The Importance of Detail: How Differences in Ligand Structures Determine Distinct Functional Responses in Integrin ?v?3

Ligand-based control of protein functional motions can provide novel opportunities in the study of fundamental biologicalmechanisms and in the development of novel therapeutics. Here, we address ligand-based modulation of integrin functions. Inhibitors ofintegrin ?v?3 are interesting anticancer agents but their molecular mechanisms are still unclear: peptides and peptidomimeticscharacterized by the RGD or isoDGR binding motifs have shown controversial agonist/antagonist effects. We investigate the differentialmechanisms of integrin activation/deactivation by three distinct ligands (cyclo-RGDf(NMe)V (Cilengitide), cyclo[DKP3-RGD],cyclo[DKP3-isoDGR]), through comparative analysis of ligand-controlled protein internal dynamics: while RGD facilitates the onset ofdynamic states leading to activation, isoDGR induces a diffuse rigidification of the complex consistent with antagonist activities.Computational predictions are experimentally probed by showing that antibody AP5, selective for active integrin conformations, bindsspecifically to the RGD complexes and not to the isoDGR one, supporting opposite functional roles for the two motifs targeting the samebinding site.