Structure of full-length porcine synovial collagenase reveals a C-terminal domain containing a calcium-linked, four-bladed beta-propeller

Matrix metalloproteinase-1 is associated with poor prognosis in oesophageal cancer

The matrix metalloproteinases (MMPs) are a family of closely related proteolytic enzymes which are involved in the degradation of different components of the extracellular matrix. There is increasing evidence to indicate that individual MMPs have an important role in tumour invasion and tumour spread. Monoclonal antibodies specific for MMP-1, MMP-2, or MMP-9 have been produced, using as immunogens peptides selected from the amino acid sequences of individual MMPs. The presence of MMP-1, MMP-2, and MMP-9 in oesophageal cancer was investigated by immunohistochemistry on formalin-fixed, wax-embedded sections of oesophageal cancers. The relationship of individual MMPs to prognosis and survival was determined. MMP-1 was present in 24 per cent of oesophageal cancers, while MMP-2 and MMP-9 were present in 78 and 70 per cent of tumours, respectively. The presence of MMP-1 was associated with a particularly poor prognosis (log rank test 8.46, P < 0.004) and was an independent prognostic factor (P = 0.02). The identification of individual MMPs in oesophageal cancer provides a rational basis for use in the treatment of oesophageal cancer of MMP inhibitors which are currently undergoing clinical trial.

Human cathepsin K cleaves native type I and II collagens at the N-terminal end of the triple helix

Cathepsin K (EC 3.4.22.38) is a recently described enzyme that has been shown to cleave type I collagen in its triple helix. The aim of this study was to determine if it also cleaves type II collagen in the triple helix and to identify the helical cleavage site(s) in types I and II collagens. Soluble human and bovine type II collagen, and rat type I collagen, were incubated with cathepsin K before the reaction was stopped with trans-epoxysuccinyl-l-leucylamido-(4-guanidino)butane (E-64). Analysis by SDS/PAGE of the collagen digests showed that optimal activity of cathepsin K against native type II collagen was between pH 5.0 and 5.5 and against denatured collagen between pH 4.0 and 7.0. The enzyme cleaved telopeptides as well as the alpha1(II) chains, generating multiple fragments in the range 90-120 kDa. The collagenolytic activity was not due to a contaminating metalloenzyme or serine proteinase as it was not inhibited by 1,10-phenanthroline, EDTA or 3,4-dichloroisocoumarin. Western blotting with anti-peptide antibodies to different regions of the alpha1(II) chain suggested that cathepsin K cleaved native alpha1(II) chains in the N-terminal region of the helical domain rather than at the well-defined collagenase cleavage site. This was confirmed by N-terminal sequencing of one of the fragments, revealing cleavage at a Gly-Lys bond, 58 residues from the N-terminus of the helical domain. By using a similar approach, cathepsin K was found to cleave native type I collagen close to the N-terminus of its triple helix. These results indicate that cathepsin K could have a role in the turnover of type II collagen, as well as type I collagen.

Matrix metalloproteinases and TIMPs: properties and implications for the rheumatic diseases

The matrix metalloproteinases (MMPs) are a unique family of metalloenzymes, which, once activated, can destroy all the components of cartilage. MMPs are found in resorbing cartilage, bone, rheumatoid and osteoarthritic synovial fluid, and adjacent soft tissues. The active enzymes are all inhibited by tissue inhibitors of metalloproteinases (TIMPs). The relative amounts of active MMPs and TIMPs are important in determining whether cartilage is broken down in joint diseases. Conventional treatments for arthritis do little to affect the underlying joint destruction, but new drugs are now available that can specifically block active MMPs. These potent inhibitors prevent the destruction of cartilage both in vitro and in animal models of arthritis. Future trials in patients will test their effectiveness in the prevention of cartilage destruction.

A study of the collagen-binding domain of a 116-kDa Clostridium histolyticum collagenase

The Clostridium histolyticum 116-kDa collagenase consists of four segments, S1, S2a, S2b, and S3. A 98-kDa gelatinase, which can degrade denatured but not native collagen, lacks the C-terminal fragment containing a part of S2b and S3. In this paper we have investigated the function of the C-terminal segments using recombinant proteins. Full-length collagenase degraded both native type I collagen and a synthetic substrate, Pz-peptide, while an 88-kDa protein containing only S1 and S2a (S1S2a) degraded only Pz-peptide. Unlike the full-length enzyme, S1S2a did not bind to insoluble type I collagen. To determine the molecular determinant of collagen binding activity, various C-terminal regions were fused to the C terminus of glutathione S-transferase. S3 as well as S2bS3 conferred collagen binding. However, a glutathione S-transferase fusion protein with a region shorter than S3 exhibited reduced collagen binding activity. S3 liberated from the fusion protein also showed collagen binding activity, but not S2aS2b or S2b. S1 had 100% of the Pz-peptidase activity but only 5% of the collagenolytic activity of the full-length collagenase. These results indicate that S1 and S3 are the catalytic and binding domains, respectively, and that S2a and S2b form an interdomain structure.

Cytochrome cd1 structure: unusual haem environments in a nitrite reductase and analysis of factors contributing to beta-propeller folds

The central tunnel of the eight-bladed beta-propeller domain of cytochrome cd1 (nitrite reductase) is seen, from a 1.28 A resolution structure, to contain hydrogen donors and acceptors that are satisfied by interaction either with water or the d1 haem. The d1 haem, although bound by an extensive network of hydrogen bonds, is not distorted in its binding pocket and is confirmed to have exactly the dioxoisobacteriochlorin structure proposed from chemical studies. A biological rationale is advanced for the undistorted structure of the d1 haem and the large number of hydrogen bonds it makes. The beta-propeller domain can be closely superimposed on that of methanol dehydrogenase despite the enzymes sharing no common sequence motifs and using a different set of interactions to "Velcro" close the propeller. The sequence and likely structural relationships between cytochrome cd1 or methanol dehydrogenase and other predicted eight-bladed beta-propeller domains in proteins, such as the pyrolloquinoline quinone-dependent alcohol dehydrogenase, are discussed and compared with other propeller proteins. From sequencing the nirS gene of Thiosphaera pantotropha, it is established that the amino acid sequence deduced previously in part from X-ray diffraction data at lower resolution was largely correct, as was the proposal that eight N-terminal amino acid residues were not seen in the structure. The unusual haem iron environments in both the c-type cytochrome domain, with His/His coordination, and the d1-type cytochrome domain with Tyr/His coordination are related to the functions of the redox centres.

A novel matrix metalloproteinase gene (XMMP) encoding vitronectin-like motifs is transiently expressed in Xenopus laevis early embryo development

To study the role of matrix metalloproteinases (MMPs) in early vertebrate development, we cloned cDNAs for six different MMPs from the frog Xenopus laevis embryos at different stages of development and describe here a novel MMP called XMMP. Xenopus XMMP has 604 amino acids including a putative signal peptide of 22 residues. At the carboxyl-terminal end of the propeptide, XMMP has a 37-amino acid-long insertion domain containing a segment that is 38% identical with a rat vitronectin sequence between residues 108-135. Following this domain is an RRKR motif, a putative cleavage site for intracellular activation by furin proteinases. XMMP lacks a proline-rich linker peptide, or hinge region, typically found in other MMPs between the catalytic domain and carboxyl-terminal "hemopexin/vitronectin-like" domain. In XMMP, the carboxyl-terminal domain is composed of four tandem repeats that are 21-33% identical to a sequence (residues 213-264) encoded by vitronectin exon-5. Interestingly, XMMP gene is transiently expressed during Xenopus embryo development. XMMP mRNA of 3.0 kilobase pairs was undetected in the blastula stage embryo, induced in gastrula embryo, expressed in neurula embryo, and then down-regulated in pretailbud embryo. In comparison, other Xenopus MMP genes that we have cloned show a different developmental regulation. In blastula embryo, the only MMP gene expressed was found to be 92-kDa type IV collagenase, which was also expressed in the gastrula, neurula, and pretailbud embryos. Expression of stromelysin-1, stromelysin-3, and two different membrane type-MMPs was first detected in the neurula and pretailbud embryos. These results suggest that MMPs and the novel XMMP reported here play a role in Xenopus early development.

The role of the C-terminal domain of human collagenase-3 (MMP-13) in the activation of procollagenase-3, substrate specificity, and tissue inhibitor of metalloproteinase interaction

Recombinant human procollagenase-3 and a C-terminal truncated form (Delta249-451 procollagenase-3) have been stably expressed in myeloma cells and purified. The truncated proenzyme could be processed by aminophenylmercuric acetate via a short-lived intermediate form (N-terminal Leu58) to the final active form (N-terminal Tyr85). The kinetics of activation were not affected by removal of the hemopexin-like C-terminal domain. The specific activities of both collagenase-3 and Delta249-451 collagenase-3 were found to be similar using two quenched fluorescent substrates, but Delta249-451 collagenase-3 failed to cleave native triple helical collagens (types I and II) into characteristic one- and three-quarter fragments. It was noted, however, that the beta1,2(I) chains of type I collagen were susceptible to Delta249-451 collagenase-3, which indicates that the catalytic domain displays telopeptidase activity, thereby generating alpha1,2(I) chains that are slightly shorter than those in native type I collagen. It can be concluded that the C-terminal domain is only essential for the triple helicase activity of collagenase-3. Binding of procollagenase-3 and active collagenase-3 to type I collagen is mediated by the C-terminal domain. Both collagenase-3 and Delta249-451 collagenase-3 hydrolyzed the large tenascin C isoform, fibronectin, recombinant fibronectin fragments, and type IV, IX, X, and XIV collagens; thus, these events were independent from C-terminal domain interactions. In contrast, the minor cartilage type XI collagen was resistant to cleavage. Kinetic analysis of the mechanism of inhibition of wild-type and Delta249-451 collagenase-3 by wild-type and mutant tissue inhibitors of metalloproteinase (TIMPs) revealed that the association rates for complex formation were influenced by both N- and C-terminal domain interactions. The C-terminal domain of wild-type collagenase-3 promoted increased association rates with the full-length inhibitors TIMP-1 and TIMP-3 and the hybrid N.TIMP-2/C.TIMP-1 by a factor of up to 33. In contrast, the association rates for complex formation with TIMP-2 and N.TIMP-1/C.TIMP-2 were only marginally affected by C-terminal domain interactions.

The hemopexin-like domain (C domain) of human gelatinase A (matrix metalloproteinase-2) requires Ca2+ for fibronectin and heparin binding. Binding properties of recombinant gelatinase A C domain to extracellular matrix and basement membrane components

The binding properties of the COOH-terminal hemopexin-like domain (C domain) of human gelatinase A (matrix metalloproteinase-2, 72-kDa gelatinase) were investigated to determine whether the C domain has binding affinity for extracellular matrix and basement membrane components. Recombinant C domain (rC domain) (Gly417-Cys631) was expressed in Escherichia coli, and the purified protein, identified using two antipeptide antibodies, was determined by electrospray mass spectrometry to have a mass of 25,925 Da, within 0.1 Da of that predicted. As assessed by microwell substrate binding assays and by column affinity chromatography, the matrix proteins laminin, denatured type I collagen, elastin, SPARC (secreted protein that is acidic and rich in cysteine), tenascin, and MatrigelTM were not bound by the rC domain. Unlike the hemopexin-like domains of collagenase and stromelysin, the rC domain also did not bind native type I collagen. Nor were native or denatured types II, IV, V, and X collagen, or the NC1 domain of type VII collagen bound. However, binding to heparin and fibronectin (Kd, 1.1 x 10(-6) M) could be disrupted by 0.58-0.76 and 0.3 M NaCl, respectively. Using nonoverlapping chymotrypsin-generated fragments of fibronectin, binding sites for the rC domain were found on both the 40-kDa heparin binding and the 120-kDa cell binding fibronectin domains (Kd values, approximately 4-6 x 10(-7) M). The Ca2+ ion, but not the potential structural Zn2+ ion, were found to be essential for maintaining the binding properties of the protein. The apo-form of the rC domain did not bind heparin, and both ethylenediaminetetraacetic acid and the specific Ca2+ ion chelator 1, 2-bis(2-aminophenoxy) ethane-N,N,N',N'-tetraacetic acid, but not the Zn2+ ion chelator 1,10-phenanthroline, eluted the holo form of the rC domain from both heparin-Sepharose and fibronectin. Inductive coupled plasma mass spectrometry also did not detect a Zn2+ ion in the rC domain. In contrast, reduction with 65 mM dithiothreitol did not interfere with heparin binding, further emphasizing the crucial structural role played by the Ca2+ ion. Together, these data demonstrate for the first time that the hemopexin-like domain of gelatinase A has a binding site for fibronectin and heparin, and that Ca2+ ions are important in maintaining the structure and function of the domain.

Structural and functional aspects of calcium binding in extracellular matrix proteins

Ca2+ ions play crucial roles in many matrix-matrix, cell-matrix and cell-cell contacts. Recent X-ray and NMR structure determinations have revealed an intriguing diversity of Ca(2+)-binding sites in extracellular proteins, ranging from the stabilization of isolated domains to intimate involvement in the superstructure of macromolecular assemblies. The central role of Ca2+ in extracellular proteins is illustrated by the molecular characterization of hereditary connective tissue disorders in humans. Point mutations of Ca(2+)-binding residues in fibrillin and cartilage oligomeric matrix protein are responsible for Marfan syndrome and pseudoachondroplasia, respectively. We also discuss the possibility that structure and function of extracellular proteins may be regulated by physiologically relevant Ca2+ gradients.

The collagen triple-helix structure

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

Relating matrix metalloproteinase structure to function: why the "hemopexin" domain?

Matrix metalloproteinases are thought to be key players in the remodelling activity of cells associated with both physiological and pathological processes. They share a relatively conserved structure with a number of identifiable modules linked to their specific functions. The structure of the individual domains of a number of matrix metalloproteinases have now been elucidated. Here we review these data in the light of complementary studies on the behaviour of these enzymes in biological systems. In particular we focus on the C-terminal hemopexin-like domain which has intriguingly specific roles in individual matrix metalloproteinases.

Cooperative binding of Ca2+ to human interstitial collagenase assessed by circular dichroism, fluorescence, and catalytic activity

Dissociation of Ca2+ from human interstitial collagenase induced either by chelation with EGTA or by dilution resulted in loss of enzyme activity, a red shifted emission maximum from 334 to 340 nm and quenching of protein fluorescence by 10% at 340 nm. Circular dichroism indicated that secondary structure was unaffected by EGTA. Ca2+ binding to the EGTA-treated enzyme as assessed by fluorescence was cooperative (Hill coefficient, 2.9; 50% saturation at 0.4 mM Ca2+). The dependence of catalytic activity on [Ca2+] was also cooperative (Hill coefficient, 1.7-2.0; midpoint [Ca2+], 0.2 mM). The Ca2+-reconstituted protein was indistinguishable from the untreated enzyme by activity and fluorescence measurements. These results demonstrate that removal of Ca2+ from full-length collagenase generates a catalytically incompetent, partially unfolded state with native secondary structure but altered tertiary structure characterized by exposure of at least one tryptophyl residue to a more polar environment.

Folding of the N-terminal, ligand-binding region of integrin alpha-subunits into a beta-propeller domain

The N-terminal approximately 440 aa of integrin alpha subunits contain seven sequence repeats. These are predicted here to fold into a beta-propeller domain. A homologous domain from the enzyme phosphatidylinositol phospholipase D is predicted to have the same fold. The domains contain seven four-stranded beta-sheets arranged in a torus around a pseudosymmetry axis. The trimeric G-protein beta subunit (G beta) appears to be the most closely related beta-propeller. Integrin ligands and a putative Mg2+ ion are predicted to bind to the upper face of the beta-propeller. This face binds substrates in beta-propeller enzymes and is used by the G protein beta subunit to bind the G protein alpha subunit. The integrin alpha subunit I domain, which is structurally homologous to the G protein alpha subunit, is tethered to the top of the beta-propeller domain by a hinge that may allow movement of the domains relative to one another. The Ca2+-binding motifs in integrin alpha subunits are on the lower face of the beta-propeller.

The recombinant catalytic domain of mouse collagenase-3 depolymerizes type I collagen by cleaving its aminotelopeptides

The sequence coding for the catalytic domain of mouse collagenase-3 (MMP-13) was amplified by polymerase chain reaction and expressed in Escherichia coli. The recombinant catalytic domain (CCD), mainly recovered as inclusion bodies, was renatured and purified to homogeneity by preparative SDS-PAGE. The purified CCD degraded gelatin, casein and a synthetic peptide. CCD was not able to cleave the triple-helical domain of type I collagen but conserved the specific property of full-length collagenase-3 to cleave the N-telopeptides. These results show that residues involved in the recognition and cleavage of the aminotelopeptides of type I collagen are located in the catalytic domain of mouse collagenase-3 and that the C-terminal domain is not required for this activity.

Protein folds in the all-beta and all-alpha classes

Analysis of the structures in the Protein Databank, released in June 1996, shows that the number of different protein folds, i.e. the number of different arrangements of major secondary structures and/or chain topologies, is 327. Of these folds, approximately 25% belong to the all-alpha class, 20% belong to the all-beta class, 30% belong to the alpha/beta class, and 25% belong to the alpha + beta class. We describe the types of folds now known for the all-beta and all-alpha classes, emphasizing those that have been discovered recently. Detailed theories for the physical determinants of the structures of most of these folds now exist, and these are reviewed.

Interleukin-4 blocks the release of collagen fragments from bovine nasal cartilage treated with cytokines

Interleukin-1 (IL-1) in combination with other cytokines can induce a reproducible release of collagen fragments from bovine nasal cartilage in culture. Over 70% of the total collagen is released by day 14 and this release is accompanied by the appearance of collagenolytic activity in the medium that cleaves collagen specifically at the one quarter/three quarter position. Interleukin-4 is able to prevent the release of collagen fragments from the tissue and this is accompanied by a reduced secretion and activation of collagenase (MMP-1) with an increase in tissue inhibitor of metalloproteinases-1 (TIMP-1). IL-4, especially in the presence of IL-1, increased TIMP secretion by bovine nasal cartilage in culture. These results suggest that IL-4 is able to specifically block cartilage collagen resorption by down-regulating the production of collagenase (MMP-1) and up-regulating TIMP-1 by chondrocytes within the cartilage.

Matrix metalloproteinases and the development of cancer

Proteolytic remodeling of the extracellular matrix is an important aspect of the creation and progression of cancer. Matrix metalloproteinases are important at several points during multi-stage neoplastic progression in tumor cells and responding blood vessels, inflammatory cells and stroma.

Cellular mechanisms for human procollagenase-3 (MMP-13) activation. Evidence that MT1-MMP (MMP-14) and gelatinase a (MMP-2) are able to generate active enzyme

Gelatinase A and membrane-type metalloproteinase (MT1-MMP) were able to process human procollagenase-3 (Mr 60,000) to the fully active enzyme (Tyr85 N terminus; Mr 48,000). MT1-MMP activated procollagenase-3 via a Mr 56,000 intermediate (Ile36 N terminus) to 48,000 which was the result of the cleavage of the Glu84-Tyr85 peptide bond. We have established that the activation rate of procollagenase-3 by MT1-MMP was enhanced in the presence of progelatinase A, thereby demonstrating a unique new activation cascade consisting of three members of the matrix metalloproteinase family. In addition, procollagenase-3 can be activated by plasmin, which cleaved the Lys38-Glu39 and Arg76-Cys77 peptide bonds in the propeptide domain. Autoproteolysis then resulted in the release of the rest of the propeptide domain generating Tyr85 N-terminal active collagenase-3. However, plasmin cleaved the C-terminal domain of collagenase-3 which results in the loss of its collagenolytic activity. Concanavalin A-stimulated fibroblasts expressing MT1-MMP and fibroblast-derived plasma membranes were able to process human procollagenase-3 via a Mr 56,000 intermediate form to the final Mr 48,000 active enzyme which, by analogy with progelatinase A activation, may represent a model system for in vivo activation. Inhibition experiments using tissue inhibitor of metalloproteinases, plasminogen activator inhibitor-2, or aprotinin demonstrated that activation in the cellular model system was due to MT1-MMP/gelatinase A and excluded the participation of serine proteinases such as plasmin during procollagenase-3 activation. We have established that progelatinase A can considerably potentiate the activation rate of procollagenase-3 by crude plasma membrane preparations from concanavalin A-stimulated fibroblasts, thus confirming our results using purified progelatinase A and MT1-MMP. This new activation cascade may be significant in human breast cancer pathology, where all three enzymes have been implicated as playing important roles.

Progress towards understanding beta-sheet structure

This review is focused on recent advances in our understanding of beta-sheet structure. It is intended to supplement previous surveys describing the early characterization and study of beta-sheet structure. The first two sections of this review provide a brief introduction to beta-sheet structure referencing the prior comprehensive reviews in this area as well as integrating new concepts. The next part outlines the typical problems encountered in solution studies on beta-sheet structures. The most useful spectroscopic and biophysical techniques used to characterize beta-sheet structures are described in the fourth section. Current hypotheses regarding the folding of predominantly beta-sheet proteins are discussed in some detail in the fifth segment. The efforts of a number of laboratories to utilize peptides or peptidomimetics to serve as small beta-sheet model systems are reviewed in the penultimate section. Finally, the efforts of a number of research groups focusing on the de novo design of beta-sheet-based proteins are outlined.

Structure and domain-domain interactions of the gelatin binding site of human 72-kilodalton type IV collagenase (gelatinase A, matrix metalloproteinase 2)

We have shown previously that all three fibronectin type II modules of gelatinase A contribute to its gelatin affinity. In the present investigation we have studied the structure and module-module interactions of this gelatin-binding domain by circular dichroism spectroscopy and differential scanning calorimetry. Comparison of the Tm values of the thermal transitions of isolated type II modules with those of bimodular or trimodular proteins has shown that the second type II module is significantly more stable in the trimodular protein coll 123 (Tm = 54 degrees C) than in the single-module protein coll 2 (Tm = 44 degrees C) or in the bimodular proteins coll 23 (Tm = 47 degrees C) and coll 12 (Tm = 48 degrees C). Analysis of the enthalpy changes associated with thermal unfolding of the second type II module suggests that it is stabilized by domain-domain interactions in coll 123. We propose that intimate contacts exist between the three tandem type 11 units and they form a single gelatin-binding site. Based on the three-dimensional structures of homologous metalloproteases and type II modules, a model is proposed in which the three type II units form an extension of the substrate binding cleft of gelatinase A.

Structure and distribution of modules in extracellular proteins

It has become standard practice to compare new amino-acid and nucleotide sequences with existing ones in the rapidly growing sequence databases. This has led to the recurring identification of certain sequence patterns, usually corresponding to less than 300 amino-acids in length. Many of these identifiable sequence regions have been shown to fold up to form a ‘domain’ structure; they are often called protein ‘modules’ (see definitions below). Proteins that contain such modules are widely distributed in biology, but they are particularly common in extracellular proteins.

X-ray structure of a hydroxamate inhibitor complex of stromelysin catalytic domain and its comparison with members of the zinc metalloproteinase superfamily

BACKGROUND

Stromelysin belongs to a family of zinc-dependent endopeptidases referred to as matrix metalloproteinases (MMPs, matrixins) because of their capacity for selective degradation of various components of the extracellular matrix. Matrixins play key roles in diseases as diverse as arthritis and cancer and hence are important targets for therapeutic intervention.

RESULTS

The crystal structure of the stromelysin catalytic domain (SCD) with bound hydroxamate inhibitor, solved by multiple isomorphous replacement, shows deep S1' specificity pocket which explains differences in inhibitors binding between the collagenases and stromelysin. The binding of calcium ions by loops at the two ends of a beta-strand which marks the boundary of the active site provides a structural rationale for the importance of these cations for stability and catalytic activity. Major differences between the matrixins are clustered in two regions forming the entrance to the active site and hence may be determinants of substrate selectivity.

CONCLUSIONS

Structural comparisons of SCD with representative members of the metalloproteinase superfamily clearly highlight the conservation of key secondary structural elements, in spite of major variations in the sequences including insertions and deletions of functional domains. However, the three-dimensional structure of SCD, which is generally closely related to the collagenases, shows significant differences not only in the peripheral regions but also in the specificity pockets; these latter differences should facilitate the rational design of specific inhibitors.

Characterization of folded, intermediate, and unfolded states of recombinant human interstitial collagenase

Recombinant interstitial collagenase (rMMP-1) forms insoluble inclusion bodies when over-expressed in Escherichia coli. We surveyed conditions for renaturation of purified rMMP-1 in 6 M guandine hydrochloride (GdnHCl) and found that optimal folding occurred when the denatured protein was diluted at 4 degrees C in approximately 2 M guanidine HCl, 20% glycerol, 2.5 mM reduced and oxidized glutathione, and 5 mM CaCl2, followed by buffer exchange to remove denaturant and thiols. The circular dichroism spectrum and catalytic constants of the refolded enzyme were similar to those of native MMP-1. The propeptide, which comprises approximately 20% of the mass of proMMP-1, was not required for folding to a functional enzyme. Size exclusion chromatography and spectroscopic measurements at intermediate [GdnHCl] revealed two intermediate folding states. The first, observed at 1 M GdnHCl, had a slightly larger Stokes' radius than the folded protein. CD and fluorescence analysis showed that it contained ordered tryptophan residues with a higher quantum yield than the fully folded state. The second intermediate, which appeared between 2 and 4 M GdnHCl, exhibited properties consistent with the molten globule, including secondary structure, lack of ordered tryptophan, exposed hydrophobic binding sites, and a Stokes' radius between that of the folded and unfolded states.

Circular assemblies

During 1994 and 1995, the structures of the serum amyloid P component, the bacterial chaperonin GroEL, the 20S proteasome, the bacterial light-harvesting complexes and the tryptophan operon RNA-binding attenuation protein have been determined. These structures all form circular assemblies in which the individual subunits are related by rotational symmetry. In most cases the circular organization generates a new biophysical property and a specific biological function which have presumably been selected by evolution.

An analysis of the conformational changes that accompany the activation and inhibition of gelatinase A

The latent precursors of the matrix metalloproteinases (MMPs) are converted by (4-aminophenylmercuric)acetate to active forms that lose their propeptide as a result of autolysis. C.D. and an active site mutant of progelatinase A (MMP2) were used to demonstrate that, although propeptide removal is accompanied by a decrease in the enzyme's beta-sheet content, the initial activation is achieved with only minor modifications to the conformation. Mixing activated gelatinase A with the natural inhibitor, TIMP-1, resulted in conformational changes that were absent when a synthetic inhibitor was used. The relevance of these results to MMP activation and inhibition is discussed.

Activation of human neutrophil procollagenase by stromelysin 2

Neutrophil procollagenase (MMP-8) was efficiently activated by incubation with active stromelysin 2 (MMP-10). A single-step activation mechanism involving the cleavage of the Gly78-Phe79 peptide bond at the end of the propeptide domain was observed. Determination of the collagenolytic activity revealed the generation of active neutrophil collagenase displaying high specific activity. When compared with the specific activity following mercurial activation, which generates active collagenase by autoproteolytic cleavage of either Phe79-Met8O or Met8O-Leu81 peptide bonds [Bläser, J., Knäuper, V., Osthues, A., Reinke, H. & Tschesche, H. (1991) Eur J. Biochem. 202, 1223-1230], the specific activity of the stromelysin-2-activated enzyme was considerably higher. Thus, human neutrophil procollagenase was 'superactivated' by stromelysin 2, as was recently shown for the stromelysin-1-activated enzyme [Knäuper, V., Wilhelm, S. M., Seperack, P. K., De Clerck, Y. A., Langley, K. E., Osthues, A. & Tschesche, H. 1993 a) Biochem. J. 295, 581-586].

The C-terminal (haemopexin-like) domain structure of human gelatinase A (MMP2): structural implications for its function

In common with most other matrix metalloproteinases, gelatinase A has a non-catalytic C-terminal domain that displays sequence homology to haemopexin. Crystals of this domain were used by molecular replacement to solve its molecular structure at 2.6 A resolution, which was refined to an R value of 17.9%. This structure has a disc-like shape, with the chain folded into a beta-propeller structure that has pseudo four-fold symmetry. Although the topology and the side-chain arrangement are very similar to the equivalent domain of fibroblast collagenase, significant differences in surface charge and contouring are observable on 1 side of the gelatinase A disc. This difference might be a factor in allowing the gelatinase A C-terminal domain to bind to natural inhibitor TIMP-2.

Human matrix metalloproteinase specificity studies using collagen sequence-based synthetic peptides

The matrix metalloproteinase (MMP)/matrixin family has been implicated in both normal tissue remodeling and a variety of diseases associated with abnormal turnover of extracellular matrix components. To better understand MMP behaviors and to aid in the design of MMP inhibitors, a variety of sequence specificity studies have been performed using collagen sequence-based peptides and MMP family members. Results of these studies have been valuable for defining the differences in MMPs and for creating fluorogenic substrates that can continuously monitor MMP activity. However, these studies have also demonstrated that these peptides may not be very good models of native MMP substrates, and that the additivity principle is not always applicable for designing synthetic MMP substrates.

Collagenase: a key enzyme in collagen turnover

The primary agents responsible for cartilage and bone destruction in joint diseases are active proteinases that degrade collagen and proteoglycan. All four main classes of proteolytic enzymes are involved in either the normal turnover of connective tissue or its pathological destruction. These proteinases are made by different cells found within the joints. Both extracellular and intracellular pathways exist and individual enzymes can be inhibited by specific proteinaceous inhibitors that block their activity. Recent research has implicated the matrix metalloproteinases (MMPs) in many of the processes involved in joint diseases. The metalloproteinases are capable of degrading all components of the extracellular matrix. This family of proteinases contains a group of at least three collagenases that are capable of degrading native fibrillar collagen. Collagen degradation within joint disease is recognized as the irreversible step in the destruction of cartilage that leads to a failure in joint function. The collagenases are the enzymes necessary to initiate collagen turnover in normal connective tissue turnover and in disease.

Metalloproteinase inhibitors and the prevention of connective tissue breakdown

The primary agents responsible for cartilage and bone destruction in joint diseases are active proteinases degrading collagen and proteoglycan. All four main classes of proteolytic enzymes are involved in either the normal turnover of connective tissue or its pathological destruction. These proteinases are made by different cells found within the joints. Both extracellular and intracellular pathways exist, and individual enzymes can be inhibited by specific proteinaceous inhibitors that block their activity. Recent research has implicated the matrix metalloproteinases in many of the processes involved in joint diseases. Conventional treatments do little to affect the underlying disease processes, and recently, the use of proteinase inhibitors has been suggested as a new therapeutic approach. A large variety of different synthetic approaches have been used and highly effective metalloproteinase inhibitors have been designed, synthesised and tested. These metalloproteinase inhibitors can prevent the destruction of animal cartilage in model systems and slow the progression of human tumours. Future patient trials will test the effectiveness of these compounds in vivo for the treatment of joint diseases.

Solution structure of the catalytic domain of human stromelysin complexed with a hydrophobic inhibitor

Stromelysin, a representative matrix metalloproteinase and target of drug development efforts, plays a prominent role in the pathological proteolysis associated with arthritis and secondarily in that of cancer metastasis and invasion. To provide a structural template to aid the development of therapeutic inhibitors, we have determined a medium-resolution structure of a 20-kDa complex of human stromelysin's catalytic domain with a hydrophobic peptidic inhibitor using multinuclear, multidimensional NMR spectroscopy. This domain of this zinc hydrolase contains a mixed beta-sheet comprising one antiparallel strand and four parallel strands, three helices, and a methionine-containing turn near the catalytic center. The ensemble of 20 structures was calculated using, on average, 8 interresidue NOE restraints per residue for the 166-residue protein fragment complexed with a 4-residue substrate analogue. The mean RMS deviation (RMSD) to the average structure for backbone heavy atoms is 0.91 A and for all heavy atoms is 1.42 A. The structure has good stereochemical properties, including its backbone torsion angles. The beta-sheet and alpha-helices of the catalytic domains of human stromelysin (NMR model) and human fibroblast collagenase (X-ray crystallographic model of Lovejoy B et al., 1994b, Biochemistry 33:8207-8217) superimpose well, having a pairwise RMSD for backbone heavy atoms of 2.28 A when three loop segments are disregarded. The hydroxamate-substituted inhibitor binds across the hydrophobic active site of stromelysin in an extended conformation. The first hydrophobic side chain is deeply buried in the principal S'1 subsite, the second hydrophobic side chain is located on the opposite side of the inhibitor backbone in the hydrophobic S'2 surface subsite, and a third hydrophobic side chain (P'3) lies at the surface.

Crystal structure of the haemopexin-like C-terminal domain of gelatinase A

The crystal structure of the haemopexin-like C-terminal domain of gelatinase A reveals that it is a four-bladed beta-propeller protein. The four blades are arranged around a channel-like opening in which Ca2+ and a Na-Cl+ ion pair are bound.

1.8 A crystal structure of the C-terminal domain of rabbit serum haemopexin

BACKGROUND

Haemopexin is a serum glycoprotein that binds haem reversibly and delivers it to the liver where it is taken up by receptor-mediated endocytosis. Haemopexin has two homologous domains, each having a characteristic fourfold internal sequence repeat. Haemopexin-type domains are also found in other proteins, including the serum adhesion protein vitronectin and various collagenases, in which they mediate protein-protein interactions.

RESULTS

We have determined the crystal structure of the C-terminal domain of haemopexin at 1.8 A resolution. The domain is folded into four beta-leaflet modules, arranged in succession around a central pseudo-fourfold axis. A funnel-shaped tunnel through the centre of this disc-shaped domain serves as an ion-binding site.

CONCLUSIONS

A model for haem binding by haemopexin is proposed, utilizing an anion-binding site at the wider end of the central tunnel, together with an associated cleft. This parallels the active-site location in other beta-propeller structures. The capacity to bind both cations and anions, together with the disc shape of the domain, suggests that such domains may be used widely for macromolecular recognition.

A helping hand for collagenases: the haemopexin-like domain

The structures of the C-terminal domains of rabbit haemopexin and full-length porcine fibroblast collagenase reveal a common beta-propeller fold with pseudo-fourfold symmetry. The interactions of these haemopexin-like domains with target proteins and ligands is, however, still a matter of debate.