The purpose of this investigation was to determine what structural changes convert 'inert' αIIbβ3 integrins into 'activated' high-affinity receptors for adhesive proteins. Light scattering, analytical ultracentrifugation, electron microscopy, and molecular modeling were used to probe the conformational states of the αIIbβ3 integrin. Isolated from human blood platelets in octyl glucoside, the αIIbβ3 complex behaved as an asymmetric 230 kDa macromolecule with a z-average translational diffusion coefficient of 2.9 F and a weight-average sedimentation coefficient of 7.7 S. Dynamic light scattering showed that ligand-mimetic peptides (RGDX, X = F, W, S) caused prompt, concentration-dependent increases in the Stokes radius (R(s)) of the αIIβ3 complex, whereas control peptides of reversed sequence (XDGR, X = F, W, S) had no significant effect. Sedimentation velocity data coupled with time-derivative analyses showed that RGDX peptides shifted the distribution of αIIbβ3 sedimenting species toward smaller s values. Sedimentation equilibrium measurements indicated that a slower increase in the αIIbβ3 molecular weight distribution took place in the presence of RGDX ligand-mimetics. Electron microscopy showed a split of αIIbβ3's globular domain into two distinct nodules in the presence of RGDX peptides; oligomers joined through their stalk regions were seen frequently. These observations suggest that receptor occupancy by ligand-mimetic RGDX peptides is tightly coupled to relatively large changes in the structure of the αIIbβ3 complex, αIIβ3 bead models were developed to describe quantitatively the ligand- induced transition from a 'closed' to an 'open' integrin conformation and the limited oligomerization that follows. This provides a new mechanistic framework for understanding integrin activation and the formation of signaling clusters on the surface of stimulated platelets.
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