TCB Publications - Abstract

David Craig, Mu Gao, Klaus Schulten, and Viola Vogel. Structural insights into how the MIDAS ion stabilizes integrin binding to an RGD peptide under force. Structure, 12:2049-2058, 2004.

CRAI2004A Integrin $\alpha_{V}\beta_{3}$ binds to extracellular matrix proteins through the tripeptide Arg-Gly-Asp (RGD). X-ray crystallography has previously revealed that this binding is mediated by a divalent cation located at the metal ion-dependent adhesion site (MIDAS) of the $\beta_3$-subunit, which coordinates to the carboxyl group of the aspartate amino acid of the RGD peptide (Asp$^{RGD}$). A dynamic picture of how the RGD- $\alpha_{V}\beta_{3}$ complex resists dissociation by mechanical forces is derived here from steered molecular dynamic (SMD) simulations. The headpiece of the $\alpha_{V}\beta_{3}$-integrin bound to an RGD-ligand is hydrated in a box of water molecules and the ligand is subsequently pulled away from its receptor. In all simulations, the major force peak correlated with the breaking of the contact between the Asp$^{RGD}$ and the MIDAS ion. RGD-binding to integrins does not involve a deep binding pocket that protects force-bearing contacts from attacks by free water but forms a shallow crevice at the interface between the $\alpha\beta$-subunits. We show here that the RGD- $\alpha_{V}\beta_{3}$ complex is stabilized from detachment by a single water molecule that is tightly coordinated to the divalent MIDAS ion, thereby blocking access of free water molecules to the critical force bearing interactions. The MIDAS motif is common to many proteins that contain the phylogenetically ancient von Willebrand A (vWA) domain, including integrins, the anthrax toxin receptor, various calcium and chloride channels, complement factors, protease inhibitors, as well as the family of vWA collagens. Whereas the regulatory functions of divalent ions have been well recognized, structural reasons of how they stabilize receptor-ligand interactions under load were not known. The functional role of single water molecules tightly coordinated to the MIDAS ion observed in the present study for $\alpha_{V}\beta_{3}$ might be a more general principle of how divalent cations stabilize protein-protein interactions against cell derived forces.

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