A novel fibronectin binding site required for fibronectin fibril growth during matrix assembly

Fibronectin (FN) assembly into a fibrillar extracellular matrix is a stepwi se process requiring participation from multiple FN domains. Fibril formati on is regulated in part by segments within the first seven type III repeats (III1-7). To define the specific function(s) of this region, recombinant F Ns (recFNs) containing an overlapping set of deletions were tested for the ability to assemble into fibrils. Surprisingly, recFN lacking type III repe at III1 (FN Delta III1), which contains a cryptic FN binding site and has b een suggested to be essential for fibril assembly, formed a matrix identica l in all respects to a native FN matrix. Similarly, displacement of the cel l binding domain in repeats III9-10 to a position close to the NH2-terminal assembly domain, as well as a large deletion spanning repeats III4-7, had no effect on assembly. in contrast, two deletions that included repeat III2 , Delta III1-2 and Delta III2-5, caused significant reductions in fibril el ongation, although binding of FN to the cell surface and initiation of asse mbly still proceeded. Using individual repeats in binding assays, we show t hat III2 but not III1 contains an FN binding site. Thus, these results pinp oint repeat III2 as an important module for FN-FN interactions during fibri l growth.