ADAM-TS8, a novel metalloprotease of the ADAM-TS family located on mouse chromosome 9 and human chromosome 11

A disintegrin-like and metalloprotease domain with thrombospondin type I modules (ADAM-TS) describes a novel family of zinc metalloendopeptidases. Its members have a common domain organization, which includes, typically, a pre-pro-metalloprotease domain, a disintegrin-like domain, and one or more thrombospondin-like (TS) modules. We describe here the complete primary structure of mouse ADAM-TS8, through cloning of Adamts8 cDNA. This novel member of the family contains two TS modules and is highly similar in sequence and domain organization to three other recently described gene products, ADAM-TS5, ADAM-TS6, and ADAM-TS7. Adamts8 is expressed at low levels throughout development and in adult mouse lung and heart. Through analysis of an interspecific backcross panel, we place the Adamts8 locus on mouse chromosome 9 at a consensus position of 11 cM and its human ortholog, recently reported as the METH2 gene, on human chromosome 11q25.     

Adamts9 is widely expressed during mouse embryo development

ADAMTS metalloproteases constitute a family of 19 secreted protein or proteoglycan processing enzymes. ADAMTS9 and its closest mammalian relative, ADAMTS20, are related to gon-1, a metalloprotease required for gonadal morphogenesis in Caenorhabditis elegans. Although expressed at generally low levels in embryonic subectodermal mesenchyme, ADAMTS20 is required for melanoblast colonization of skin. Mutations in Adamts20 cause Belted, one of several white spotting alleles in the mouse. In contrast to Adamts20, we previously showed by Northern blotting that Adamts9 was expressed highly throughout mouse development. Using RNA in situ hybridization, we determined the spatial and temporal regulation of Adamts9 during mouse embryogenesis. At 7.5 dpc Adamts9 is expressed in the allantois, trophoblast, parietal endoderm and decidual tissue. At 9.5 dpc it is expressed in head mesoderm and in the developing heart. From 11.5 to 12.5 dpc, Adamts9 is strongly expressed in posterior mesoderm, in the craniofacial region, ventral body wall and diaphragm. After 14.5 dpc, Adamts9 was highly expressed in the mesenchyme of developing lung, kidney, and mesentery. It is expressed during skeletogenesis, being present from 13.5 dpc in perichondrium, in the proliferation zone of growth plates after 15.5 dpc and it is highly expressed in newly formed bone. It is expressed in vascular endothelium and during formation of the pituitary and cochlea, but expression in the central nervous system is limited to the floor plate of the diencephalon, to the ventricular zone of the cerebral cortex and to the choroid plexus.     

Regulation of ADAMTS9 secretion and enzymatic activity by its propeptide

ADAMTS9 is a secreted, cell-surface-binding metalloprotease that cleaves the proteoglycans versican and aggrecan. Unlike most precursor proteins, the ADAMTS9 zymogen (pro-ADAMTS9) is resistant to intracellular processing. Instead, pro-ADAMTS9 is processed by furin at the cell surface. Here, we investigated the role of the ADAMTS9 propeptide in regulating its secretion and proteolytic activity. Removal of the propeptide abrogated secretion of the ADAMTS9 catalytic domain, and secretion was inefficiently restored by expression of the propeptide in trans. Substitution of Ala for Asn residues within each of three consensus N-linked glycosylation sites in the propeptide abrogated ADAMTS9 secretion. Thus, the propeptide is an intramolecular chaperone whose glycosylation is critical for secretion of the mature enzyme. In addition to two previously identified furin-processing sites (Arg74 downward arrow and Arg287 downward arrow) the ADAMTS9 propeptide was also furin-processed at Arg209. Substitution of Ala for Arg74, Arg209, and Arg287 resulted in secretion of an unprocessed zymogen. Unexpectedly, versican incubated with cells expressing this pro-ADAMTS9 was processed to a greater extent than when incubated with cells expressing wild-type, furin-processable ADAMTS9. Moreover, cells and medium treated with the proprotein convertase inhibitor decanoyl-Arg-Val-Lys-Arg-chloromethyl ketone had greater versican-cleaving activity than untreated cells. Following furin processing of pro-ADAMTS9, propeptide fragments maintained a non-covalent association with the catalytic domain. Collectively, these observations suggest that, unlike other metalloproteases, furin processing of the ADAMTS9 propeptide reduces its catalytic activity. Thus, the propeptide is a key functional domain of ADAMTS9, mediating an unusual regulatory mechanism that may have evolved to ensure maximal activity of this protease at the cell surface.     

Biosynthesis and Expression of a Disintegrin-like and Metalloproteinase Domain with Thrombospondin-1 Repeats-15: A NOVEL VERSICAN-CLEAVING PROTEOGLYCANASE*

The proteoglycanase clade of the ADAMTS superfamily shows preferred proteolytic activity toward the hyalectan/lectican proteoglycans as follows: aggrecan, brevican, neurocan, and versican. ADAMTS15, a member of this clade, was recently identified as a putative tumor suppressor gene in colorectal and breast cancer. However, its biosynthesis, substrate specificity, and tissue expression are poorly described. Therefore, we undertook a detailed study of this proteinase and its expression. We report propeptide processing of the ADAMTS15 zymogen by furin activity, identifying RAKR²¹²↓ as a major furin cleavage site within the prodomain. ADAMTS15 was localized on the cell surface, activated extracellularly, and required propeptide processing before cleaving V1 versican at position ⁴⁴¹E↓A⁴⁴². In the mouse embryo, Adamts15 was expressed in the developing heart at E10.5 and E11.5 days post-coitum and in the musculoskeletal system from E13.5 to E15.5 days post-coitum, where it was co-localized with hyaluronan. Adamts15 was also highly expressed in several structures within the adult mouse colon. Our findings show overlapping sites of Adamts15 expression with other members of ADAMTS proteoglycanases during embryonic development, suggesting possible cooperative roles during embryogenesis, consistent with other ADAMTS proteoglycanase combinatorial knock-out mouse models. Collectively, these data suggest a role for ADAMTS15 in a wide range of biological processes that are potentially mediated through the processing of versican.     

Characterization of ADAMTS-9 and ADAMTS-20 as a distinct ADAMTS subfamily related to Caenorhabditis elegans GON-1

We demonstrate that in humans, two metalloproteases, ADAMTS-9 (1935 amino acids) and ADAMTS-20 (1911 amino acids) are orthologs of GON-1, an ADAMTS protease required for gonadal morphogenesis in Caenorhabditis elegans. ADAMTS-9 and ADAMTS-20 have an identical modular structure, are distinct in possessing 15 TSRs and a unique C-terminal domain, and have a similar gene structure, suggesting that they comprise a new subfamily of human ADAMTS proteases. ADAMTS20 is very sparingly expressed, although it is detectable in epithelial cells of the breast and lung. However, ADAMTS9 is highly expressed in embryonic and adult tissues, and therefore we characterized the ADAMTS-9 protein further. Although the ADAMTS-9 zymogen has many proprotein convertase processing sites, pulse-chase analysis, site-directed mutagenesis, and amino acid sequencing demonstrated that maturation to the active form occurs by selective proprotein convertase (e.g. furin) cleavage of the Arg(287)-Phe(288) bond. Although lacking a transmembrane sequence, ADAMTS-9 is retained near the cell surface as well as in the ECM of transiently transfected COS-1 and 293 cells. COS-1 cells transfected with ADAMTS9 (but not vector-transfected cells) proteolytically cleaved bovine versican and aggrecan core proteins at the Glu(441)-Ala(442) bond of versican V1 and the Glu(1771)-Ala(1772) bond of aggrecan, respectively. In contrast, the ADAMTS-9 catalytic domain alone was neither localized to the cell surface nor able to confer these proteolytic activities on cells, demonstrating that the ancillary domains of ADAMTS-9, including the TSRs, are required both for specific extracellular localization and for its versicanase and aggrecanase activities.     

Adamts5, the gene encoding a proteoglycan-degrading metalloprotease,is expressed by specific cell lineages during mouse embryonic development and in adult tissues

The secreted metalloprotease ADAMTS5 is implicated in destruction of the cartilage proteoglycan aggrecan in arthritis, but its physiological functions are unknown. Its expression profile during embryogenesis and in adult tissues is therefore of considerable interest. β-Galactosidase (β-gal) histochemistry, enabled by a LacZ cassette inserted in the Adamts5 locus, and validated by in situ hybridization with an Adamts5 cRNA probe and ADAMTS5 immunohistochemistry, was used to profile Adamts5 expression during mouse embryogenesis and in adult mouse tissues. Embryonic expression was scarce prior to 11.5 days of gestation (E11.5) and noted only in the floor plate of the developing brain at E9.5. After E11.5 there was continued expression in brain, especially in the choroid plexus, peripheral nerves, dorsal root ganglia, cranial nerve ganglia, spinal and cranial nerves, and neural plexuses of the gut. In addition to nerves, developing limbs have Adamts5 expression in skeletal muscle (from E13.5), tendons (from E16.5), and inter-digital mesenchyme of the developing autopod (E13.5–15.5). In adult tissues, there is constitutive Adamts5 expression in arterial smooth muscle cells, mesothelium lining the peritoneal, pericardial and pleural cavities, smooth muscle cells in bronchi and pancreatic ducts, glomerular mesangial cells in the kidney, dorsal root ganglia, and in Schwann cells of the peripheral and autonomic nervous system. Expression of Adamts5 during neuromuscular development and in smooth muscle cells coincides with the broadly distributed proteoglycan versican, an ADAMTS5 substrate. These observations suggest the major contexts in which developmental and physiological roles could be sought for this protease.     

A disintegrin-like and metalloprotease (reprolysin type) with thrombospondin type 1 motifs: the ADAMTS family

ADAMTS proteases are complex secreted enzymes containing a prometalloprotease domain of the reprolysin type attached to an ancillary domain with a highly conserved structure that includes at least one thrombospondin type 1 repeat. Known functions of ADAMTS proteases include processing of procollagens and von Willebrand factor as well as catabolism of aggrecan, versican and brevican. They have been demonstrated to have important roles in connective tissue organization, coagulation, inflammation, arthritis, angiogenesis and cell migration. ADAMTS can be grouped into distinct clades within which there is conservation of modular organization, protein sequence, gene structure and possibly, of substrate preference. ADAMTS proteases are synthesized as zymogens, with constitutive proprotein convertase removal of the propeptide occurring prior to secretion. Their enzymatic specificity is heavily influenced by their ancillary domain, which plays a critical role in directing these enzymes to their substrates, the cell surface and the extracellular matrix.     

Cell-surface Processing of the Metalloprotease Pro-ADAMTS9 Is Influenced by the Chaperone GRP94/gp96

A disintegrin-like and metalloprotease domain with thrombospondin type 1 motifs 9 (ADAMTS9) is a highly conserved metalloprotease that has been identified as a tumor suppressor gene and is required for normal mouse development. The secreted ADAMTS9 zymogen undergoes proteolytic excision of its N-terminal propeptide by the proprotein convertase furin. However, in contrast to other metalloproteases, propeptide excision occurs at the cell surface and leads to decreased activity of the zymogen. Here, we investigated the potential cellular mechanisms regulating ADAMTS9 biosynthesis and cell-surface processing by analysis of molecular complexes formed by a construct containing the propeptide and catalytic domain of pro-ADAMTS9 (Pro-Cat) in HEK293F cells. Cross-linking of cellular proteins bound to Pro-Cat followed by mass spectrometric analysis identified UDP-glucose:glycoprotein glucosyltransferase I, heat shock protein gp96 (GRP94), BiP (GRP78), and ERdj3 (Hsp40 homolog) as associated proteins. gp96 and BiP were present at the cell surface in an immunoprecipitable complex with pro-ADAMTS9 and furin. Treatment with geldanamycin, an inhibitor of the HSP90α family (including gp96), led to decreased furin processing of pro-ADAMTS9 and accumulation of the unprocessed pro-ADAMTS9 at the cell surface. gp96 siRNA down-regulated the levels of cell-surface pro-ADAMTS9 and furin, whereas the levels of cell-surface pro-ADAMTS9, but not of cell-surface furin, were decreased upon treatment with BiP siRNA. These data identify for the first time the cellular chaperones associated with secretion of an ADAMTS protease and suggest a role for gp96 in modulating pro-ADAMTS9 processing.     

Regulation of procollagen amino-propeptide processing during mouse embryogenesis by specialization of homologous ADAMTS proteases: insights on collagen biosynthesis and dermatosparaxis

Mutations in ADAMTS2, a procollagen amino-propeptidase, cause severe skin fragility, designated as dermatosparaxis in animals, and a subtype of the Ehlers-Danlos syndrome (dermatosparactic type or VIIC) in humans. Not all collagen-rich tissues are affected to the same degree, which suggests compensation by the ADAMTS2 homologs ADAMTS3 and ADAMTS14. In situ hybridization of Adamts2, Adamts3 and Adamts14, and of the genes encoding the major fibrillar collagens, Col1a1, Col2a1 and Col3a1, during mouse embryogenesis, demonstrated distinct tissue-specific, overlapping expression patterns of the protease and substrate genes. Adamts3, but not Adamts2 or Adamts14, was co-expressed with Col2a1 in cartilage throughout development, and with Col1a1 in bone and musculotendinous tissues. ADAMTS3 induced procollagen I processing in dermatosparactic fibroblasts, suggesting a role in procollagen I processing during musculoskeletal development. Adamts2, but not Adamts3 or Adamts14, was co-expressed with Col3a1 in many tissues including the lungs and aorta, and Adamts2(-/-) mice showed widespread defects in procollagen III processing. Adamts2(-/-) mice had abnormal lungs, characterized by a decreased parenchymal density. However, the aorta and collagen fibrils in the aortic wall appeared normal. Although Adamts14 lacked developmental tissue-specific expression, it was co-expressed with Adamts2 in mature dermis, which possibly explains the presence of some processed skin procollagen in dermatosparaxis. The data show how evolutionarily related proteases with similar substrate preferences may have distinct biological roles owing to tissue-specific gene expression, and provide insights into collagen biosynthesis and the pathobiology of dermatosparaxis.     

A defect in a novel ADAMTS family member is the cause of the belted white-spotting mutation

Several features of the pigment defect in belted (bt) mutant mice suggest that it occurs as a result of a defect in melanocyte development that is unique from those described for other classical white-spotting mutations. We report here that bt mice carry mutations in Adamts20, a novel member of the ADAMTS family of secreted metalloproteases. Adamts20 shows a highly dynamic pattern of expression in the developing embryo that generally precedes the appearance of melanoblasts in the same region, and is not expressed in the migrating cells themselves. Adamts20 shows remarkable homology with GON-1, an ADAMTS family protease required for distal tip cell migration in C. elegans. Our results suggest that the role of ADAMTS proteases in the regulation of cell migration has been conserved in mammalian development.     

The proprotein convertase PC5A and a metalloprotease are involved in the proteolytic processing of the neural adhesion molecule L1

The transmembrane and multidomain neural adhesion molecule L1 plays important functional roles in the developing and adult nervous system. L1 is proteolytically processed at two distinct sites within the extracellular domain, leading to the generation of different fragments. In this report, we present evidence that the proprotein convertase PC5A is the protease that cleaves L1 in the third fibronectin type III domain, whereas the proprotein convertases furin, PC1, PC2, PACE4, and PC7 are not effective in cleaving L1. Analysis of mutations revealed Arg(845) to be the site of cleavage generating the N-terminal 140-kDa fragment. This fragment was present in the hippocampus, which expresses PC5A, but was not detectable in the cerebellum, which does not express PC5A. The 140-kDa L1 fragment was found to be tightly associated with the full-length 200-kDa L1 molecule. The complex dissociated from the membrane upon cleavage by a protease acting at a more membrane-proximal site of full-length L1. This proteolytic cleavage was inhibited by the metalloprotease inhibitor GM 6001 and enhanced by a calmodulin inhibitor. L1-dependent neurite outgrowth of cerebellar neurons was inhibited by GM 6001, suggesting that proteolytic processing of L1 by a metalloprotease is involved in neurite outgrowth.     

Cooperation of two ADAMTS metalloproteases in closure of the mouse palate identifies a requirement for versican proteolysis in regulating palatal mesenchyme proliferation

We have identified a role for two evolutionarily related, secreted metalloproteases of the ADAMTS family, ADAMTS20 and ADAMTS9, in palatogenesis. Adamts20 mutations cause the mouse white-spotting mutant belted (bt), whereas Adamts9 is essential for survival beyond 7.5 days gestation (E7.5). Functional overlap of Adamts9 with Adamts20 was identified using Adamts9(+/-);bt/bt mice, which have a fully penetrant cleft palate. Palate closure was delayed, although eventually completed, in both Adamts9(+/-);bt/+ and bt/bt mice, demonstrating cooperation of these genes. Adamts20 is expressed in palatal mesenchyme, whereas Adamts9 is expressed exclusively in palate microvascular endothelium. Palatal shelves isolated from Adamts9(+/-);bt/bt mice fused in culture, suggesting an intact epithelial TGFβ3 signaling pathway. Cleft palate resulted from a temporally specific delay in palatal shelf elevation and growth towards the midline. Mesenchyme of Adamts9(+/-);bt/bt palatal shelves had reduced cell proliferation, a lower cell density and decreased processing of versican (VCAN), an extracellular matrix (ECM) proteoglycan and ADAMTS9/20 substrate, from E13.5 to E14.5. Vcan haploinsufficiency led to greater penetrance of cleft palate in bt mice, with a similar defect in palatal shelf extension as Adamts9(+/-);bt/bt mice. Cell density was normal in bt/bt;Vcan(hdf)(/+) mice, consistent with reduced total intact versican in ECM, but impaired proliferation persisted in palate mesenchyme, suggesting that ADAMTS-cleaved versican is required for cell proliferation. These findings support a model in which cooperative versican proteolysis by ADAMTS9 in vascular endothelium and by ADAMTS20 in palate mesenchyme drives palatal shelf sculpting and extension.     

Intracellular maturation and transport of tumor necrosis factor alpha converting enzyme

The tumor necrosis factor alpha converting enzyme (TACE) activity is required for the shedding of a variety of biologically active membrane bound precursors. The activation of TACE necessitates the proteolytic cleavage of its prodomain, a process that was suggested to be catalyzed by the proprotein convertase furin. However, the involvement of furin in this activation process has never been experimentally demonstrated. We have shown that the furinlike cleavage site (R-V-K-R(214)) localized between the prodomain and the metalloprotease domain of TACE is the sole site that can be in vitro cleaved by furin. In Cos7 cells, the release of TACE-processed substrates was reduced by the overexpression of the furin-specific proprotein convertase inhibitor Portland alpha1-antitrypsin inhibitor, but the release of TACE-processed substrates was increased by overexpression of furin in LoVo cells (deficient in furin activity) in which a mature form of TACE was identified. The immature form of TACE was detected at the surface of LoVo cells and at the surface of Cos7 and HT29 cells upon proprotein convertase inhibition. These results suggest that furin is the major proprotein convertase involved in the maturation/activation of TACE which is not a prerequisite for its cell-surface expression.     

ADAMTSL-3/punctin-2, a novel glycoprotein in extracellular matrix related to the ADAMTS family of metalloproteases.

The complete primary structure of ADAMTSL-3/punctin-2, a novel member of the family designated ADAMTSL (a disintegrin-like and metalloprotease domain with thrombospondin type I motifs-like), was determined by cDNA cloning from a human placenta library. The predicted open reading frame encodes a protein of 1690 amino acids that has considerable similarity to ADAMTSL-1/punctin-1. These multi-domain proteins lack both a protease domain and a disintegrin-like domain but are remarkably similar in their domain organization to the ADAMTS proteases, hence the name ADAMTS-like. Punctin-2 contains thrombospondin type 1 repeats (TSRs), a cysteine-rich domain and a cysteine-free spacer domain in the precise order in which they occur in the ADAMTS proteases. However, the number and organization of the TSRs in punctin-2 is unique with respect to the ADAMTS proteases. Punctin-2 contains 13 TSRs arranged in two arrays separated by a region containing three immunoglobulin-like repeats. Northern blot analysis of RNA from human adult tissues demonstrated that ADAMTSL3 is widely expressed, with highest expression in liver, kidney, heart and skeletal muscle, whereas it is expressed at low levels in mouse embryos. We characterized two punctin-2 polyclonal antisera. Using these and a monoclonal antibody to a C-terminal myc tag, we show that in transfected COS-7 cells, punctin-2 is expressed as a 210-kDa glycoprotein that is located in the extracellular matrix. The domain structure of punctin-2 and its matrix localization suggest that it might play a role in cell-matrix interactions or in assembly of specific extracellular matrices.     

The secreted metalloprotease ADAMTS20 is required for melanoblast survival

ADAMTS20 (Adisintegrin-like and metalloprotease domain with thrombospondin type-1 motifs) is a member of a family of secreted metalloproteases that can process a variety of extracellular matrix (ECM) components and secreted molecules. Adamts20 mutations in belted (bt) mice cause white spotting of the dorsal and ventral torso, indicative of defective neural crest (NC)-derived melanoblast development. The expression pattern of Adamts20 in dermal mesenchymal cells adjacent to migrating melanoblasts led us to initially propose that Adamts20 regulated melanoblast migration. However, using a Dct-LacZ transgene to track melanoblast development, we determined that melanoblasts were distributed normally in whole mount E12.5 bt/bt embryos, but were specifically reduced in the trunk of E13.5 bt/bt embryos due to a seven-fold higher rate of apoptosis. The melanoblast defect was exacerbated in newborn skin and embryos from bt/bt animals that were also haploinsufficient for Adamts9, a close homolog of Adamts20, indicating that these metalloproteases functionally overlap in melanoblast development. We identified two potential mechanisms by which Adamts20 may regulate melanoblast survival. First, skin explant cultures demonstrated that Adamts20 was required for melanoblasts to respond to soluble Kit ligand (sKitl). In support of this requirement, bt/bt;Kit(tm1Alf)/+ and bt/bt;Kitl(Sl)/+ mice exhibited synergistically increased spotting. Second, ADAMTS20 cleaved the aggregating proteoglycan versican in vitro and was necessary for versican processing in vivo, raising the possibility that versican can participate in melanoblast development. These findings reveal previously unrecognized roles for Adamts proteases in cell survival and in mediating Kit signaling during melanoblast colonization of the skin. Our results have implications not only for understanding mechanisms of NC-derived melanoblast development but also provide insights on novel biological functions of secreted metalloproteases.     

ADAM-TS5, ADAM-TS6, and ADAM-TS7, novel members of a new family of zinc metalloproteases. General features and genomic distribution of the ADAM-TS family.

We report the primary structure of three novel, putative zinc metalloproteases designated ADAM-TS5, ADAM-TS6, and ADAM-TS7. All have a similar domain organization, comprising a preproregion, a reprolysin-type catalytic domain, a disintegrin-like domain, a thrombospondin type-1 (TS) module, a cysteine-rich domain, a spacer domain without cysteine residues, and a COOH-terminal TS module. These genes are differentially regulated during mouse embryogenesis and in adult tissues, with Adamts5 highly expressed in the peri-implantation period in embryo and trophoblast. These proteins are similar to four other cognate gene products, defining a distinct family of human reprolysin-like metalloproteases, the ADAM-TS family. The other members of the family are ADAM-TS1, an inflammation-induced gene, the procollagen I/II amino-propeptide processing enzyme (PCINP, ADAM-TS2), and proteins predicted by the KIAA0366 and KIAA0688 genes (ADAM-TS3 and ADAM-TS4). Individual ADAM-TS members differ in the number of COOH-terminal TS modules, and some have unique COOH-terminal domains. The ADAM-TS genes are dispersed in human and mouse genomes.     

ADAMTS7B, the full-length product of the ADAMTS7 gene, is a chondroitin sulfate proteoglycan containing a mucin domain

We have characterized ADAMTS7B, the authentic full-length protein product of the ADAMTS7 gene. ADAMTS7B has a domain organization similar to that of ADAMTS12, with a total of eight thrombospondin type 1 repeats in its ancillary domain. Of these, seven are arranged in two distinct clusters that are separated by a mucin domain. Unique to the ADAMTS family, ADAMTS7B is modified by attachment of the glycosaminoglycan chondroitin sulfate within the mucin domain, thus rendering it a proteoglycan. Glycosaminoglycan addition has potentially important implications for ADAMTS7B cellular localization and for substrate recognition. Although not an integral membrane protein, ADAMTS7B is retained near the cell surface of HEK293F cells via interactions involving both the ancillary domain and the prodomain. ADAMTS7B undergoes removal of the prodomain by a multistep furin-dependent mechanism. At least part of the final processing event, i.e. cleavage following Arg(220) (mouse sequence annotation), occurs at the cell surface. ADAMTS7B is an active metalloproteinase as shown by its ability to cleave alpha(2)-macroglobulin, but it does not cleave specific peptide bonds in versican and aggrecan attacked by ADAMTS proteases. Together with ADAMTS12, whose primary structure also predicts a mucin domain, ADAMTS7B constitutes a unique subgroup of the ADAMTS family.