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

Fibronectin (FN) assembly into a fibrillar extracellular matrix is a stepwise process requiring participation from multiple FN domains. Fibril formation is regulated in part by segments within the first seven type III repeats (III_1–7). To define the specific function(s) of this region, recombinant FNs (recFNs) containing an overlapping set of deletions were tested for the ability to assemble into fibrils. Surprisingly, recFN lacking type III repeat III₁ (FNΔIII₁), which contains a cryptic FN binding site and has been suggested to be essential for fibril assembly, formed a matrix identical in all respects to a native FN matrix. Similarly, displacement of the cell binding domain in repeats III_9–10 to a position close to the NH₂-terminal assembly domain, as well as a large deletion spanning repeats III_4–7, had no effect on assembly. In contrast, two deletions that included repeat III₂, ΔIII_1–2 and ΔIII_2–5, caused significant reductions in fibril elongation, although binding of FN to the cell surface and initiation of assembly still proceeded. Using individual repeats in binding assays, we show that III₂ but not III₁ contains an FN binding site. Thus, these results pinpoint repeat III₂ as an important module for FN–FN interactions during fibril growth.