Type-III procollagen assembly in semi-intact cells: chain association, nucleation and triple-helix folding do not require formation of inter-chain disulphide bonds but triple-helix nucleation does require hydroxylation

Designing collagens to shed light on the multi-scale structure-function mapping of matrix disorders

Collagens are the most abundant structural proteins in the extracellular matrix of animals and play crucial roles in maintaining the structural integrity and mechanical properties of tissues and organs while mediating important biological processes. Fibrillar collagens have a unique triple helix structure with a characteristic repeating sequence of (Gly-X-Y)n. Variations within the repetitive sequence can cause misfolding of the triple helix, resulting in heritable connective tissue disorders. The most common variations are single-point missense mutations that lead to the substitution of a glycine residue with a bulkier amino acid (Gly → X). In this review, we will first discuss the importance of collagen's triple helix structure and how single Gly substitutions can impact its folding, structure, secretion, assembly into higher-order structures, and biological functions. We will review the role of "designer collagens," i.e., synthetic collagen-mimetic peptides and recombinant bacterial collagen as model systems to include Gly → X substitutions observed in collagen disorders and investigate their impact on structure and function utilizing in vitro studies. Lastly, we will explore how computational modeling of collagen peptides, especially molecular and steered molecular dynamics, has been instrumental in probing the effects of Gly substitutions on structure, receptor binding, and mechanical stability across multiple length scales.

A cysteine-based molecular code informs collagen C-propeptide assembly

Fundamental questions regarding collagen biosynthesis, especially with respect to the molecular origins of homotrimeric versus heterotrimeric assembly, remain unanswered. Here, we demonstrate that the presence or absence of a single cysteine in type-I collagen’s C-propeptide domain is a key factor governing the ability of a given collagen polypeptide to stably homotrimerize. We also identify a critical role for Ca²⁺ in non-covalent collagen C-propeptide trimerization, thereby priming the protein for disulfide-mediated covalent immortalization. The resulting cysteine-based code for stable assembly provides a molecular model that can be used to predict, a priori, the identity of not just collagen homotrimers, but also naturally occurring 2:1 and 1:1:1 heterotrimers. Moreover, the code applies across all of the sequence-diverse fibrillar collagens. These results provide new insight into how evolution leverages disulfide networks to fine-tune protein assembly, and will inform the ongoing development of designer proteins that assemble into specific oligomeric forms.

Collagen proteins assemble into trimers from distinct monomers with high specificity, yet the molecular basis for this specificity remains unclear. Here the authors demonstrate the crucial role of conserved C-terminal domain cysteine residues and calcium in homotrimeric procollagen assembly.

Substitutions for arginine at position 780 in triple helical domain of the α1(I) chain alter folding of the type I procollagen molecule and cause osteogenesis imperfecta

OI is a clinically and genetically heterogeneous disorder characterized by bone fragility. More than 90% of patients are heterozygous for mutations in type I collagen genes, COL1A1 and COL1A2, and a common mutation is substitution for an obligatory glycine in the triple helical Gly-X-Y repeats. Few non-glycine substitutions in the triple helical domain have been reported; most result in Y-position substitutions of arginine by cysteine. Here, we investigated leucine and cysteine substitutions for one Y-position arginine, p.Arg958 (Arg780 in the triple helical domain) of proα1(I) chains that cause mild OI. We compared their effects with two substitutions for glycine located in close proximity. Like substitutions for glycine, those for arginine reduced the denaturation temperature of the whole molecule and caused asymmetric posttranslational overmodification of the chains. Circular dichroism and increased susceptibility to cleavage by MMP1, MMP2 and catalytic domain of MMP1 revealed significant destabilization of the triple helix near the collagenase cleavage site. On a cellular level, we observed slower triple helix folding and intracellular collagen retention, which disturbed the Endoplasmic Reticulum function and affected matrix deposition. Molecular dynamic modeling suggested that Arg780 substitutions disrupt the triple helix structure and folding by eliminating hydrogen bonds of arginine side chains, in addition to preventing HSP47 binding. The pathogenic effects of these non-glycine substitutions in bone are probably caused mostly by procollagen misfolding and its downstream effects.

Molecular assembly of recombinant chicken type II collagen in the yeast Pichia pastoris

Effective treatment of rheumatoid arthritis can be mediated by native chicken type II collagen (nCCII), recombinant peptide containing nCCII tolerogenic epitopes (CTEs), or a therapeutic DNA vaccine encoding the full-length CCOL2A1 cDNA. As recombinant CCII (rCCII) might avoid potential pathogenic virus contamination during nCCII preparation or chromosomal integration and oncogene activation associated with DNA vaccines, here we evaluated the importance of propeptide and telopeptide domains on rCCII triple helix molecular assembly. We constructed pC- and pN-procollagen (without N- or C-propeptides, respectively) as well as CTEs located in the triple helical domain lacking both propeptides and telopeptides, and expressed these in yeast Pichia pastoris host strain GS115 (his4, Mut⁺) simultaneously with recombinant chicken prolyl-4-hydroxylase α and β subunits. Both pC- and pN-procollagen monomers accumulated inside P. pastoris cells, whereas CTE was assembled into homotrimers with stable conformation and secreted into the supernatants, suggesting that the large molecular weight pC-or pN-procollagens were retained within the endoplasmic reticulum whereas the smaller CTEs proceeded through the secretory pathway. Furthermore, resulting recombinant chicken type II collagen pCα1(II) can induced collagen-induced arthritis (CIA) rat model, which seems to be as effective as the current standard nCCII. Notably, protease digestion assays showed that rCCII could assemble in the absence of C- and N-propeptides or telopeptides. These findings provide new insights into the minimal structural requirements for rCCII expression and folding.

L-ascorbic acid: A true substrate for HIF prolyl hydroxylase?

L-Ascorbate (L-Asc), but not D-isoascorbate (D-Asc) and N-acetylcysteine (NAC) suppress HIF1 ODD-luc reporter activation induced by various inhibitors of HIF prolyl hydroxylase (PHD). The efficiency of suppression by L-Asc was sensitive to the nature of HIF PHD inhibitor chosen for reporter activation. In particular, the inhibitors developed to compete with alpha-ketoglutarate (αKG), were less sensitive to suppression by the physiological range of L-Asc (40-100 μM) than those having a strong iron chelation motif. Challenging those HIF activators in the reporter system with D-Asc demonstrated that the D-isomer, despite exhibiting the same reducing potency with respect to ferric iron, had almost no effect compared to L-Asc. Similarly, no effect on reporter activation was observed with cell-permeable reducing agent NAC up to 1 mM. Docking of L-Asc and D-Asc acid into the HIF PHD2 crystal structure showed interference of Tyr310 with respect to D-Asc. This suggests that L-Asc is not merely a reducing agent preventing enzyme inactivation. Rather, the overall results identify L-Asc as a co-substrate of HIF PHD that may compete for the binding site of αKG in the enzyme active center. This conclusion is in agreement with the results obtained recently in cell-based systems for TET enzymes and jumonji histone demethylases, where L-Asc has been proposed to act as a co-substrate and not as a reducing agent preventing enzyme inactivation.

New strategy for expression of recombinant hydroxylated human collagen α1(III) chains in Pichia pastoris GS115

Type III collagen is one of the most abundant proteins in the human body, which forms collagen fibrils and provides the stiff, resilient characteristics of many tissues. In this paper, a new method for secretory expression of recombinant hydroxylated human collagen α1(III) chain in Pichia pastoris GS115 was applied. The gene encoding for full-length human collagen α1(III) chain (COL3A1) without N-terminal propeptide and C-terminal propeptide was cloned in the pPIC9K expression vector. The prolyl 4-hydroxylase (P4H, EC 1.14.11.2) α-subunit (P4Hα) and β-subunit (P4Hβ) genes were cloned in the same expression vector, pPICZB. Fluorogenic quantitative PCR indicates that COL3A1 and P4H genes have been expressed in mRNA level. SDS-PAGE shows that secretory expression of recombinant human collagen α1(III) chain was successfully achieved in P. pastoris GS115. In addition, the result of amino acids composition analysis shows that the recombinant human collagen α1(III) chain contains hydroxyproline by coexpression with the P4H. Furthermore, liquid chromatography coupled with tandem mass spectrometry analysis demonstrates that proline residues of the recombinant human collagen α1(III) chain were hydroxylated in the X or Y positions of Gly-X-Y triplets.

The pH sensitivity of murine heat shock protein 47 (HSP47) binding to collagen is affected by mutations in the breach histidine cluster

Heat shock protein 47 (HSP47) is a single-substrate molecular chaperone crucial for collagen biosynthesis. Although its function is well established, the molecular mechanisms that govern binding to procollagen peptides and triple helices in the endoplasmic reticulum (followed by controlled release in the Golgi) are unclear. HSP47 binds procollagen at a neutral pH but releases at a pH similar to the pK(a) of the imidazole side chain of histidine residues. It thus seems likely that these residues are involved in this pH-dependent mechanism. Murine HSP47 has 14 histidine residues grouped into three clusters, known as the breach, gate, and shutter. Here, we report the use of histidine mutagenesis to demonstrate the relative contribution of these three clusters to HSP47 structure and the "pH switch." Many of the tested mutants are silent; however, breach mutants H197A and H198A show binding but no apparent pH switch and are unable to control release. Another breach mutant, H191A, shows perturbed collagen release characteristics, consistent with observed perturbations in pH-driven trans-conformational changes. Thus, His-198, His-197 and His-191 are important (if not central) to HSP47 mechanism of binding/release to collagen. This is consistent with the breach cluster residues being well conserved across the HSP47 family.

Growth hormone doping in sports: a critical review of use and detection strategies

GH is believed to be widely employed in sports as a performance-enhancing substance. Its use in athletic competition is banned by the World Anti-Doping Agency, and athletes are required to submit to testing for GH exposure. Detection of GH doping is challenging for several reasons including identity/similarity of exogenous to endogenous GH, short half-life, complex and fluctuating secretory dynamics of GH, and a very low urinary excretion rate. The detection test currently in use (GH isoform test) exploits the difference between recombinant GH (pure 22K-GH) and the heterogeneous nature of endogenous GH (several isoforms). Its main limitation is the short window of opportunity for detection (~12-24 h after the last GH dose). A second test to be implemented soon (the biomarker test) is based on stimulation of IGF-I and collagen III synthesis by GH. It has a longer window of opportunity (1-2 wk) but is less specific and presents a variety of technical challenges. GH doping in a larger sense also includes doping with GH secretagogues and IGF-I and its analogs. The scientific evidence for the ergogenicity of GH is weak, a fact that is not widely appreciated in athletic circles or by the general public. Also insufficiently appreciated is the risk of serious health consequences associated with high-dose, prolonged GH use. This review discusses the GH biology relevant to GH doping; the virtues and limitations of detection tests in blood, urine, and saliva; secretagogue efficacy; IGF-I doping; and information about the effectiveness of GH as a performance-enhancing agent.

Faithful chaperones

This review describes the properties of some rare eukaryotic chaperones that each assist in the folding of only one target protein. In particular, we describe (1) the tubulin cofactors, (2) p47, which assists in the folding of collagen, (3) α-hemoglobin stabilizing protein (AHSP), (4) the adenovirus L4-100 K protein, which is a chaperone of the major structural viral protein, hexon, and (5) HYPK, the huntingtin-interacting protein. These various-sized proteins (102–1,190 amino acids long) are all involved in the folding of oligomeric polypeptides but are otherwise functionally unique, as they each assist only one particular client. This raises a question regarding the biosynthetic cost of the high-level production of such chaperones. As the clients of faithful chaperones are all abundant proteins that are essential cellular or viral components, it is conceivable that this necessary metabolic expenditure withstood evolutionary pressure to minimize biosynthetic costs. Nevertheless, the complexity of the folding pathways in which these chaperones are involved results in error-prone processes. Several human disorders associated with these chaperones are discussed.

Effects of mechanical loading on collagen propeptides processing in cartilage repair

Injured articular cartilage has poor reparative capabilities and if left untreated may develop into osteoarthritis. Unsatisfactory results with conventional treatment methods have brought as an alternative treatment the development of matrix autologous chondrocyte transplants (MACTs). Recent evidence proposes that the maintenance of the original phenotype by isolated chondrocytes grown in a scaffold transplant is linked to mechanical compression, because macromolecules, particularly collagen, of the extracellular matrix have the ability to 'self-assemble'. In load-bearing tissues, collagen is abundantly present and mechanical properties depend on the collagen fibre architecture. Study of the active changes in collagen architecture is the focus of diverse fields of research, including developmental biology, biomechanics and tissue engineering. In this review, the structural model of collagen assembly is presented in order to understand how scaffold geometry plays a critical role in collagen propeptide processing and chondrocyte development. When physical forces are applied to different cell-based scaffolds, the resulting specific twist of the scaffolds might be accompanied by changes in the fibril pattern synthesis of the new collagen. The alteration in the scaffolds due to mechanical stress is associated with cellular signalling communication and the preservation of N-terminus procollagen moieties, which would regulate both the collagen synthesis and the diameter of the fibre. The structural difference would also affect actin stabilization, cytoskeleton remodelling and proteoglycan assembly. These effects seemed to be dependent on the magnitude and duration of the physical stress. This review will contribute to the understanding of mechanisms for collagen assembly in both a natural and an artificial environment.

The fibrillar collagen family

Collagens, or more precisely collagen-based extracellular matrices, are often considered as a metazoan hallmark. Among the collagens, fibrillar collagens are present from sponges to humans, and are involved in the formation of the well-known striated fibrils. In this review we discuss the different steps in the evolution of this protein family, from the formation of an ancestral fibrillar collagen gene to the formation of different clades. Genomic data from the choanoflagellate (sister group of Metazoa) Monosiga brevicollis, and from diploblast animals, have suggested that the formation of an ancestral alpha chain occurred before the metazoan radiation. Phylogenetic studies have suggested an early emergence of the three clades that were first described in mammals. Hence the duplication events leading to the formation of the A, B and C clades occurred before the eumetazoan radiation. Another important event has been the two rounds of "whole genome duplication" leading to the amplification of fibrillar collagen gene numbers, and the importance of this diversification in developmental processes. We will also discuss some other aspects of fibrillar collagen evolution such as the development of the molecular mechanisms involved in the formation of procollagen molecules and of striated fibrils.

Protein disulphide isomerase family members show distinct substrate specificity: P5 is targeted to BiP client proteins

At least 17 members of the protein disulphide isomerase (PDI) family of oxidoreductases are present in the endoplasmic reticulum (ER) of mammalian cells. They are thought to catalyse disulphide formation to aid folding or to regulate protein function; however, little is known about their individual functions. Here, we show that some proteins that enter the ER are clients for single oxidoreductases, whereas others are clients for several PDI-like enzymes. We previously identified potential substrates for ERp57, and here identify substrates for ERp18 and ERp46. In addition, we analysed the specificity of substrates towards PDI, ERp72, ERp57, ERp46, ERp18 and P5. Strikingly, ERp18 shows specificity towards a component of the complement cascade, pentraxin-related protein PTX3, whereas ERp46 has specificity towards peroxiredoxin-4, a thioredoxin peroxidase. By contrast, most PDI family members react with Ero1alpha. Moreover, P5 forms a non-covalent complex with immunoglobulin heavy chain binding protein (BiP) and shows specificity towards BiP client proteins. These findings highlight cooperation between BiP and P5, and demonstrate that individual PDI family members recognise specific substrate proteins.

Ex vivo conditions for self-replication of human hepatic stem cells

Human hepatic stem cells (hHpSCs), identifiable by a unique antigenic profile, have been isolated from human livers and established ex vivo under expansion conditions permissive for self-replication. The conditions consist of a substratum of type III collagen, ideally on Transwell inserts, and Kubota's medium, a serum-free medium developed for hepatic progenitors. Under these conditions the cells demonstrated a doubling time of approximately 24 h, generating at least a 16-fold increase in cell number within 7-10 days; were stable at confluence for up to 2 weeks; could be passaged, if on type III collagen, to initiate colonies that went through log-phase growth and saturation density kinetics; and expressed telomerase, indicative of regenerative capacity. The hHpSC colonies remained morphologically and phenotypically stable throughout expressing epithelial cell adhesion molecule, neural cell adhesion molecule, albumin, cytokeratins 8, 18, and 19, but not alpha-fetoprotein, or intercellular adhesion molecule-1 (ICAM-1). Those maintained under self-replication conditions for more than a month were transplanted and found to engraft in the livers of SCID/nod mice yielding human liver tissue expressing adult liver-specific proteins. The conditions for self-replication should offer ideal culture conditions for generating large numbers of hHpSCs for use in commercial and clinical programs.

Direct and continuous assay for prolyl 4-hydroxylase

Prolyl 4-hydroxylase (P4H) is a nonheme iron dioxygenase that catalyzes the posttranslational hydroxylation of (2S)-proline (Pro) residues in protocollagen strands. The resulting (2S,4R)-4-hydroxyproline (Hyp) residues are essential for the folding, secretion, and stability of the collagen triple helix. P4H uses alpha-ketoglutarate and O2 as cosubstrates, and forms succinate and CO2 as well as Hyp. Described herein is the first assay for P4H that continuously and directly detects turnover of the proline-containing substrate. This assay is based on (2S,4S)-4-fluoroproline (flp), a proline analogue that is transformed into (2S)-4-ketoproline (Kep) and inorganic fluoride by P4H. The fluoride ion, and thus turnover by P4H, is detected by a fluoride ion-selective electrode. Using this assay, steady-state kinetic parameters for the human P4H-catalyzed turnover of a flp-containing peptide were determined and found to be comparable to those obtained with a discontinuous HPLC-based assay. In addition, this assay can be used to characterize P4H variants, as demonstrated by a comparison of catalysis by D414A P4H and the wild-type enzyme. Finally, the use of the assay to identify small-molecule inhibitors of P4H was verified by an analysis of catalysis in the presence of 2,4-pyridine dicarboxylate, an analogue of alpha-ketoglutarate. Thus, the assay described herein could facilitate biochemical analyses of this essential enzyme.

Substrate specificity of the oxidoreductase ERp57 is determined primarily by its interaction with calnexin and calreticulin

The formation of disulfides within proteins entering the secretory pathway is catalyzed by the protein disulfide isomerase family of endoplasmic reticulum localized oxidoreductases. One such enzyme, ERp57, is thought to catalyze the isomerization of non-native disulfide bonds formed in glycoproteins with unstructured disulfide-rich domains. Here we investigated the mechanism underlying ERp57 specificity toward glycoprotein substrates and the interdependence of ERp57 and the calnexin cycle for their correct folding. Our results clearly show that ERp57 must be physically associated with the calnexin cycle to catalyze isomerization reactions with most of its substrates. In addition, some glycoproteins only require ERp57 for correct disulfide formation if they enter the calnexin cycle. Hence, the specificity of ER oxidoreductases is not only determined by the physical association of enzyme and substrate but also by accessory factors, such as calnexin and calreticulin in the case of ERp57. These conclusions suggest that the calnexin cycle has evolved with a specialized oxidoreductase to facilitate native disulfide formation in complex glycoproteins.

Crystal structure of human type III collagen Gly991-Gly1032 cystine knot-containing peptide shows both 7/2 and 10/3 triple helical symmetries

Type III collagen is a critical collagen that comprises extensible connective tissue such as skin, lung, and the vascular system. Mutations in the type III collagen gene, COL3A1, are associated with the most severe forms of Ehlers-Danlos syndrome. A characteristic feature of type III collagen is the presence of a stabilizing C-terminal cystine knot. Crystal structures of collagen triple helices reported so far contain artificial sequences like (Gly-Pro-Pro)(n) or (Gly-Pro-Hyp)(n). To gain insight into the structural properties exhibited by the natural type III collagen triple helix, we synthesized, crystallized, and determined the structure of a 12-triplet repeating peptide containing the natural type III collagen sequence from residues 991 to 1032 including the C-terminal cystine knot region, to 2.3A resolution. This represents the longest collagen triple helical structure determined to date with a native sequence. Strikingly, the Gly(991)-Gly(1032) structure reveals that the central non-imino acid-containing region adopts 10/3 superhelical properties, whereas the imino acid rich N- and C-terminal regions adhere to a 7/2 superhelical conformation. The structure is consistent with two models for the cystine knot; however, the poor density for the majority of this region suggests that multiple conformations may be adopted. The structure shows that the multiple non-imino acids make several types of direct intrahelical as well as interhelical contacts. The looser superhelical structure of the non-imino acid region of collagen triple helices combined with the extra contacts afforded by ionic and polar residues likely play a role in fibrillar assembly and interactions with other extracellular components.

Conformational preferences of substrates for human prolyl 4-hydroxylase

Prolyl 4-hydroxylase (P4H) catalyzes the posttranslational hydroxylation of (2 S)-proline (Pro) residues in procollagen strands. The resulting (2 S,4 R)-4-hydroxyproline (Hyp) residues are essential for the folding, secretion, and stability of the collagen triple helix. Even though its product (Hyp) differs from its substrate (Pro) by only a single oxygen atom, no product inhibition has been observed for P4H. Here, we examine the basis for the binding and turnover of substrates by human P4H. Synthetic peptides containing (2 S,4 R)-4-fluoroproline (Flp), (2 S,4 S)-4-fluoroproline (flp), (2 S)-4-ketoproline (Kep), (2 S)-4-thiaproline (Thp), and 3,5-methanoproline (Mtp) were evaluated as substrates for P4H. Peptides containing Pro, flp, and Thp were found to be excellent substrates for P4H, forming Hyp, Kep, and (2 S,4 R)-thiaoxoproline, respectively. Thus, P4H is tolerant to some substitutions on C-4 of the pyrrolidine ring. In contrast, peptides containing Flp, Kep, or Mtp did not even bind to the active site of P4H. Each proline analogue that does bind to P4H is also a substrate, indicating that discrimination occurs at the level of binding rather than turnover. As the iron(IV)-oxo species that forms in the active site of P4H is highly reactive, P4H has an imperative for forming a snug complex with its substrate and appears to do so. Most notably, those proline analogues with a greater preference for a C (gamma)- endo pucker and cis peptide bond were the ones recognized by P4H. As Hyp has a strong preference for C (gamma)- exo pucker and trans peptide bond, P4H appears to discriminate against the conformation of proline residues in a manner that diminishes product inhibition during collagen biosynthesis.

Trimerization of collagen IX alpha-chains does not require the presence of the COL1 and NC1 domains

Collagen IX is a heterotrimer of three alpha-chains, which consists of three COL domains (collagenous domains) (COL1-COL3) and four NC domains (non-collagenous domains) (NC1-NC4), numbered from the C-terminus. Although collagen IX chains have been shown to associate via their C-terminal NC1 domains and form a triple helix starting from the COL1 domain, it is not known whether chain association can occur at other sites and whether other collagenous and non-collagenous regions are involved. To address this question, we prepared five constructs, two long variants (beginning at the NC4 domain) and three short variants (beginning at the COL2 domain), all ending at the NC2 domain (or NC2 replaced by NC1), to study association and selection of collagen IX alpha-chains. Both long variants were able to associate with NC1 or NC2 at the C-terminus and form various disulfide-bonded trimers, but the specificity of chain selection was diminished compared with full-length chains. Trimers of the long variant ending at NC2 were shown to be triple helical by CD. Short variants were not able to assemble into disulfide-bonded trimers even in the presence of both conserved cysteine residues from the COL1-NC1 junction. Our results demonstrate that collagen IX alpha-chains can associate in the absence of COL1 and NC1 domains to form a triple helix, but the COL2-NC2 region alone is not sufficient for trimerization. The results suggest that folding of collagen IX is a co-operative process involving multiple COL and NC domains and that the COL1-NC1 region is important for chain specificity.

Protein disulfide isomerase activity is essential for viability and extracellular matrix formation in the nematode Caenorhabditis elegans

Protein disulfide isomerase (PDI) is a multifunctional protein required for many aspects of protein folding and transit through the endoplasmic reticulum. A conserved family of three PDIs has been functionally analysed using genetic mutants of the model organism Caenorhabditis elegans. PDI-1 and PDI-3 are individually non-essential, whereas PDI-2 is required for normal post-embryonic development. In combination, all three genes are synergistically essential for embryonic development in this nematode. Mutations in pdi-2 result in severe body morphology defects, uncoordinated movement, adult sterility, abnormal molting and aberrant collagen deposition. Many of these phenotypes are consistent with a role in collagen biogenesis and extracellular matrix formation. PDI-2 is required for the normal function of prolyl 4-hydroxylase, a key collagen-modifying enzyme. Site-directed mutagenesis indicates that the independent catalytic activity of PDI-2 may also perform an essential developmental function. PDI-2 therefore performs two critical roles during morphogenesis. The role of PDI-2 in collagen biogenesis can be restored following complementation of the mutant with human PDI.

Three arginine to cysteine substitutions in the pro-alpha (I)-collagen chain cause Ehlers-Danlos syndrome with a propensity to arterial rupture in early adulthood

Mutations in the COL1A1 and COL1A2 genes, encoding the proalpha1 and 2 chains of type I collagen, cause osteogenesis imperfecta (OI) or Ehlers-Danlos syndrome (EDS) arthrochalasis type. Although the majority of missense mutations in the collagen type I triple helix affect glycine residues in the Gly-Xaa-Yaa repeat, few nonglycine substitutions have been reported. Two arginine-to-cysteine substitutions in the alpha1(I)-collagen chain are associated with classic EDS [R134C (p.R312C)] or autosomal dominant Caffey disease with mild EDS features [R836C (p.R1014C)]. Here we show alpha1(I) R-to-C substitutions in three unrelated patients who developed iliac or femoral dissection in early adulthood. In addition, manifestations of classic EDS in Patient 1 [c.1053C>T; R134C (p.R312C); X-position] or osteopenia in Patients 2 [c.1839C>T; R396C (p.R574C); Y-position] and 3 [c.3396C>T; R915C (p.R1093C); Y-position] are seen. Dermal fibroblasts from the patients produced disulfide-bonded alpha1(I)-dimers in approximately 20% of type I collagen, which were efficiently secreted into the medium in case of the R396C and R915C substitution. Theoretical stability calculations of the collagen type I heterotrimer and thermal denaturation curves of monomeric mutant alpha1(I)-collagen chains showed minor destabilization of the collagen helix. However, dimers were shown to be highly unstable. The R134C and R396C caused delayed procollagen processing by N-proteinase. Ultrastructural findings showed collagen fibrils with variable diameter and irregular interfibrillar spaces, suggesting disturbed collagen fibrillogenesis. Our findings demonstrate that R-to-C substitutions in the alpha1(I) chain may result in a phenotype with propensity to arterial rupture in early adulthood. This broadens the phenotypic range of nonglycine substitutions in collagen type I and has important implications for genetic counseling and follow-up of patients carrying this type of mutation.

Type I collagen in Hsp47-null cells is aggregated in endoplasmic reticulum and deficient in N-propeptide processing and fibrillogenesis

Heat-shock protein of 47 kDa (Hsp47) is a molecular chaperone that recognizes collagen triple helices in the endoplasmic reticulum (ER). Hsp47-knockout mouse embryos are deficient in the maturation of collagen types I and IV, and collagen triple helices formed in the absence of Hsp47 show increased susceptibility to protease digestion. We show here that the fibrils of type I collagen produced by Hsp47-/- cells are abnormally thin and frequently branched. Type I collagen was highly accumulated in the ER of Hsp47-/- cells, and its secretion rate was much slower than that of Hsp47+/+ cells, leading to accumulation of the insoluble aggregate of type I collagen within the cells. Transient expression of Hsp47 in the Hsp47-/- cells restored normal extracellular fibril formation and intracellular localization of type I collagen. Intriguingly, type I collagen with unprocessed N-terminal propeptide (N-propeptide) was secreted from Hsp47-/- cells and accumulated in the extracellular matrix. These results indicate that Hsp47 is required for correct folding and prevention of aggregation of type I collagen in the ER and that this function is indispensable for efficient secretion, processing, and fibril formation of collagen.

Production of human prolyl 4-hydroxylase in Escherichia coli

Prolyl 4-hydroxylase (P4H) catalyzes the post-translational hydroxylation of proline residues in collagen strands. The enzyme is an alpha2beta2 tetramer in which the alpha subunits contain the catalytic active sites and the beta subunits (protein disulfide isomerase) maintain the alpha subunits in a soluble and active conformation. Heterologous production of the native alpha2beta2 tetramer is challenging and had not been reported previously in a prokaryotic system. Here, we describe the production of active human P4H tetramer in Escherichia coli from a single bicistronic vector. P4H production requires the relatively oxidizing cytosol of Origami B(DE3) cells. Induction of the wild-type alpha(I) cDNA in these cells leads to the production of a truncated alpha subunit (residues 235-534), which assembles with the beta subunit. This truncated P4H is an active enzyme, but has a high Km value for long substrates. Replacing the Met235 codon with one for leucine removes an alternative start codon and enables production of full-length alpha subunit and assembly of the native alpha2beta2 tetramer in E. coli cells to yield 2 mg of purified P4H per liter of culture (0.2 mg/g of cell paste). We also report a direct, automated assay of proline hydroxylation using high-performance liquid chromatography. We anticipate that these advances will facilitate structure-function analyses of P4H.

Procollagen trafficking, processing and fibrillogenesis

Collagen fibrils in the extracellular matrix allow connective tissues such as tendon, skin and bone to withstand tensile forces. The fibrils are indeterminate in length, insoluble and form elaborate three-dimensional arrays that extend over numerous cell lengths. Studies of the molecular basis of collagen fibrillogenesis have provided insight into the trafficking of procollagen (the precursor of collagen) through the cellular secretory pathway, the conversion of procollagen to collagen by the procollagen metalloproteinases, and the directional deposition of fibrils involving the plasma membrane and late secretory pathway. Fibril-associated molecules are targeted to the surface of collagen fibrils, and these molecules play an important role in regulating the diameter and interactions between the fibrils.

Assembly of stable human type I and III collagen molecules from hydroxylated recombinant chains in the yeast Pichia pastoris. Effect of an engineered C-terminal oligomerization domain foldon

The C-propeptides of the pro alpha chains of type I and type III procollagens are believed to be essential for correct chain recognition and chain assembly in these molecules. We studied here whether the 30-kDa C-propeptides of the human pC alpha 1(I), pC alpha 2(I), and pC alpha 1(III) chains, i.e. pro alpha chains lacking their N-propeptides, can be replaced by foldon, a 29-amino acid sequence normally located at the C terminus of the polypeptide chains in the bacteriophage T4 fibritin. The alpha foldon chains were expressed in Pichia pastoris cells that also expressed the two types of subunit of human prolyl 4-hydroxylase; the foldon domain was subsequently removed by pepsin treatment, which also digests non-triple helical collagen chains, whereas triple helical collagen molecules are resistant to it. The foldon domain was found to be very effective in chain assembly, as expression of the alpha 1(I)foldon or alpha 1(III)foldon chains gave about 2.5-3-fold the amount of pepsin-resistant type I or type III collagen homotrimers relative to those obtained using the authentic C-propeptides. In contrast, expression of chains with no oligomerization domain led to very low levels of pepsin-resistant molecules. Expression of alpha 2(I)foldon chains gave no pepsin-resistant molecules at all, indicating that in addition to control at the level of the C-propeptide other restrictions at the level of the collagen domain exist that prevent the formation of stable [alpha 2(I)]3 molecules. Co-expression of alpha 1(I)foldon and alpha 2(I)foldon chains led to an efficient assembly of heterotrimeric molecules, their amounts being about 2-fold those obtained with the authentic C-propeptides and the alpha 1(I) to alpha 2(I) ratio being 1.91 +/- 0.31 (S.D.). As the foldon sequence contains no information for chain recognition, our data indicate that chain assembly is influenced not only by the C-terminal oligomerization domain but also by determinants present in the alpha chain domains.

NMR and CD spectroscopy show that imino acid restriction of the unfolded state leads to efficient folding

Protein folding is determined by molecular features in the unfolded state, as well as the native folded structure. In the unfolded state, imino acids both restrict conformational space and present cis-trans isomerization barriers to folding. Because of its high proline and hydroxyproline content, the collagen triple-helix offers an opportunity to characterize the impact of imino acids on the unfolded state and folding kinetics. Here, NMR and CD spectroscopy are used to characterize the role of imino acids in a triple-helical peptide, T1-892, which contains an 18-residue sequence from type I collagen and a C-terminal (Gly-Pro-Hyp)(4) domain. The replacement of Pro or Hyp by an Ala in the (Gly-Pro-Hyp)(4) region significantly decreases the folding rate at low but not high concentrations, consistent with less efficient nucleation. To understand the molecular basis of the decreased folding rate, changes in the unfolded as well as the folded states of the peptides were characterized. While the trimer states of the peptides are all similar, NMR dynamics studies show monomers with all trans (Gly-Pro-Hyp)(4) are less flexible than monomers containing Pro --> Ala or Hyp --> Ala substitutions. Nucleation requires all trans bonds in the (Gly-Pro-Hyp)(4) domain and the constrained monomer state of the all trans nucleation domain in T1-892 increases its competency to initiate triple-helix formation and illustrates the impact of the unfolded state on folding kinetics.

Identification, characterization and expression analysis of a new fibrillar collagen gene, COL27A1

The fibrillar collagens provide structural scaffolding and strength to the extracellular matrices of connective tissues. We identified a partial sequence of a new fibrillar collagen gene in the NCBI databases and completed the sequence with bioinformatic approaches and 5' RACE. This gene, designated COL27A1, is approximately 156 kbp long and has 61 exons located on chromosome 9q32-33. The homologous mouse gene is located on chromosome 4. The gene encodes amino- and carboxyl-terminal propeptides similar to those in the 'minor' fibrillar collagens. The triple-helical domain is, however, shorter and contains 994 amino acids with two imperfections of the Gly-Xaa-Yaa repeat pattern. There were three sites of alternative RNA splicing, only one of which led to the intact mRNA that encodes this full-length collagen proalpha chain. Phylogenetic analyses indicated that COL27A1 forms a clade with COL24A1 that is distinct from the two known lineages of fibrillar collagens. Expression analyses of the mouse col27a1 gene demonstrated high expression in cartilage, the eye and ear, but also in lung and colon. It is likely that the major protein product of COL27A1, proalpha1(XXVII), is a component of the extracellular matrices of cartilage and these other tissues. Study of this collagen should yield insights into normal chondrogenesis, and provide clues to the pathogenesis of some chondrodysplasias and disorders of other tissues in which this gene is expressed.

Transgenic silkworms produce recombinant human type III procollagen in cocoons

We describe the generation of transgenic silkworms that produce cocoons containing recombinant human collagen. A fusion cDNA was constructed encoding a protein that incorporated a human type III procollagen mini-chain with C-propeptide deleted, a fibroin light chain (L-chain), and an enhanced green fluorescent protein (EGFP). This cDNA was ligated downstream of the fibroin L-chain promoter and inserted into a piggyBac vector. Silkworm eggs were injected with the vectors, producing worms displaying EGFP fluorescence in their silk glands. The cocoons emitted EGFP fluorescence, indicating that the promoter and fibroin L-chain cDNAs directed the synthesized products to be secreted into cocoons. The presence of fusion proteins in cocoons was demonstrated by immunoblotting, collagenase-sensitivity tests, and amino acid sequencing. The fusion proteins from cocoons were purified to a single electrophoretic band. This study demonstrates the viability of transgenic silkworms as a tool for producing useful proteins in bulk.

Sequence-specific recognition of collagen triple helices by the collagen-specific molecular chaperone HSP47

HSP47 is a molecular chaperone that plays an unknown role during the assembly and transport of procollagen. Our previous studies showed that, unlike most chaperones, HSP47 interacts with a correctly folded substrate. We suggested that HSP47 either stabilizes the correctly folded collagen helix from heat denaturation or prevents lateral aggregation prior to its transport from the endoplasmic reticulum. In this study we have addressed the role of triple helix stability in the binding of HSP47 to procollagen by expressing procollagen molecules with differing thermal stabilities and analyzing their ability to interact with HSP47 within the endoplasmic reticulum. Our results show that HSP47 interacts with thermostable procollagen molecules, suggesting that helix stabilization is not the primary function of HSP47 and that the interaction of HSP47 with procollagen depends upon the presence of a minimum of one Gly-X-Arg triplet within the triple helical domain. Interestingly, procollagen chains containing high proportions of stabilizing triplets formed triple helices and interacted with HSP47 even in the absence of proline hydroxylation, demonstrating that recognition does not depend upon this modification. Our results support the view that HSP47 functions early in the secretory pathway by preventing the lateral aggregation of procollagen chains.

Characterization of the nucleation step and folding of a collagen triple-helix peptide

Peptide T1-892 is a triple-helical peptide designed to include two distinct domains: a C-terminal (Gly-Pro-Hyp)(4) sequence, together with an N-terminal 18-residue sequence from the alpha1(I) chain of type I collagen. Folding experiments of T1-892 using CD spectroscopy were carried out at varying concentrations and temperatures, and fitting of kinetic models to the data was used to obtain information about the folding mechanism and to derive rate constants. Proposed models include a heterogeneous population of monomers with respect to cis-trans isomerization and a third-order folding reaction from competent monomer to the triple helix. Fitting results support a nucleation domain composed of all or most of the (Gly-Pro-Hyp)(4) sequence, which must be in trans form before the monomer is competent to initiate triple-helix formation. The folding of competent monomer to a triple helix is best described by an all-or-none third-order reaction. The temperature dependence of the third-order rate constant indicates a negative activation energy and provides information about the thermodynamics of the trimerization step. These CD studies complement NMR studies carried out on the same peptide at high concentrations, illustrating how the rate-limiting folding step is affected by changes in concentration. This sequence preference of repeating Gly-Pro-Hyp units for the initiation of triple-helix formation in peptide T1-892 may be related to features in the triple-helix folding of collagens.

A single amino acid substitution (D1441Y) in the carboxyl-terminal propeptide of the proalpha1(I) chain of type I collagen results in a lethal variant of osteogenesis imperfecta with features of dense bone diseases

Osteogenesis imperfecta (OI) is characterised by brittle bones and caused by mutations in the type I collagen genes, COL1A1 and COL1A2. We identified a mutation in the carboxyl-terminal propeptide coding region of one COL1A1 allele in an infant who died with an OI phenotype that differed from the usual lethal form and had regions of increased bone density. The newborn female had dysmorphic facial features, including loss of mandibular angle. Bilateral upper and lower limb contractures were present with multiple fractures in the long bones and ribs. The long bones were not compressed and their ends were radiographically dense. She died after a few hours and histopathological studies identified extramedullary haematopoiesis in the liver, little lamellar bone formation, decreased osteoclasts, abnormally thickened bony trabeculae with retained cartilage in long bones, and diminished marrow spaces similar to those seen in dense bone diseases such as osteopetrosis and pycnodysostosis. The child was heterozygous for a COL1A1 4321G-->T transversion in exon 52 that changed a conserved aspartic acid to tyrosine (D1441Y). Abnormal proalpha1(I) chains were slow to assemble into dimers and trimers, and abnormal molecules were retained intracellularly for an extended period. The secreted type I procollagen molecules synthesised by cultured dermal fibroblasts were overmodified along the full length but had normal thermal stability. These findings suggest that the unusual phenotype reflected both a diminished amount of secreted type I procollagen and the presence of a population of stable and overmodified molecules that might support increased mineralisation or interfere with degradation of bone.

The molecular interactions of heat shock protein 47 (Hsp47) and their implications for collagen biosynthesis

Heat shock protein 47 (Hsp47) is a procollagen/collagen-specific molecular chaperone protein derived from the serpin family of proteins and essential for the early stages of collagen biosynthesis. In this paper, the results of an extensive biophysical analysis of mature recombinant mouse Hsp47 show the existence of both a structurally mesostable monomer with a 5-strand A-sheet and/or a hyperstable trimer; both states have biological activity. It is also demonstrated that Hsp47 is able to bind to a monomeric and partially folded conformation collagen mimic peptide (PPG)(10). Upon peptide binding Hsp47 has the capacity to induce the peptide backbone to fold into a polyproline type II conformation. Induction of this conformation results in (PPG)(10) peptides associating into higher order assemblies with increased stability compared with the monomeric peptide alone. These assemblies are similar to those observed by others (Go, N., and Suezaki, Y. (1973) Biopolymers 12, 1927-1930; Engel, J., Chen, H. T., Prockop, D. J., and Klump, H. (1977) Biopolymers 16, 601-622) when the peptide is dissolved at high concentration and are proposed to be long chains of peptides in a collagen-like configuration. Examination of the biophysical characteristics of both monomeric and trimeric Hsp47-(PPG)(10) complexes is also used to determine that the peptide-binding site does not reside in strand 4 position of sheet A, as observed for other serpins (Skinner, R., Chang, W. S. W., Jin, L., Pei, X., Huntington, J. A., Abrahams, J. P., Carrell, R. W., and Lomas, D. A. (1998) J. Mol. Biol. 283, 9-14), leaving earlier proposals of a binding site near helix A viable.

Deletions and duplications of Gly-Xaa-Yaa triplet repeats in the triple helical domains of type I collagen chains disrupt helix formation and result in several types of osteogenesis imperfecta

Triple helix formation is a prerequisite for the passage of type I procollagen from the endoplasmic reticulum and secretion from the cell to form extracellular fibrils that will support mineral deposition in bone. Analysis of cDNA from 11 unrelated individuals with osteogenesis imperfecta (OI) revealed the presence of 11 novel, short in-frame deletions or duplications of three, nine, or 18 nucleotides in the helical coding regions of the COL1A1 and COL1A2 collagen genes. Triple helix formation was impaired, type I collagen alpha chains were post-translationally overmodified, and extracellular secretion was markedly reduced. With one exception, the obligate Gly-Xaa-Yaa repeat pattern of amino acids in the helical domains was not altered, but the Xaa- and Yaa position residues were out of register relative to the amino acid sequences of adjacent chains in the triple helix. Thus, the identity of these amino acids, in addition to third position glycines, is important for normal helix formation. These findings expand the known repertoire of uncommon in-frame deletions and duplications in OI, and provide insight into normal collagen biosynthesis and collagen triple helix formation.

Inhibitors of procollagen C-terminal proteinase block gastrulation and spicule elongation in the sea urchin embryo

In the sea urchin embryo, inhibition of collagen processing and deposition affects both gastrulation and embryonic skeleton (spicule) formation. It has been found that cell-free extracts of gastrula-stage embryos of Strongylocentrotus purpuratus contain a procollagen C-terminal proteinase (PCP) activity. A rationally designed non-peptidic organic hydroxamate, which is a potent and specific inhibitor of human recombinant PCP (FG-HL1), inhibited both the sea urchin PCP as well as purified chick embryo tendon PCP. In the sea urchin embryo, FG-HL1 inhibited gastrulation and blocked spicule elongation, but not spicule nucleation. A related compound with a terminal carboxylate rather than a hydroxamate (FG-HL2) did not inhibit either chick PCP or sea urchin PCP activity in a procollagen-cleavage assay. However, FG-HL2 did block spicule elongation without affecting spicule nucleation or gastrulation. Neither compound was toxic, because their effects were reversible on removal. It was shown that the inhibition of gastrulation and spicule elongation were independent of tissue specification events, because both the endoderm specific marker Endo1 and the primary mesenchyme cell specific marker SM50 were expressed in embryos treated with FG-HL1 and FG-HL2. These results suggest that disruption of the fibrillar collagen deposition in the blastocoele blocks the cell movements of gastrulation and may disrupt the positional information contained within the extracellular matrix, which is necessary for spicule formation.

Collagenous Sequence Governs the Trimeric Assembly of Collagen XII

A minicollagen containing the COL1 and NC1 domains of chicken collagen XII has been produced in insect cells. Significant amounts of trimers contain a triple-helical domain in which the cysteines are not involved in inter- but in intrachain bonds. In reducing conditions, providing that the triple-helix is maintained, disulfide exchange between intra- and interchain bonding is observed, suggesting that the triple-helix forms first and that in favorable redox conditions interchain bonding occurs to stabilize the molecule. This hypothesis is verified by in vitro reassociation studies performed in the presence of reducing agents, demonstrating that the formation of interchain disulfide bonds is not a prerequisite to the trimeric association and triple-helical folding of the collagen XII molecule. Shortening the COL1 domain of minicollagen XII to its five C-terminal GXY triplets results in an absence of trimers. This can be explained by the presence of a collagenous domain that is too short to form a stable triple-helix. In contrast, the presence of five additional C-terminal triplets in COL1 allows the formation of triple-helical disulfide-bonded trimers, suggesting that the presence of a triple-helix is essential for the assembly of collagen XII.

Disruption of one intra-chain disulphide bond in the carboxyl-terminal propeptide of the proalpha1(I) chain of type I procollagen permits slow assembly and secretion of overmodified, but stable procollagen trimers and results in mild osteogenesis imperfecta

Type I procollagen is a heterotrimer comprised of two proalpha1(I) chains and one proalpha2(I) chain. Chain recognition, association, and alignment of proalpha chains into correct registration are thought to occur through interactions between the C-terminal propeptide domains of the three chains. The C-propeptide of each chain contains a series of cysteine residues (eight in proalpha1(I) and seven in proalpha2(I)), the last four of which form intra-chain disulphide bonds. The remaining cysteine residues participate in inter-chain stabilisation. Because these residues are conserved, they are thought to be important for folding and assembly of procollagen. We identified a mutation (3897C-->G) that substituted tryptophan for the cysteine at position 1299 in proalpha1(I) (C1299W, the first cysteine that participates in intra-chain bonds) and resulted in mild osteogenesis imperfecta. The patient was born with a fractured clavicle and four rib fractures. By 18 months of age he had had no other fractures and was on the 50th centile for length and weight. The proband's mother, maternal aunt, and grandfather had the same mutation and had few fractures, white sclerae, and discoloured teeth, but their heights were within the normal range. In the patient's cells the defective chains remained as monomers for over 80 minutes (about four times normal) and were overmodified. Some secreted procollagens were also overmodified but had normal thermal stability, consistent with delayed, but normal helix formation. This intra-chain bond may stabilise the C-propeptide and promote rapid chain association. Other regions of the C-propeptide thus play more prominent roles in chain registration and triple helix nucleation.

Hsp47: a molecular chaperone that interacts with and stabilizes correctly-folded procollagen

Hsp47 is a heat-shock protein that interacts transiently with procollagen during its folding, assembly and transport from the endoplasmic reticulum (ER) of mammalian cells. It has been suggested to carry out a diverse range of functions, such as acting as a molecular chaperone facilitating the folding and assembly of procollagen molecules, retaining unfolded molecules within the ER, and assisting the transport of correctly folded molecules from the ER to the Golgi apparatus. Here we define the substrate recognition of Hsp47, demonstrating that it interacts preferentially with triple-helical procollagen molecules. The association of Hsp47 with procollagen coincides with the formation of a collagen triple helix. This demonstrates that Hsp47's role in procollagen folding and assembly is distinct from that of prolyl 4-hydroxylase. These results indicate that Hsp47 acts as a novel molecular chaperone, potentially stabilizing the correctly folded collagen helix from heat denaturation before its transport from the ER.

Site-specific NMR monitoring of cis-trans isomerization in the folding of the proline-rich collagen triple helix

Understanding the folding of the proline-rich collagen triple helix requires consideration of the effects of proline cis-trans isomerization and may shed light on the misfolding of collagen in connective tissue diseases. Folding was monitored in real time by heteronuclear 2D NMR spectroscopy for the (15)N labeled positions in the triple-helical peptide T1-892 [GPAGPAGPVGPAGARGPAGPOGPOGPOGPOGV]. In the equilibrium unfolded monomer form, each labeled residue showed multiple peaks with interconversion rates consistent with cis-trans isomerization of Gly-Pro and Pro-Hyp bonds. Real-time NMR studies on the folding of T1-892 showed slow decay of monomer peaks and a concomitant increase in trimer peaks. Gly25 in the C-terminal rich (Gly-Pro-Hyp)(4) domain folds first, consistent with its being a nucleation domain. Analysis of the kinetics indicates that the folding of Gly25 is biphasic and the slower step represents cis-trans isomerization of imino acids. This illustrates that nucleation is limited by cis-trans isomerization. Monitoring Gly6, Gly10, Ala12, and Gly13 monomer and trimer peaks captures the C- to N-terminal propagation of the triple helix, which is also limited by Gly-Pro cis-trans isomerization events. The zipper-like nature of the propagation process is confirmed by the slower rate of folding of Ala6 compared to Gly13, reflecting the larger number of isomerization events encountered by the more N-terminal Ala6. The cis-trans isomerization events at multiple proline residues is a complex statistical process which can be visualized by these NMR studies.

Type XIII collagen forms homotrimers with three triple helical collagenous domains and its association into disulfide-bonded trimers is enhanced by prolyl 4-hydroxylase

Type XIII collagen is a type II transmembrane protein predicted to consist of a short cytosolic domain, a single transmembrane domain, and three collagenous domains flanked by noncollagenous sequences. Previous studies on mRNAs indicate that the structures of the collagenous domain closest to the cell membrane, COL1, the adjacent noncollagenous domain, NC2, and the C-terminal domains COL3 and NC4 are subject to alternative splicing. In order to extend studies of type XIII collagen from cDNAs to the protein level we have produced it in insect cells by means of baculoviruses. Type XIII collagen alpha chains were found to associate into disulfide-bonded trimers, and hydroxylation of proline residues dramatically enhanced this association. This protein contains altogether eight cysteine residues, and interchain disulfide bonds could be located in the NC1 domain and possibly at the junction of COL1 and NC2, while the two cysteine residues in NC4 are likely to form intrachain bonds. Pepsin and trypsin/chymotrypsin digestions indicated that the type XIII collagen alpha chains form homotrimers whose three collagenous domains are in triple helical conformation. The thermal stabilities (T(m)) of the COL1, COL2, and COL3 domains were 38, 49 and 40 degrees C, respectively. The T(m) of the central collagenous domain is unusually high, which in the light of this domain being invariant in terms of alternative splicing suggests that the central portion of the molecule may have an important role in the stability of the molecule. All in all, most of the type XIII collagen ectodomain appears to be present in triple helical conformation, which is in clear contrast to the short or highly interrupted triple helical domains of the other known collagenous transmembrane proteins.

Triple helix assembly and processing of human collagen produced in transgenic tobacco plants

The use of tobacco plants as a novel expression system for the production of human homotrimeric collagen I is presented in this report. Constructs were engineered from cDNA encoding the human proalpha1(I) chain to generate transgenic tobacco plants expressing collagen I. The recombinant proalpha1(I) chains were expressed as disulfide-bonded trimers and were shown to fold into a stable homotrimeric triple helix. Moreover, the recombinant procollagen was subsequently processed to collagen as it occurs in animals. Large amounts of recombinant collagen were purified from field grown plant material. The data suggest that plants are a valuable alternative for the recombinant production of collagen for various medical and scientific purposes.

Accumulation of properly folded human type III procollagen molecules in specific intracellular membranous compartments in the yeast Pichia pastoris

It was recently reported that co-expression of the proalpha1(III) chain of human type III procollagen with the subunits of human prolyl 4-hydroxylase in Pichia pastoris produces fully hydroxylated and properly folded recombinant type III procollagen molecules (Vuorela, A., Myllyharju, J., Nissi, R., Pihlajaniemi, T., Kivirikko, K.I., 1997. Assembly of human prolyl 4-hydroxylase and type III collagen in the yeast Pichia pastoris: formation of a stable enzyme tetramer requires coexpression with collagen and assembly of a stable collagen requires coexpression with prolyl 4-hydroxylase. EMBO J. 16, 6702-6712). These properly folded molecules accumulated inside the yeast cell, however, only approximately 10% were found in the culture medium. We report here that replacement of the authentic signal sequence of the human proalpha1(III) with the Saccharomyces cerevisiae alpha mating factor prepro sequence led only to a minor increase in the amount secreted. Immunoelectron microscopy studies indicated that the procollagen molecules accumulate in specific membranous vesicular compartments that are closely associated with the nuclear membrane. Prolyl 4-hydroxylase, an endoplasmic reticulum (ER) lumenal enzyme, was found to be located in the same compartments. Non-helical proalpha1(III) chains produced by expression without recombinant prolyl 4-hydroxylase likewise accumulated within these compartments. The data indicate that properly folded recombinant procollagen molecules accumulate within the ER and do not proceed further in the secretory pathway. This may be related to the large size of the procollagen molecule.

Intracellular retention of procollagen within the endoplasmic reticulum is mediated by prolyl 4-hydroxylase

The correct folding and assembly of proteins within the endoplasmic reticulum (ER) are prerequisites for subsequent transport from this organelle to the Golgi apparatus. The mechanisms underlying the ability of the cell to recognize and retain unassembled or malfolded proteins generally require binding to molecular chaperones within the ER. One classic example of this process occurs during the biosynthesis of procollagen. Here partially folded intermediates are retained and prevented from secretion, leading to a build up of unfolded chains within the cell. The accumulation of these partially folded intermediates occurs during vitamin C deficiency due to incomplete proline hydroxylation, as vitamin C is an essential co-factor of the enzyme prolyl 4-hydroxylase. In this report we show that this retention is tightly regulated with little or no secretion occurring under conditions preventing proline hydroxylation. We studied the molecular mechanism underlying retention by determining which proteins associate with partially folded procollagen intermediates within the ER. By using a combination of cross-linking and sucrose gradient analysis, we show that the major protein binding to procollagen during its biosynthesis is prolyl 4-hydroxylase, and no binding to other ER resident proteins including Hsp47 was detected. This binding is regulated by the folding status rather than the extent of hydroxylation of the chains demonstrating that this enzyme can recognize and retain unfolded procollagen chains and can release these chains for further transport once they have folded correctly.

Assembly of the type 1 procollagen molecule: selectivity of the interactions between the alpha 1(I)- and alpha 2(I)-carboxyl propeptides

Assembly of the heterotrimeric procollagen I molecule is initiated by interactions between the carboxyl propeptide domains of the completed nascent pro alpha chains. The [pro alpha 1(I)]2[pro alpha 2(I)] heterotrimer is the predominant molecule, with much smaller amounts of stable [pro alpha 1(I)]3 homotrimer also being formed. However, the [pro alpha 2(1)]3 homotrimer has not been detected, raising questions as to the mechanism of chain assembly and why [pro alpha2(1)]3 homotrimers are not formed. These questions have been examined here by expressing the intact and amino- or carboxyl-terminal truncated C-propeptides of the pro alpha chains recombinantly in bacteria and in a coupled transcription/translation reticulocyte lysate system. Their interactions were studied in vitro by binding analyses and in vivo by using the yeast two-hybrid system. The C-pro alpha 1(I) interacted with itself, and with C-pro alpha 2(I), as expected. Surprisingly, the C-pro alpha 2(I) also interacted with itself, both in vitro and in vivo. While the interaction of C-pro alpha 2(I) with itself and C-pro alpha 1(I) in vitro was strong, these interactions were weaker in vivo. Deletion of 36 amino acids from the C-terminal domain of C-pro alpha 1 had no effect on its binding to intact self or intact C-pro alpha 2, but the same deletion in C-pro alpha 2 completely abolished its binding to intact C-pro alpha 2 and to C-pro alpha 1. Comparable N-terminal deletions in C-pro alpha 1 or C-pro alpha 2 diminished, but did not abolish, their binding to intact C-pro alpha 1 and C-pro alpha 2. In the yeast two-hybrid system, C-pro alpha 2 interacted with itself more weakly than with C-pro alpha 1. Molecular modeling and circular dichroism analyses showed that C-pro alpha 1 and C-pro alpha 2 have different folded structures and stability. Studies with antibodies specific to the C-pro alpha1 and alpha2 peptides showed them to precipitate different, specific, and distinct cell proteins from fibroblast lysates. The C-pro alpha 2(I) antibody complexed with more cell proteins. We hypothesize that the lack of pro alpha 2(I) homotrimers is not due to the inability of the C-pro alpha 2(I) to interact with itself, but rather to the competing presence of other cell proteins. The specificity of these interactions may reside in conformational differences in N- and C-terminal sequences of the two propeptides or in their different folding patterns.

Expression of an engineered form of recombinant procollagen in mouse milk

We have examined the suitability of the mouse mammary gland for expression of novel recombinant procollagens that can be used for biomedical applications. We generated transgenic mouse lines containing cDNA constructs encoding recombinant procollagen, along with the alpha and beta subunits of prolyl 4-hydroxylase, an enzyme that modifies the collagen into a form that is stable at body temperature. The lines expressed relatively high levels (50-200 micrograms/ml) of recombinant procollagen in milk. As engineered, the recombinant procollagen was shortened and consisted of a pro alpha 2(I) chain capable of forming a triple-helical homotrimer not normally found in nature. Analysis of the product demonstrated that (1) the pro alpha chains formed disulphide-linked trimers, (2) the trimers contained a thermostable triple-helical domain, (3) the N-propeptides were aligned correctly, and (4) the expressed procollagen was not proteolytically processed to collagen in milk.

Folding and assembly of type X collagen mutants that cause metaphyseal chondrodysplasia-type schmid. Evidence for co-assembly of the mutant and wild-type chains and binding to molecular chaperones

Schmid metaphyseal chondrodysplasia results from mutations within the COOH-terminal globular domain (NC1) of type X collagen, a short chain collagen expressed in the hypertrophic region of the growth plate cartilage. Previous in vitro studies have proposed that mutations prevent the association of the NC1 domain of constituent chains of the trimer based upon a lack of formation of a trimeric structure that is resistant to dissociation with sodium dodecyl sulfate. To examine the effect of mutations on folding and assembly within a cellular context, bovine type X cDNAs containing analogous disease causing mutations Y598D, N617K, W651R, and wild-type were expressed in semi-permeabilized cells. We assessed trimerization of the mutant chains by their ability to form a collagen triple helix. Using this approach, we demonstrate that although there is an apparent lower efficiency of association of the mutant NC1 domains, they can drive the formation of correctly aligned triple helices with the same thermal stability as the wild-type collagen. When epitope-tagged mutant and wild-type collagen were co-expressed, heterotrimers could be detected by sequential immunoprecipitation. Both wild-type and mutant type X chains were found in association with the molecular chaperones protein disulfide isomerase and Hsp 47. The implications of these findings on the likely mechanism of Schmid metaphyseal chondrodysplasia will be discussed.

Epitope-specific monoclonal antibodies against human C-terminal procollagen alpha1(III)-propeptide

We have generated monoclonal antibodies against recombinant C-terminal human procollagen alpha1(III) propeptide (PIIICP), produced in E. coli in high yields. The monoclonal antibodies were screened for epitope specificity using recombinant truncated PIIICP. Several antibodies were identified which recognized different regions of the PIIICP molecule. The ability of the antibodies to detect PIIICP antigens in human cell line lysates and supernatants was demonstrated. As PIIICP antigens are a key marker of extracellular matrix metabolism, the monoclonal antibodies described here should be of value for clinical and basic research.