Assembly of a novel cartilage matrix protein filamentous network: Molecular basis of differential requirement of von Willebrand factor A domains

Cartilage matrix protein (CMP) is the prototype of the newly discovered mat rilin family, all of which contain von Willebrand factor A domains. Althoug h the function of matrilins remain unclear, we have shown that, in primary chondrocyte cultures, CMP (matrilin-1) forms a filamentous network which is made up of two types of filaments, a collagen-dependent one and a collagen -independent one. In this study, we demonstrate that the collagen-independe nt CMP filaments are enriched in pericellular compartments, extending direc tly from chondrocyte membranes. Their morphology can be distinguished from that of collagen filaments by immunogold electron microscopy, and mimicked by that of self-assembled purified CMP. The assembly of CMP filaments can o ccur from transfection of a wild-type CMP transgene alone in skin fibroblas ts, which do not produce endogenous CMP. Conversely, assembly of endogenous CMP filaments by chondrocytes can be inhibited specifically by dominant ne gative CMP transgenes. The two A domains within CMP serve essential but dif ferent functions during network formation. Deletion of the A2 domain conver ts the trimeric CMP into a mixture of monomers, dimers, and trimers, wherea s deletion of the Al domain does not affect the trimeric configuration. Thi s suggests that the A2 domain modulates multimerization of CMP. Absence of either A domain from CMP abolishes its ability to form collagen-independent filaments. in particular, Asp(22) in A1 and Asp(255) in A2 are essential; double point mutation of these residues disrupts CMP network formation. The se residues are part of the metal ion-dependent adhesion sites, thus a meta l ion-dependent adhesion site-mediated adhesion mechanism may be applicable to matrilin assembly. Taken together, our data suggest that CMP is a bridg ing molecule that connects matrix components in cartilage to form an integr ated matrix network.