STRUCTURAL REQUIREMENTS FOR FIBROMODULIN BINDING TO COLLAGEN AND THE CONTROL OF TYPE-I COLLAGEN FIBRILLOGENESIS - CRITICAL ROLES FOR DISULFIDE BONDING AND THE C-TERMINAL REGION

Fibromodulin belongs to the family of small, leucine-rich proteoglycan s which have been reported to interact with collagens and to inhibit t ype I collagen fibrillogenesis. Decorin and fibromodulin exhibit a not iceable degree of sequence similarity. However, as previously reported [Font,., Eichenberger, D., Rosenberg, L. M. & van der Rest, M. (1996) Matrix Biol. 15, 341-348] the domains of these molecules implicated i n the interactions with type XII and type XIV collagens are different, these being the dermatan sulphate/chondroitin sulphate chain for deco rin and the core protein for fibromodulin. At the present time the fib romodulin domains implicated in the interactions with fibrillar collag ens remain unknown. In experiments reported here, we have sought to id entify the structural requirements for fibromodulin interaction with c ollagen and for the control of type I collagen fibrillogenesis. Circul ar dichroism spectra and fibrillogenesis inhibition studies show that fibromodulin structure and its collagen fibrillogenesis control functi on are strictly dependent on the presence of intact disulphide bridge( s). In addition, we show that the binding of fibromodulin (or fibromod ulin-derived fragments) to type I collagen is not necessarily correlat ed with fibrillogenesis inhibition. To isolate fibromodulin domains, t he native proteoglycan was submitted to mild proteolysis. We have isol ated an a-chymotrypsin-resistant fragment which contains the bulk of t he N-terminal and central region of the molecule including the leucine -rich repeats 4 and 6 reported for decorin to be involved in type I co llagen binding. This fragment does not bind to type I collagen. Using enzymes with different specificities, a number of large fragments of f ibromodulin were obtained, suggesting a compact structure for this mol ecule which is relatively resistant to proteolysis. None of these N-gl ycosylated fragments were able to bind to type I collagen in co-sedime ntation experiments. Taken together these results suggest that fibromo dulin-type I collagen interactions leading to fibrillogenesis inhibiti on require more than one binding domain. One of these domains could be the C-terminal end of the molecule containing the disulphide loop whi ch is absent in the chymotrypsin-resistant fragment.