Deletion of Epidermal Growth Factor-like Domains Converts Mammalian Tolloid into a Chordinase and Effective Procollagen C-proteinase

Structural and evolutionary insights into astacin metallopeptidases

The astacins are a family of metallopeptidases (MPs) that has been extensively described from animals. They are multidomain extracellular proteins, which have a conserved core architecture encompassing a signal peptide for secretion, a prodomain or prosegment and a zinc-dependent catalytic domain (CD). This constellation is found in the archetypal name-giving digestive enzyme astacin from the European crayfish Astacus astacus. Astacin catalytic domains span ∼200 residues and consist of two subdomains that flank an extended active-site cleft. They share several structural elements including a long zinc-binding consensus sequence (HEXXHXXGXXH) immediately followed by an EXXRXDRD motif, which features a family-specific glutamate. In addition, a downstream SIMHY-motif encompasses a "Met-turn" methionine and a zinc-binding tyrosine. The overall architecture and some structural features of astacin catalytic domains match those of other more distantly related MPs, which together constitute the metzincin clan of metallopeptidases. We further analysed the structures of PRO-, MAM, TRAF, CUB and EGF-like domains, and described their essential molecular determinants. In addition, we investigated the distribution of astacins across kingdoms and their phylogenetic origin. Through extensive sequence searches we found astacin CDs in > 25,000 sequences down the tree of life from humans beyond Metazoa, including Choanoflagellata, Filasterea and Ichtyosporea. We also found < 400 sequences scattered across non-holozoan eukaryotes including some fungi and one virus, as well as in selected taxa of archaea and bacteria that are pathogens or colonizers of animal hosts, but not in plants. Overall, we propose that astacins originate in the root of Holozoa consistent with Darwinian descent and that the latter genes might be the result of horizontal gene transfer from holozoan donors.

Extracellular BMP1 is the major proteinase for COOH-terminal proteolysis of type I procollagen in lung fibroblasts

Proteolytic processing of procollagens is a central step during collagen fibril formation. Bone morphogenic protein 1 (BMP1) is a metalloprotease that plays an important role in the cleavage of carboxy-terminal (COOH-terminal) propeptides from procollagens. Although the removal of propeptides is required to generate mature collagen fibrils, the contribution of BMP1 to this proteolytic process and its action site remain to be fully determined. In this study, using postnatal lung fibroblasts as a model system, we showed that genetic ablation of Bmp1 in primary murine lung fibroblasts abrogated COOH-terminal cleavage from type I procollagen as measured by COOH-terminal propeptide of type I procollagen (CICP) production. We also showed that inhibition of BMP1 by siRNA-mediated knockdown or small-molecule inhibitor reduced the vast majority of CICP production and collagen deposition in primary human lung fibroblasts. Furthermore, we discovered and characterized two antibody inhibitors for BMP1. In both postnatal lung fibroblast and organoid cultures, BMP1 blockade prevented CICP production. Together, these findings reveal a nonredundant role of extracellular BMP1 to process CICP in lung fibroblasts and suggest that development of antibody inhibitors is a viable pharmacological approach to target BMP1 proteinase activity in fibrotic diseases.

Circulating autoantibodies against neuroblastoma suppressor of tumorigenicity 1 (NBL1): A potential biomarker for coronary artery disease in patients with obstructive sleep apnea

Objective

Although severe obstructive sleep apnea (OSA) is an important risk factor for atherosclerosis-related diseases including coronary artery disease (CAD), there is no reliable biomarker of CAD risks in patients with OSA. This study aimed to test our hypothesis that circulating autoantibodies against neuroblastoma suppressor of tumorigenicity 1 (NBL1-Abs) are associated with the prevalence of CAD in patients with OSA.

Methods

Eighty-two adults diagnosed with OSA by polysomnography, 96 patients with a diagnosis of acute coronary syndrome (ACS) and 64 healthy volunteers (HVs) were consecutively enrolled. Serum samples were collected from patients with OSA at diagnostic polysomnography and from patients with ACS at disease onset. Serum NBL1-Ab level was measured by amplified luminescence proximity homogeneous assay and its association with clinical variables related to atherosclerosis was evaluated.

Results

NBL1-Ab level was significantly elevated in patients with both OSA and ACS compared with HVs. Subgroup analyses showed that NBL1-Ab level was markedly higher in patients with severe OSA and OSA patients with a history of CAD. Weak associations were observed between NBL1-Ab level and apnea-hypopnea index, age, mean SpO₂ and arousal index, whereas significantly higher NBL1-Ab levels were observed in OSA patients with a history of CAD than in those without a history of CAD. Sensitivity analysis using a logistic regression model also demonstrated that increased NBL1-Ab levels were associated with the previous history of CAD in patients with OSA.

Conclusions

Elevated NBL1-Ab levels may be associated with the prevalence of CAD in patients with OSA, which needs to be confirmed further.

Fell Muir Lecture: Collagen fibril formation in vitro and in vivo

It is a great honour to be awarded the Fell Muir Prize for 2016 by the British Society of Matrix Biology. As recipient of the prize, I am taking the opportunity to write a minireview on collagen fibrillogenesis, which has been the focus of my research for 33 years. This is the process by which triple helical collagen molecules assemble into centimetre-long fibrils in the extracellular matrix of animals. The fibrils appeared a billion years ago at the dawn of multicellular animal life as the primary scaffold for tissue morphogenesis. The fibrils occur in exquisite three-dimensional architectures that match the physical demands of tissues, for example orthogonal lattices in cornea, basket weaves in skin and blood vessels, and parallel bundles in tendon, ligament and nerves. The question of how collagen fibrils are formed was posed at the end of the nineteenth century. Since then, we have learned about the structure of DNA and the peptide bond, understood how plants capture the sun's energy, cloned animals, discovered antibiotics and found ways of editing our genome in the pursuit of new cures for diseases. However, how cells generate tissues from collagen fibrils remains one of the big unsolved mysteries in biology. In this review, I will give a personal account of the topic and highlight some of the approaches that my research group are taking to find new insights.

Diversity between mammalian tolloid proteinases: Oligomerisation and non-catalytic domains influence activity and specificity

The mammalian tolloid family of metalloproteinases is essential for tissue patterning and extracellular matrix assembly. The four members of the family: bone morphogenetic protein-1 (BMP-1), mammalian tolloid (mTLD), tolloid-like (TLL)-1 and TLL-2 differ in their substrate specificity and activity levels, despite sharing similar domain organization. We have previously described a model of substrate exclusion by dimerisation to explain differences in the activities of monomeric BMP-1 and dimers of mTLD and TLL-1. Here we show that TLL-2, the least active member of the tolloid family, is predominantly monomeric in solution, therefore it appears unlikely that substrate exclusion via dimerisation is a mechanism for regulating TLL-2 activity. X-ray scattering and electron microscopy structural and biophysical analyses reveal an elongated shape for the monomer and flexibility in the absence of calcium. Furthermore, we show that TLL-2 can cleave chordin in vitro, similar to other mammalian tolloids, but truncated forms of TLL-2 mimicking BMP-1 are unable to cleave chordin. However, both the N- and C-terminal non-catalytic domains from all mammalian tolloids bind chordin with high affinity. The mechanisms underlying substrate specificity and activity in the tolloid family are complex with variation between family members and depend on both multimerisation and substrate interaction.

Metalloproteinases in Drosophila to humans that are central players in developmental processes

Many secreted proteins are synthesized as precursors with propeptides that must be cleaved to yield the mature functional form of the molecule. In addition, various growth factors occur in extracellular latent complexes with protein antagonists and are activated upon cleavage of such antagonists. Research in the separate fields of embryonic patterning and extracellular matrix formation has identified members of the BMP1/Tolloid-like family of metalloproteinases as key players in these types of biosynthetic processing events in species ranging from Drosophila to humans.

Strongyloides stercoralis excretory/secretory protein strongylastacin specifically recognized by IgE antibodies in infected human sera

The infective, microscopic Strongyloides stercoralis larvae in contaminated soil can penetrate human skin with the help of excretory/secretory proteases. These proteases play a critical role in infection and transmigration of the parasite to the intestines. Strongylastacin is similar to astacin (from the digestive gland of the crayfish Astacus astacus), a multi-domain protein with a signal peptide, a pro-enzyme, a catalytic domain containing the zinc binding consensus astacin family signature sequence HEXXHXXGFXHEXXRXDR, and a second conserved zinc binding motif SIMHY at N- terminal region. An EGF-1 like domain and a CUB domain are located at the COOH- terminal. In this study, the excretory/secretory Strongylastacin gene from S. stercoralis infective larval stage was cloned and expressed as a 45 kDa in Escherichia coli. Immunoblot analysis showed the presence of natural IgG antibodies against strongylastacin in six infected and six non-endemic normal sera. These findings were confirmed in an ELISA of 32 S. stercoralis infected and 32 presumed normal human sera; all contained natural anti-strongylastacin IgG antibodies. By contrast, IgE antibodies specific to strongylastacin were present in sera from individuals infected with S. stercoralis but not in uninfected control sera. Moreover, recombinant strongylastacin did not cross-react with IgE antibodies either from patients infected with filaria or patients with tropical pulmonary eosinophilic (TPE) who had increased IgE antibodies. The present authors conclude that strongylastacin, an excretory/secretory antigen, elicits specific IgE antibodies in S. stercoralis infected humans. Non-specific IgG antibodies to strongylastacin are present in both infected and normal humans. Further investigation is needed to understand the role of the host protective response against strongylastacin.

First evidence of bone morphogenetic protein 1 expression and activity in sheep ovarian follicles

Bone morphogenetic protein (BMP) 1 is a vertebrate metalloproteinase of the astacin family. BMP1 plays a key role in regulating the formation of the extracellular matrix (ECM), particularly by processing the C-propeptide of fibrillar procollagens. BMP1 also promotes BMP signaling by releasing BMP signaling molecules from complexes with the BMP-antagonist chordin. As a result of BMP1's dual role in both ECM formation and BMP signaling, we hypothesized that BMP1 could play a role in ovarian physiology. Using the sheep ovary as a model system, we showed that BMP1 was expressed in the ovary throughout early fetal stages to adulthood. Furthermore, in adult ovaries, BMP1 was expressed along with chordin, BMP4, and twisted gastrulation, which together form an extracellular regulatory complex for BMP signaling. Within ovine ovaries, immunohistochemical localization demonstrated that BMP1 was present in granulosa cells at all stages of follicular development, from primordial to large antral follicles, and that the levels of BMP1 were not affected by the final follicle selection mechanism. In cultured granulosa cells, BMP1 expression was not affected by gonadotropins, but BMP4 and activin A had opposing effects on the levels of BMP1 mRNA. BMP1 appeared to be secreted into the follicular fluid of antral follicles, where it is able to exert procollagen C-proteinase and chordinase activities. Interestingly, BMP1 activity in follicular fluid decreased with follicular growth.

Dentin sialophosphoprotein (DSPP) is cleaved into its two natural dentin matrix products by three isoforms of bone morphogenetic protein-1 (BMP1)

The protease that cleaves the most abundant non-collagenous protein of dentin matrix, dentin sialophosphoprotein (DSPP), into its two final dentin matrix products, dentin sialoprotein (DSP) and dentin phosphoprotein (DPP), has not been directly identified. In this study, full-length recombinant mouse DSPP was made for the first time in furin-deficient mammalian LoVo cells and used to test the ability of three different isoforms of one candidate protease, bone morphogenetic protein-1 (BMP1) to cleave DSPP at the appropriate site. Furthermore, two reported enhancers of BMP1/mTLD activity (procollagen C-endopeptidase enhancer-1, PCPE-1, and secreted frizzled-related protein-2, sFRP2) were tested for their abilities to modulate BMP1-mediated processing of both DSPP and another SIBLING family member with a similar cleavage motif, dentin matrix protein-1 (DMP1). Three splice variants of BMP1 (classic BMP1, the full-length mTolloid (mTLD), and the shorter isoform lacking the CUB3 domain, BMP1-5) were all shown to cleave the recombinant DSPP in vitro although mTLD was relatively inefficient at processing both DSPP and DMP1. Mutation of the MQGDD peptide motif to IEGDD completely eliminated the ability of all three recombinant isoforms to process full-length recombinant DSPP in vitro thereby verifying the single predicted cleavage site. Furthermore when human bone marrow stromal cells (which naturally express furin-activated BMP1) were transduced with the adenovirus-encoding either wild-type or mutant DSPP, they were observed to fully cleave wild-type DSPP but failed to process the mutant DSPP(MQDeltaIE) during biogenesis. All three BMP1 isoforms were shown to process type I procollagen as well as DSPP and DMP1 much more efficiently in low-salt buffer (< or = 50 mM NaCl) compared to commonly used normal saline buffers (150 mM NaCl). Neither PCPE-1 nor sFRP2 were able to enhance any of the three BMP1 isoforms in cleaving either DSPP or DMP1 under either low or normal saline conditions. Interestingly, we were unable to reproduce sFRP2's reported ability to enhance the processing of type I procollagen by BMP1/mTLD. In summary, three isoforms of BMP1 process both DSPP and DMP1 at the MQX/DDP motif, but the identity of a protein that can enhance the cleavage of the two SIBLING proteins remains elusive.

Structural and functional evidence for a substrate exclusion mechanism in mammalian tolloid like-1 (TLL-1) proteinase

Bone morphogenetic protein-1 (BMP-1)/tolloid proteinases are fundamental to regulating dorsal ventral patterning and extracellular matrix deposition. In mammals there are four proteinases, the splice variants BMP-1 and mammalian tolloid (mTLD), and tolloid like-1 and -2 (TLL-1/2). BMP-1 has the highest catalytic activity and lacks three non-catalytic domains. We demonstrate that TLL-1, which has intermediate activity, forms a calcium-ion dependent dimer with monomers stacked side-by-side. In contrast, truncated TLL-1 molecules having the same shorter structure as BMP-1 are monomers and have improved activity towards their substrate chordin. The increased activity exceeds not only that of full-length TLL-1 but also BMP-1.

Structured summary

MINT-7386098: BMP-1 (uniprotkb:P13497) cleaves (MI:0194) Chordin (uniprotkb:Q9H2X0) by protease assay (MI:0435)

Role of dimerization and substrate exclusion in the regulation of bone morphogenetic protein-1 and mammalian tolloid

The bone morphogenetic protein (BMP)-1/tolloid metalloproteinases are evolutionarily conserved enzymes that are fundamental to dorsal-ventral patterning and tissue morphogenesis. The lack of knowledge regarding how these proteinases recognize and cleave their substrates represents a major hurdle to understanding tissue assembly and embryonic patterning. Although BMP-1 and mammalian tolloid (mTLD) are splice variants, it is puzzling why BMP-1, which lacks 3 of the 7 noncatalytic domains present in all other family members, is the most effective proteinase. Using a combination of single-particle electron microscopy, small-angle X-ray scattering, and other biophysical measurements in solution, we show that mTLD, but not BMP-1, forms a calcium-ion-dependent dimer under physiological conditions. Using a domain deletion approach, we provide evidence that EGF2, which is absent in BMP-1, is critical to the formation of the dimer. Based on a combination of structural and functional data, we propose that mTLD activity is regulated by a substrate exclusion mechanism. These results provide a mechanistic insight into how alternative splicing of the Bmp1 gene produces 2 proteinases with differing biological activities and have broad implications for regulation of BMP-1/mTLD and related proteinases during BMP signaling and tissue assembly.

Secreted Frizzled-related protein 2 is a procollagen C proteinase enhancer with a role in fibrosis associated with myocardial infarction

Secreted frizzled related proteins (sFRPs) have emerged as key regulators of a wide range of developmental and disease processes, with virtually all known functions of mammalian sFRPs attributed to their ability to antagonize Wnt signaling. Recently however, the Xenopus and zebrafish sFRP, Sizzled, was shown to function as an antagonist of Chordin processing by Tolloid-like metalloproteinases, leading to the proposal that sFRPs may function as evolutionarily-conserved antagonists of the chordinase activities of this class of proteinases. Herein, in contrast to this proposal, we show that the mammalian sFRP, sFRP2, does not affect Chordin processing, but instead can serve as a direct enhancer of the procollagen C-proteinase activity of Tolloid-like metalloproteinases. We further show that the level of fibrosis, in which procollagen processing by Tolloid-like proteinases plays a rate-limiting role, is markedly reduced in sFRP2-null mice subjected to myocardial infarction. Importantly, this reduced level of fibrosis is accompanied by significantly improved cardiac function. This study thus uncovers a novel function for sFRP2 and a potential therapeutic application for sFRP2 antagonism in controlling fibrosis in the infarcted heart.

Mutations in mammalian tolloid-like 1 gene detected in adult patients with ASD

Atrial septal defect (ASD) is an incomplete septation of atria in human heart causing circulatory problems. Its frequency is estimated at one per 10 000. Actions of numerous genes have been linked to heart development. However, no single gene defect causing ASD has yet been identified. Incomplete heart septation similar to ASD was reported in transgenic mice with both inactive alleles of gene encoding mammalian zinc metalloprotease a mammalian tolloid-like 1 (tll1). Here, we have screened 19 ASD patients and 15 healthy age-matched individuals for mutations in TLL1 gene. All 22 exons were analyzed exon by exon for heteroduplex formation. Subsequently, DNA fragments forming heteroduplexes were sequenced. In four nonrelated patients, three missense mutations in coding sequence, and one single base change in the 5'UTR have been detected. Two mutations (Met182Leu, and Ala238Val) were detected in ASD patients with the same clinical phenotype. As the second mutation locates immediately upstream of the catalytic zinc-binding signature, it might change the enzyme substrate specificity. The third change, Leu627Val in the CUB3 domain, has been found in an ASD patient with interatrial septum aneurysm in addition to ASD. The CUB3 domain is important for substrate-specific recognition. In the remaining 15 patients as well as in 15 reference samples numerous base substitutions, deletions, and insertions have been detected, but no mutations changing the coding sequence have been found. Lack of mutations in relation to ASD of these patients could possibly be because of genetic heterogeneity of the syndrome.

Meprins, membrane-bound and secreted astacin metalloproteinases

The astacins are a subfamily of the metzincin superfamily of metalloproteinases. The first to be characterized was the crayfish enzyme astacin. To date more than 200 members of this family have been identified in species ranging from bacteria to humans. Astacins are involved in developmental morphogenesis, matrix assembly, tissue differentiation and digestion. Family members include the procollagen C-proteinase (BMP1, bone morphogenetic protein 1), tolloid and mammalian tolloid-like, HMP (Hydra vulgaris metalloproteinase), sea urchin BP10 (blastula protein) and SPAN (Strongylocentrotus purpuratus astacin), the 'hatching' subfamily comprising alveolin, ovastacin, LCE, HCE ('low' and 'high' choriolytic enzymes), nephrosin (from carp head kidney), UVS.2 from frog, and the meprins. In the human and mouse genomes, there are six astacin family genes (two meprins, three BMP1/tolloid-like, one ovastacin), but in Caenorhabditis elegans there are 40. Meprins are the only astacin proteinases that function on the membrane and extracellularly by virtue of the fact that they can be membrane-bound or secreted. They are unique in their domain structure and covalent subunit dimerization, oligomerization propensities, and expression patterns. They are normally highly regulated at the transcriptional and post-translational levels, localize to specific membranes or extracellular spaces, and can hydrolyse biologically active peptides, cytokines, extracellular matrix (ECM) proteins and cell-surface proteins. The in vivo substrates of meprins are unknown, but the abundant expression of these proteinases in the epithelial cells of the intestine, kidney and skin provide clues to their functions.

Activity of ulilysin, an archaeal PAPP-A-related gelatinase and IGFBP protease

Human growth and development are conditioned by insulin-like growth factors (IGFs), which have also implications in pathology. Most IGF molecules are sequestered by IGF-binding proteins (IGFBPs) so that exertion of IGF activity requires disturbance of these complexes. This is achieved by proteolysis mediated by IGFBP proteases, among which the best characterised is human PAPP-A, the first member of the pappalysin family of metzincins. We have previously identified and studied the only archaeal homologue found to date, Methanosarcina acetivorans ulilysin. This is a proteolytically functional enzyme encompassing a pappalysin catalytic domain and a pro-domain involved in maintenance of latency of the zymogen, proulilysin. Once activated, the protein hydrolyses IGFBP-2 to -6 and insulin chain beta in vitro. We report here that ulilysin is also active against several other substrates, viz (azo)casein, azoalbumin, and extracellular matrix components. Ulilysin has gelatinolytic but not collagenolytic activity. Moreover, the proteolysis-resistant skeletal proteins actin and elastin are also cleaved, as is fibrinogen, but not plasmin and alpha1-antitrypsin from the blood coagulation cascade. Ulilysin develops optimal activity at pH 7.5 and strictly requires peptide bonds preceding an arginine residue, as determined by means of a novel fluorescence resonance energy transfer assay, thus pointing to biotechnological applications as an enzyme complementary to trypsin.

The protease domain of procollagen C-proteinase (BMP1) lacks substrate selectivity, which is conferred by non-proteolytic domains

Procollagen C-proteinase (PCP) removes the C-terminal pro-peptides of procollagens and also processes other matrix proteins. The major splice form of the PCP is termed BMP1 (bone morphogenetic protein 1). Active BMP1 is composed of an astacin-like protease domain, three CUB (complement, sea urchin Uegf, BMP1) domains and one EGF-like domain. Here we compare the recombinant human full-length BMP1 with its isolated proteolytic domain to further unravel the functional influence of the CUB and EGF domains. We show that the protease domain alone cleaves truncated procollagen VII within the short telopeptide region into fragments of similar size as the full-length enzyme does. However, unlike full-length BMP1, the protease domain does not stop at this point, but degrades its substrate completely. Moreover, the protease domain cleaves other matrix proteins such as fibronectin, collagen I and collagen IV, which are left intact by the full-length enzyme. In addition, we show for the first time that thrombospondin-1 is differently cleaved by both BMP1 and its catalytic domain. In summary, our data support the concept that the C-terminal domains of BMP1 are important for substrate recognition and for controlling and restricting its proteolytic activity via exosite binding.

The bone morphogenetic protein 1/Tolloid-like metalloproteinases

A decade ago, bone morphogenetic protein 1 (BMP1) was shown to provide the activity necessary for proteolytic removal of the C-propeptides of procollagens I-III: precursors of the major fibrillar collagens. Subsequent studies have shown BMP1 to be the prototype of a small group of extracellular metalloproteinases that play manifold roles in regulating formation of the extracellular matrix (ECM). Soon after initial cloning of BMP1, genetic studies showed the related Drosophila proteinase Tolloid (TLD) to be necessary for the formation of the dorsal-ventral axis in early embryogenesis. It is now clear that the BMP1/TLD-like proteinases, conserved in species ranging from Drosophila to humans, act in dorsal-ventral patterning via activation of transforming growth factor beta (TGFbeta)-like proteins BMP2, BMP4 (vertebrates) and decapentaplegic (arthropods). More recently, it has become apparent that the BMP1/TLD-like proteinases are activators of a broader subset of the TGFbeta superfamily of proteins, with implications that these proteinases may be key in orchestrating the formation of ECM with growth factor activation and BMP signaling in morphogenetic processes.

The interaction of recombinant subdomains of the procollagen C-proteinase with procollagen I provides a quantitative explanation for functional differences between the two splice variants, mammalian tolloid and bone morphogenetic protein 1

The procollagen C-proteinase (PCP) is a zinc peptidase of the astacin family and the metzincin superfamily. The enzyme removes the C-terminal propeptides of fibrillar procollagens and activates other matrix proteins. Besides its catalytic protease domain, the procollagen C-proteinase contains several C-terminal CUB modules (named after complement factors C1r and C1s, the sea urchin UEGF protein, and BMP-1) and EGF-like domains. The two major splice forms of the C-proteinase differ in their overall domain composition. The longer variant, termed mammalian tolloid (mTld, i.e., PCP-2), has the protease-CUB1-CUB2-EGF1-CUB3-EGF2-CUB4-CUB5 composition, whereas the shorter form termed bone morphogenetic protein 1 (BMP-1, i.e., PCP-1) ends after the CUB3 domain. Two related genes encode proteases similar to mTld in humans and have been termed mammalian tolloid like-1 and -2 (mTll-1 and mTll-2, respectively). For mTll-1, it has been shown that it has C-proteinase activity. We demonstrate that recombinant EGF1-CUB3, CUB3, CUB3-EGF2, EGF2-CUB4, and CUB4-CUB5 modules of the procollagen C-proteinase can be expressed in bacteria and adopt a functional antiparallel beta-sheet conformation. As shown by surface plasmon resonance analysis, the modules bind to procollagen I in a 1:1 stoichiometry with dissociation constants (K(D)) ranging from 622.0 to 1.0 nM. Their binding to mature collagen I is weaker by at least 1 order of magnitude. Constructs containing EGF domains bind more strongly than those consisting of CUB domains only. This suggests that a combination of CUB and EGF domains serves as the minimal functional unit. The binding affinities of the EGF-containing modules for procollagen increase in the order EGF1-CUB3 < CUB3-EGF2 < EGF2-CUB4. In the context of the full length PCP, this implies that a given module has an affinity that continues to increase the more C-terminally the module is located within the PCP. The tightest binding module, EGF2-CUB4 (K(D) = 1.0 nM), is only present in mTld, which might provide a quantitative explanation for the different efficiencies of BMP-1 and mTld in procollagen C-proteinase activity.

A Complete Domain Structure of Drosophila Tolloid Is Required for Cleavage of Short Gastrulation

Drosophila tolloid (TLD) is a member of a family of proteinases that play important roles in development and includes mammalian tolloid (mTLD) and bone morphogenetic protein (BMP)-1. TLD accentuates the activity of decapentaplegic (DPP), a transforming growth factor beta superfamily growth factor, by cleaving its antagonist Short gastrulation (Sog). Similarly, the activity of BMP-2/4 (vertebrate homologues of DPP) is augmented by cleavage of chordin. However, whereas TLD is an effective Sogase, mTLD is a poor chordinase and is functionally replaced by its smaller splice variant BMP-1, which lacks the most C-terminal epidermal growth factor (EGF)-like and CUB domains of mTLD. Moreover, the minimal chordinase activity resides in the N-terminal half of BMP-1. This study showed that the proteolytic activity of TLD is considerably enhanced by Ca2+ and tested the hypothesis that the Sogase activity of TLD resides in the N-terminal half of the proteinase. Unexpectedly, it was found that TLD lacking the CUB4 and CUB5 domains and/or the EGF-like domains was unable to cleave Sog. Loss of function mutations have been reported in the tld gene that result in amino acid substitutions at E835K (in CUB4), S915L (in CUB5), and N760I (in EGF2) in TLD. The CUB mutants were found to be ineffective Sogases, but the activity of the EGF2 mutant was unchanged. The results show that substrate recognition and cleavage by Drosophila tolloid and mTLD are different despite their identical domain structure and homologous functions in patterning. The result that the N760I mutant has full Sogase activity suggests that novel substrates for TLD exist.

Mammalian Tolloid-like 1 Binds Procollagen C-proteinase Enhancer Protein 1 and Differs from Bone Morphogenetic Protein 1 in the Functional Roles of Homologous Protein Domains

Bone morphogenetic protein 1 (BMP1) is the prototype of a subgroup of metalloproteinases with manifold roles in morphogenesis. Four mammalian subgroup members exist, including BMP1 and mammalian Tolloid-like 1 (mTLL1). Subgroup members have a conserved protein domain structure: an NH2-terminal astacin-like protease domain, followed by a fixed order of CUB and epidermal growth factor-like protein-protein interaction motifs. Previous structure/function studies have documented those BMP1 protein domains necessary for secretion, and activity against various substrates. Here we demonstrate that, in contradiction to previous reports, the most NH2-terminal CUB domain (CUB1) is not required for BMP1 secretion nor is the next CUB domain (CUB2) required for enzymatic activity. The same is true for mTLL1. In fact, secreted protease domains of BMP1 and mTLL1, devoid of CUB or epidermal growth factor-like domains, have procollagen C-proteinase (pCP) activity and activity for biosynthetic processing of biglycan, the latter with kinetics superior to those of the full-length proteins. Structure-function analyses herein also suggest differences in the functional roles played by some of the homologous domains in BMP1 and mTLL1. Surprisingly, although BMP1 has long been known to be Ca2+-dependent, a property previously assumed to apply to all members of the subgroup, mTLL1 is demonstrated to be independent of Ca2 levels in its ability to cleave some, but not all, substrates. We also show that pCP activities of only versions of BMP1 and mTLL1 with intact COOH termini are enhanced by the procollagen C-proteinase enhancer 1 (PCOLCE1) and that mTLL1 binds PCOLCE1, thus suggesting reappraisal of the accepted paradigm for how PCOLCE1 enhances pCP activities.

Developmental roles of the BMP1/TLD metalloproteinases

The astacin family (M12A) of the metzincin subclan MA(M) of metalloproteinases has been detected in developing and mature individuals of species that range from hydra to humans. Functions of this family of metalloproteinase vary from digestive degradation of polypeptides, to biosynthetic processing of extracellular proteins, to activation of growth factors. This review will focus on a small subgroup of the astacin family; the bone morphogenetic protein 1 (BMP1)/Tolloid (TLD)-like metalloproteinases. In vertebrates, the BMP1/TLD-like metalloproteinases play key roles in regulating formation of the extracellular matrix (ECM) via biosynthetic processing of various precursor proteins into mature functional enzymes, structural proteins, and proteins involved in initiating mineralization of the ECM of hard tissues. Roles in ECM formation include: processing of the C-propeptides of procollagens types I-III, to yield the major fibrous components of vertebrate ECM; proteolytic activation of the enzyme lysyl oxidase, necessary to formation of covalent cross-links in collagen and elastic fibers; processing of NH2-terminal globular domains and C-propeptides of types V and XI procollagen chains to yield monomers that are incorporated into and control the diameters of collagen type I and II fibrils, respectively; processing of precursors for laminin 5 and collagen type VII, both of which are involved in securing epidermis to underlying dermis; and maturation of small leucine-rich proteoglycans. The BMP1/TLD-related metalloproteinases are also capable of activating the vertebrate transforming growth factor-beta (TGF-beta)-like "chalones" growth differentiation factor 8 (GDF8, also known as myostatin), and GDF11 (also known as BMP11), involved in negative feedback inhibition of muscle and neural tissue growth, respectively; by freeing them from noncovalent latent complexes with their cleaved prodomains. BMP1/TLD-like proteinases also liberate the vertebrate TGF-beta-like morphogens BMP2 and 4 and their invertebrate ortholog decapentaplegic, from latent complexes with the vertebrate extracellular antagonist chordin and its invertebrate ortholog short gastrulation (SOG), respectively. The result is formation of the BMP signaling gradients that form the dorsal-ventral axis in embryogenesis. Thus, BMP1/TLD-like proteinases appear to be key to regulating and orchestrating formation of the ECM and signaling by various TGF-beta-like proteins in morphogenetic and homeostatic events.

Identification of the Minimal Domain Structure of Bone Morphogenetic Protein-1 (BMP-1) for Chordinase Activity

Bone morphogenetic protein 1 (BMP-1), which is a tolloid member of the astacin-like family of zinc metalloproteinases, is a highly effective procollagen C-proteinase (PCP) and chordinase. On the other hand, mammalian tolloid like-2 (mTLL-2) does not cleave chordin or procollagen; procollagen is cleaved by mTLL-2 in the presence of high levels of procollagen C-proteinase enhancer-1 (PCPE-1), for reasons that are unknown. We used these differences in activity between BMP-1 and mTLL-2 to narrow in on the domains in BMP-1 that specify PCP and chordinase activity. Using a domain swap approach, we showed that: 1) the metalloproteinase and CUB2 domains of BMP-1 are absolutely required for PCP activity; swaps with either of the corresponding domains in BMP-1 and mTLL-2 did not result in procollagen cleavage and 2) the proteinase domain of mTLL-2 can cleave chordin if coupled to the CUB1 domain of BMP-1. Therefore, the minimal structure for chordinase activity comprises a metalloproteinase domain (either from BMP-1 or from mTLL-2) and the CUB1 domain of BMP-1 (the CUB1 domain of mTLL-2 cannot substitute for the CUB1 domain of BMP-1). We showed that the minimal procollagen C-proteinase (BMP-1 lacking the EGF and CUB3 domain) was enhanced by PCPE-1 but not as well as BMP-1 retaining the CUB3 domain. Further studies showed that PCPE-1 had no effect on the ability of BMP-1 to cleave chordin. The data support a previously suggested mechanism of PCPE-1 whereby PCPE-1 interacts with procollagen, but in addition, the CUB3 domain of BMP-1 appears to augment the interaction.

Substrate-specific Modulation of a Multisubstrate Proteinase

Members of the bone morphogenetic protein-1/tolloid (BMP-1/Tld) family of metalloproteinases, also known as procollagen C-proteinases (PCPs), control multiple biological events (including matrix assembly, cross-linking, cell adhesion/migration and pattern formation) through enzymatic processing of several extracellular substrates. PCP activities on fibrillar procollagens can be stimulated by another family of extracellular proteins, PCP enhancers (PCPE-1, PCPE-2), which lack intrinsic enzymatic activity. While PCPs have multiple substrates, the extent to which PCPEs is involved in the processing of proteins other than fibrillar procollagens is unknown. In the experiments reported here, PCPE-1 was found to have no effect on the in vitro BMP-1 processing of procollagen VII, the procollagen V N-propeptide, the laminin 5 gamma2 chain, osteoglycin, prolysyl oxidase, or chordin. In contrast, PCPE-1 enhanced C-terminal processing of human fibrillar procollagen III but only when this substrate was in its native, disulfide-bonded conformation. Surprisingly, processing of procollagen III continued to be enhanced when essentially all the triple-helical region was removed. These and previous results (Ricard-Blum, S., Bernocco, S., Font, B., Moali, C., Eichenberger, D., Farjanel, J., Burchardt, E. R., van der Rest, M., Kessler, E., and Hulmes, D. J. S. (2002) J. Biol. Chem. 277, 33864-33869; Bernocco, S., Steiglitz, B. M., Svergun, D. I., Petoukhov, M. V., Ruggiero, F., Ricard-Blum, S., Ebel, C., Geourjon, C., Deleage, G., Font, B., Eichenberger, D., Greenspan, D. S., and Hulmes, D. J. S. (2003) J. Biol. Chem. 278, 7199-7205) indicate that the mechanism of PCPE-1 action involves recognition sites in both the C-propeptide domain and in the C-telopeptide region of the procollagen molecule. PCPEs therefore define a new class of extracellular adaptor proteins that stimulate proteinase activity in a substrate-specific manner, thereby providing a new target for the selective regulation of PCP activity on fibrillar procollagen substrates.