The metalloproteinase ADAMTS-5 (A disintegrin and metalloproteinase with thrombospondin motifs) degrades aggrecan, a proteoglycan essential for cartilage structure and function. ADAMTS-5 is the major aggrecanase in mouse cartilage, and is also likely to be the major aggrecanase in humans. ADAMTS-5 is a multidomain enzyme, but the function of the C-terminal ancillary domains is poorly understood. We show that mutant ADAMTS-5 lacking the catalytic domain, but with a full suite of ancillary domains inhibits wild type ADAMTS activity, in vitro and in vivo, in a dominant-negative manner. The data suggest that mutant ADAMTS-5 binds to wild type ADAMTS-5; thus we tested the hypothesis that ADAMTS-5 associates to form oligomers. Co-elution, competition, and in situ PLA experiments using full-length and truncated recombinant ADAMTS-5 confirmed that ADAMTS-5 molecules interact, and showed that the catalytic and disintegrin-like domains support these intermolecular interactions. Cross-linking experiments revealed that recombinantADAMTS-5formedlarge,reduction-sensitiveoligomers with a nominal molecular mass of ϳ400 kDa. The oligomers were unimolecular and proteolytically active. ADAMTS-5 truncates comprising the disintegrin and/or catalytic domains were able to competitively block full-length ADAMTS-5-mediated aggrecan cleavage, measured by production of the G1-EGE 373 neoepitope. These results show that ADAMTS-5 oligomerization is required for full aggrecanase activity, and they provide evidence that blocking oligomerization inhibits ADAMTS-5 activity. The data identify the surface provided by the catalytic and disintegrin-like domains of ADAMTS-5 as a legitimate target for the design of aggrecanase inhibitors.ADAMTS-5 (TS5) is one member of a family of secreted, multidomain, zinc-dependent proteinases known as A disintegrin and metalloproteinase with thrombospondin motifs (ADAMTS).2 TS5 is a major aggrecan-degrading enzyme (aggrecanase) (1) mediating proteolysis and loss of aggrecan from articular cartilage in arthritic diseases. TS5 inhibitors are currently in development for arthritis therapies (2, 3), although ADAMTS-4 (TS4) might also have a role in human joint disease (4 -6). LGHG (7, 8). Fragments arising from proteolysis at these sites have been detected in cartilage and synovial fluids from patients with arthritic diseases including rheumatoid arthritis, osteoarthritis, and juvenile idiopathic arthritis (9 -12).TS5 is a multidomain enzyme comprising an N-terminal pro (Pro)-domain, a catalytic (Cat) domain, a disintegrin (Dis)-like domain, two thrombospondin (TS)-type 1 motifs, a cysteinerich (CysR) region, and a spacer (Sp) domain as reviewed previously (13-15). It is expressed as a zymogen that requires cleavage by pro-protein convertases (16, 17) at a polyarginine consensus sequence to remove the pro-domain. Further processing at the C terminus by autocatalysis gives rise to multiple forms of TS5, and recombinant C-terminal truncates have altered enzyme activity and substrate specificity (18,19). The active and ...