NC4 Domain of Cartilage-specific Collagen IX Inhibits Complement Directly Due to Attenuation of Membrane Attack Formation and Indirectly through Binding and Enhancing Activity of Complement Inhibitors C4B-binding Protein and Factor H

Collagen IX containing the N-terminal noncollagenous domain 4 (NC4) is unique to cartilage and a member of the family of fibril-associated collagens with both collagenous and noncollagenous domains. Collagen IX is located at the surface of fibrils formed by collagen II and a minor proportion of collagen XI, playing roles in tissue stability and integrity. The NC4 domain projects out from the fibril surface and provides sites for interaction with...