to mammalian collagenases, and surprisingly sensitive to trypsin treatment. Collagens V and XI are usually buried within the major collagen fibrils, although they have both cell adhesion and heparin binding sites which could be of crucial importance in physiological processes such as development and wound healing. It has became evident that several molecules are in fact heterotypic associations of chains from both collagens V and XI, demonstrating that these two collagens are not distinct types but a single type which can be called collagen V/XI.

The energy of formation of internal loops in triple-helical collagen polypeptides

The sequence-dependent local destabilization in the interior of the collagen triple helix has been evaluated by means of conformational energy computations. Using a model poly(Gly-Pro-Pro) triple helix as the reference state, a method was developed for generating local loops, i.e., internal deformations, and analyzing their conformations. A seven-residue Gly-Pro-Pro- Gly-Pro-Pro-Gly fragment was replaced by the Gly-Pro-Ala-Gly-Ala-Ala-Gly sequence in one, two, or all three of the strands of the loop region. A set of loop conformations was generated in which the ends of the loop were initially fixed in the triple-helical structure. The potential energy of the entire deformed triple helix was then minimized, resulting in a variety of structures that contained deformed loops. The conformations of the triple helices at the two ends of the loops remained essentially unchanged in many of the low-energy conformations. In numerous high-energy conformations, however, the triple-helical segments were also partially or totally disrupted. The minimum-energy conformations of the whole structures were compared in terms of rms deviations of atomic coordinates with respect to the original triple helix, and of the shapes of the loops (using a distance function derived from differential geometry). Three new geometrical parameters-stretch S, kink K, and unwinding U-were defined to describe the changes in the overall orientation of the triple helices at the two ends of the loop. It is shown that, when the number of Pro residues in a short fragment is reduced, the triple helical structure can accomodate internal loops (i.e., distortions) within a 5 kcal/mol cutoff from the essentially unperturbed triple helical structure. For structures with a Gly-Pro-Ala-Gly-Ala-Ala-Gly sequence in all three strands, the probability of finding conformations with internal loops is small, i.e., 0.06. Internal loops affect the overall orientation of these structures, as measured by the helix-distortion parameters S, K, and U.

Alternative exon splicing within the amino-terminal nontriple-helical domain of the rat pro-alpha 1(XI) collagen chain generates multiple forms of the mRNA transcript which exhibit tissue-dependent variation

Type XI collagen is an integral, although minor component of cartilage collagen fibrils. We have established that alternative exon usage is a mechanism for increasing structural diversity within the amino-terminal nontriple helical domain of the pro-alpha 1(XI) collagen gene. cDNA clones spanning the amino-terminal domain were selected from a rat chondrosarcoma library, and were shown to contain two major sequence differences from the previously reported human sequence. The first difference was the replacement of sequence encoding an acidic domain of 39 amino acids in length by a sequence encoding a 51-amino acid basic domain with a predicted pI of 11.9. The second difference was the absence of a sequence that would translate into a highly acidic 85-amino acid sequence downstream from the first variation. These two changes, expressed together, result in the replacement of most of the acidic domain with one that is smaller and basic. These two sequence differences serve to identify subdomains of a variable region, designated V1 and V2, respectively. V1a is defined as the acidic 39-amino acid sequence element and V1b is defined as the 51-amino acid basic sequence. Analysis of genomic DNA revealed that both V1a and V1b are encoded by separate adjacent exons in the rat genome and V2 is also encoded in a single exon downstream. Analysis of mRNA from cartilage-derived sources revealed a complex pattern of alpha 1(XI) transcript expression due to differential exon usage. In non-cartilage sources, the pattern is less complex; the most prevalent form is the one containing the two acidic sequences, V1a and V2.

Thermal stability and folding of the collagen triple helix and the effects of mutations in osteogenesis imperfecta on the triple helix of type I collagen

Osteogenesis imperfecta (OI) is an inherited disease in which 90% of the cases result from mutations in the 2 genes, pro alpha 1 and pro alpha 2, coding for type I collagen. Type I collagen is a trimeric molecule, (alpha 1)2 alpha 2, which is dominated both structurally and functionally by the 300 nm triple-helical domain. Most OI mutations occur in this domain and almost all point mutations result in the substitution of other amino acids for the obligate glycine which occurs at every third residue. The phenotypic effects of these mutations are frequently attributed in part to alterations in the stability and rate of folding of the triple helix. In order to better understand the relationship between glycine substitutions and stability we review current concepts of the forces governing triple helical stability, denaturational and predenaturational unfolding, and the techniques of measuring stability. From observations on the stability of several collagen types as well as synthetic tripeptides, we present a model for stability based on the contribution of individual and neighboring tripeptide units to the local stability. Although in preliminary form, this empirical model can account for the observed shifts in the Tm of many of the point mutations described. The folding of the triple helix is reviewed. The involvement of peptidyl prolyl cis-trans isomerase in this process in vivo is demonstrated by the inhibition of collagen folding in fibroblasts by cyclosporin A. An hypothesis based on the relationship between the thermal stability at the site of mutation and the propensity for renucleation of folding is proposed.

The human rhabdomyosarcoma cell line A204 lays down a highly insoluble matrix composed mainly of alpha 1 type-XI and alpha 2 type-V collagen chains

The biosynthesis of collagen by the A204 cell line was examined using polyclonal antibodies raised against collagen type V and type XI. The study of the pepsin-digested collagen showed that it is composed mainly of alpha 1(XI) and alpha 2(V) collagen chains in an apparent 2:1 ratio, suggesting the formation of heterotypic molecules [alpha 1(XI)]2 alpha 2(V). The existence of this chain stoichiometry was further demonstrated by immunoprecipitation of the molecule with an antibody recognizing alpha 2(V) but not alpha 1(XI) collagen chains. Electron microscopy analyses of 24-h cultures showed that this matrix is composed of thin fibrils, that can be decorated with immunogold-labelled anti-(type-V collagen) IgG, but not with anti-(type-XI collagen) IgG. The collagen matrix laid down by A204 cells is highly insoluble. In the presence of beta-aminopropionitrile, an inhibitor of lysyl oxidase, only a small proportion of intact collagen could be extracted without proteolytic treatment. Immunoblotting of intact medium collagen from cultures performed in the presence of beta-aminopropionitrile showed four distinct bands with each antibody. The migration of the bands, stained with anti-(type-V collagen) IgG, had apparent molecular masses of 127, 149, 161 and 198 kDa (compared to globular standards) while the bands stained with anti-(type-XI collagen) IgG had apparent masses of 145, 182, 207 and 225 kDa. These data indicate that type-V and type-XI collagen chains can assemble in heterotypic isoforms. In this system, the synthesized isoforms are able to aggregate into a highly cohesive matrix and they undergo a proteolytic processing closely similar to that of other fibrillar collagens.

Sequence specific thermal stability of the collagen triple helix

Theoretical calculations of the thermal stability of collagen triple helices using empirical values for the contribution of individual tripeptide units are presented and compared with direct measurements of the thermal stability of various types of collagens. Relative stabilities are assigned to the positions of the tripeptide units in the amino acid sequence along the length of the collagen molecule. The sequence specific relative stabilities of type I and type XI collagens are compared. These offer insight into the reasons for the existence of unfolding intermediates in type XI collagen that are absent in type I collagen. The pattern of relative stabilities calculated for mouse type IV collagen is consistent with experimental results which indicate that the amino terminal region is very stable and that the interruptions cause increased flexibility and independently unfolding domains. Mutations in the triple helical domain of human type I procollagen occurring in brittle bone disease (osteogenesis imperfecta) show varying effects on the thermal stability of the molecule. The sequence specific thermal stability calculations shed some light on why some mutations of cysteine for glycine have greater effects on the thermal stability than others.