Implications for collagen binding from the crystallographic structure of fibronectin 6FnI1-2FnII7FnI

Collagen and fibronectin (FN) are two abundant and essential components of the vertebrate extracellular matrix; they interact directly with cellular receptors and affect cell adhesion and migration. Past studies identified a FN fragment comprising six modules, 6FnI1-2FnII7-9FnI, and termed the gelatin binding domain (GBD) as responsible for collagen interaction. Recently, we showed that GBD binds tightly to a specific site within type I collagen and determined the structure of domains 8-9FnI in complex with a peptide from that site. Here, we present the crystallographic structure of domains 6FnI1-2FnII7FnI, which form a compact, globular unit through interdomain interactions. Analysis of NMR titrations with single-stranded collagen peptides reveals a dominant collagen interaction surface on domains 2FnII and 7FnI; a similar surface appears involved in interactions with triple-helical peptides. Models of the complete GBD, based on the new structure and the 8-9FnI-collagen complex show a continuous putative collagen binding surface. We explore the implications of this model using long collagen peptides and discuss our findings in the context of FN interactions with collagen fibrils.