Structure-function analysis of Arg-Gly-Asp helix motifs in αvβ6 integrin ligands

Data relating to the structural basis of ligand recognition by integrins are limited. Here we describe the physical requirements for high affinity binding of ligands to αvβ6. By combining a series of structural analyses with functional testing, we show that 20-mer peptide ligands, derived from high affinity ligands of αvβ6 (foot-and-mouth-disease virus, latency associated peptide), have a common structure comprising an Arg-Gly-Asp motif at the tip of a hairpin turn followed immediately by a C-terminal helix. This arrangement allows two conserved Leu/ Ile residues at Asp⁺¹ and Asp⁺⁴ to be presented on the outside face of the helix enabling a potential hydrophobic interaction with the αvβ6 integrin, in addition to the Arg-Gly-Asp interaction. The extent of the helix determines peptide affinity for αvβ6 and potency as an αvβ6 antagonist. A major role of this C-terminal helix is likely to be the correct positioning of the Asp⁺¹ and Asp⁺⁴ residues. These data suggest an explanation for several biological functions of αvβ6 and provide a structural platform for design of αvβ6 antagonists.