Association of chondroadherin with collagen type II

Chondroadherin is a cell binding, leucine-rich repeat protein found in the territorial matrix of articular cartilage. Several members of the leucine-r ich repeat protein family present in the extracellular matrix of e.g. carti lage. have been shown to interact with collagen and influence collagen fibr illogenesis. We show that complexes of monomeric collagen type II and chond roadherin can be released under non-denaturing conditions from articular ca rtilage treated with p-aminophenylmercuric acetate to activate resident mat rix metalloproteinases. Purified complexes as well as complexes formed in v itro between recombinant chondroadherin and collagen type II were studied b y electron microscopy. Chondroadherin was shown to bind to two sites on col lagen type II. The interaction was characterized by surface plasmon resonan ce analysis showing KD values in the nanomolar range. Both chondroadherin a nd collagen interact with chondrocytes, partly via the same receptor, but g ive rise to different cellular responses. By also interacting with each oth er, a complex system is created which may be of functional importance for t he communication between the cells and its surrounding matrix and/or in the regulation of collagen fibril assembly.