The platelet integrin ?IIb?3 binds to a KQAGDV motif at the fibrinogen ?-chain C terminus and to RGD motifs present in loops in many extracellular matrix proteins. These ligands bind in a groove between the integrin ? and ?-subunits; the basic Lys or Arg side chain hydrogen bonds to the ?IIb-subunit, and the acidic Asp side chain coordinates to a metal ion held by the ?3-subunit. Ligand binding induces headpiece opening, with conformational change in the ?-subunit. During this opening, RGD slides in the ligand-binding pocket toward ?IIb, with movement of the ?I-domain ?1-?1 loop toward ?IIb, enabling formation of direct, charged hydrogen bonds between the Arg side chain and ?IIb. Here we test whether ligand interactions with ?3 suffice for stable ligand binding and headpiece opening. We find that the AGDV tetrapeptide from KQAGDV binds to the ?IIb?3 headpiece with affinity comparable with the RGDSP peptide from fibronectin. AGDV induced complete headpiece opening in solution as shown by increase in hydrodynamic radius. Soaking of AGDV into closed ?IIb?3 headpiece crystals induced intermediate states similarly to RGDSP. AGDV has very little contact with the ?-subunit. Furthermore, as measured by epitope exposure, AGDV, like the fibrinogen ? C-terminal peptide and RGD, caused integrin extension on the cell surface. Thus, pushing by the ?3-subunit on Asp is sufficient for headpiece opening and ligand sliding, and no pulling by the ?IIb subunit on Arg is required.