CARTILAGE MATRIX PROTEIN FORMS A TYPE-II COLLAGEN-INDEPENDENT FILAMENTOUS NETWORK - ANALYSIS IN PRIMARY-CELL CULTURES WITH A RETROVIRUS EXPRESSION SYSTEM

Cartilage matrix protein (CMP) is expressed specifically in mature car tilage and consists of two von Willebrand factor A domains (CMP-A1 and CMP-A2) that are separated by an epidermal growth factor-like domain, and a coiled-coil tail domain at the carboxyl terminal end. We have s hown previously that CMP interacts with type II collagen-containing fi brils in cartilage. In this study, we describe a type II collagen-inde pendent CMP filament and we analyze the structural requirement for the formation of this type of filament. Recombinant wild-type CMP and two mutant forms were expressed in chick primary cell cultures using a re trovirus expression system. In chondrocytes, the wildtype virally enco ded CMP is able to form disulfide bonded trimers and to assemble into filaments. Filaments also form with CMP whose Cys(455) and Cys(457) in the tail domain were mutagenized to prevent interchain disulfide bond formation. Therefore, intermolecular disulfide bonds are not necessar y for the assembly of CMP into filaments. Both the wild-type and the d ouble cysteine mutant also form filaments in fibroblasts, indicating t hat chondrocyte-specific factors are not required for filament formati on. A truncated form of CMP that consists only of the CMP-A2 domain an d the tail domain can form trimers but fails to form filaments, indica ting that the deleted CMP-A1 domain and/or the epidermal growth factor domain are necessary for filament assembly but not for trimer formati on. Furthermore, the expression of the virally encoded truncated CMP i n chondrocyte culture disrupts endogenous CMP filament formation. Toge ther these data suggest a role for CMP in cartilage matrix assembly by forming filamentous networks that require participation and coordinat ion of individual domains of CMP.