Defining the topology of integrin α5β1-fibronectin interactions using inhibitory anti-α5 and anti-β1 monoclonal antibodies: Evidence that the synergy sequence of fibronectin is recognized by the amino-terminal repeats of the α5 subunit

The high affinity interaction of integrin α5β1 with the central cell binding domain (CCBD) of fibronectin requires both the Arg-Gly-Asp (RGD) sequence (in the 10th type III repeat) and a second site (in the adjacent 9th type III repeat) which synergizes with RGD. We have attempted to map the fibronectin binding interface on α5β1 using monoclonal antibodies (mAbs) that inhibit ligand recognition. The binding of two anti-α5 mAbs (P1D6 and JBS5) to α5β1 was strongly inhibited by a tryptic CCBD fragment of fibronectin (containing both synergy sequence and RGD) but not by GRGDS peptide. Using recombinant wild type and mutated fragments of the CCBD, we show that the synergy region of the 9th type III repeat is involved in blocking the binding of P1D6 and JBS5 to α5β1. In contrast, binding of the anti-β1 mAb P4C10 to α5β1 was inhibited to a similar extent by GRGDS peptide, the tryptic CCBD fragment, or recombinant proteins lacking the synergy region, indicating that the RGD sequence is involved in blocking P4C10 binding. P1D6 inhibited the interaction of a wild type CCBD fragment with α5β1 but bad no effect on the binding of a mutant fragment that lacked the synergy region. The epitopes of P1D6 and JBS5 mapped to the NH₂- terminal repeats of the α5 subunit. Our results indicate that the synergy region is recognized primarily by the α5 subunit (in particular by its NH₂- terminal repeats) but that the β1 subunit plays the major role in binding of the RGD sequence. These findings provide new insights into the mechanisms, specificity, and topology of integrin-ligand interactions.