Difference in interaction of fibronectin with type I collagen and type IV collagen

In our studies on fibronectin, difference in binding to type I collagen and type IV collagen was observed and analysed. Four different fragments, which consist of I6-II1-II2-I7-I8-I9, I6-II1-II2-I7, I6-II1-II2, and I8-I9 within the collagen binding domain, have been isolated from proteolytic digests of fibronectin. The N-terminal fragments of the collagen binding domain showed the binding affinity to both of type I and type IV collagens. On the other hand, the C-terminal portion of the domain, I8-I9, only bound to type I collagen. The newly developed monoclonal antibody FN 40, which recognizes type I9 homology repeat of the collagen binding domain, inhibited the binding of fibronectin to type I collagen in a dose-dependent manner. Three overlapping fragments, I6-II1-II2-I7, I6-II1-II2-I7-I8, and I7-I8-I9, which have been expressed in Escherichia coli, showed the similar binding affinity to type IV collagen, whereas the fragment containing type I9 repeat (I7-I8-I9) showed the significantly stronger binding activity to type I collagen. In addition, the expressed fragment lacking I9 competitively inhibited the binding of fibronectin to type IV collagen. In contrast, the interference of this fragment to type I collagen binding activity of fibronectin was not significant. These data indicate that the collagen binding domain contains at least 2 sites for interaction with each type of collagen and that type I9 repeat is important for type I collagen binding, whereas I7 is for type IV collagen binding. Chemical modification studies revealed that the three-dimensional structure of fibronectin is more important for type I collagen binding.