Integrins, which are central to cell adhesion, are cell-surface molecules comprising a and b transmembrane subunits. Their extracellular domains bind to external ligands containing the Arg-Gly-Asp tripeptide motif, and this results in the canonical "outside-in" signaling common to many cell-surface receptors: Internal signals transmitted to their intracellular domains alter the ligand-binding competency of the external regions. Xiong et al. present the structure of the complex between the extracellular portions of the integrin aV-b3 and a cyclic peptide. The contacts made by the Arg-Gly-Asp residues lead to conformational changes in the relative orientations of the a and b subunits.
J. P. Xiong, T. Stehle, R. Zhang, A. Joachimiak, M. Frech, S. L. Goodman, M. A. Arnaout, Crystal structure of the extracellular segment of integrin V3 in complex with an Arg-Gly-Asp ligand. Science 296, 151-155 (2002). [Abstract] [Full Text]
