We have characterized ADAMTS7B, the authentic fulllength 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 ␣ 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.
The extracellular matrix (ECM)1 is an information-rich assembly influencing cell proliferation, apoptosis, and cell migration. Proteases have an essential role in modulating the environmental cues that ECM provides for tissue morphogenesis, homeostasis, and disease progression. Metalloproteases, especially matrix metalloproteases, have a conspicuous role in ECM degradation as well as in proteolysis of cell-surface and soluble proteins (1, 2). Another metalloprotease family, ADAM (a disintegrin and metalloprotease domain), contains transmembrane enzymes with a major role in ectodomain shedding of cell-surface molecules, but a negligible function in ECM proteolysis (3). The active site of ADAM proteases, unlike that of the matrix metalloproteases, is of the reprolysin (snake venom zinc metalloprotease) type (3). The recent discovery of the ADAMTS (a disintegrin-like and metalloprotease domain with thrombospondin type 1 motif) family brought to light metalloproteases that contain a reprolysintype catalytic site, but, unlike the ADAM proteases, are secreted enzymes with a prominent role in ECM proteolysis.ADAMTS proteases have a characteristic modular structure whose hallmark is the presence of one or more thrombospondin type 1 repeats (TSRs) (4). In the short period of time since the discovery of ADAMTS1 in 1997 (4), important functions have been attributed to a number of family members, and mutations of two of these enzymes have been shown to cause human genetic disorders. Inactivating mutations of ADAMTS13 cause inherited thrombocytopenic purpura due to a failure to process von Willebrand factor (5). Mutations of ADAMTS2, a procollagen aminopropeptidase, cause skin fragility in a var...