Abstract
Chondroadherin is a cell binding, leucine-rich repeat protein found in the territorial matrix of articular cartilage. Several members of the leucine-rich repeat protein family present in the extracellular matrix of e.g. cartilage have been shown to interact with collagen and influence collagen fibrillogenesis. We show that complexes of monomeric collagen type II and chondroadherin can be released under non-denaturing conditions from articular cartilage treated with p-aminophenylmercuric acetate to activate resident matrix metalloproteinases. Purified complexes as well as complexes formed in vitro between recombinant chondroadherin and collagen type II were studied by electron microscopy. Chondroadherin was shown to bind to two sites on collagen type II. The interaction was characterized by surface plasmon resonance analysis showing K(D) values in the nanomolar range. Both chondroadherin and collagen interact with chondrocytes, partly via the same receptor, but give rise to different cellular responses. By also interacting with each other, a complex system is created which may be of functional importance for the communication between the cells and its surrounding matrix and/or in the regulation of collagen fibril assembly.
Highlights
The major constituents of the extracellular matrix of cartilage are type II collagen, proteoglycans, and a number of proteins including some of the minor collagens
We show that complexes of monomeric collagen type II and chondroadherin can be released under non-denaturing conditions from articular cartilage treated with p-aminophenylmercuric acetate to activate resident matrix metalloproteinases
The matrix macromolecules are assembled in multicomponent networks where the interactions between the matrix constituents are essential for the maintenance of tissue integrity [1]
Summary
The major constituents of the extracellular matrix of cartilage are type II collagen, proteoglycans, and a number of proteins including some of the minor collagens. In chondroadherin the leucine-rich sequence is repeated 11 times This protein differs from the other members of this family of extracellular matrix proteins having two disulfide-bonded loops in the C-terminal region and by lacking any N-terminal extension. The surface of the fibrils as well as of the collagen molecules themselves has properties allowing tight binding of a number of other matrix proteins. These include LRR proteins such as decorin [13], fibromodulin [14], and lumican [15] as well as other collagens such as collagen type IX [16].
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