CARTILAGE MATRIX PROTEIN FORMS A TYPE-II COLLAGEN-INDEPENDENT FILAMENTOUS NETWORK - ANALYSIS IN PRIMARY-CELL CULTURES WITH A RETROVIRUS EXPRESSION SYSTEM
Q. Chen et al., CARTILAGE MATRIX PROTEIN FORMS A TYPE-II COLLAGEN-INDEPENDENT FILAMENTOUS NETWORK - ANALYSIS IN PRIMARY-CELL CULTURES WITH A RETROVIRUS EXPRESSION SYSTEM, Molecular biology of the cell, 6(12), 1995, pp. 1743-1753
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.