Modeling the αIIbβ3 integrin solution conformation

The α_IIbβ₃ platelet integrin is the prototypical member of a widely distributed class of transmembrane receptors formed by the noncovalent association of α and β subunits. Electron microscopic (EM) images of the α_IIbβ₃ complex show an asymmetric particle with a globular domain from which two extended regions protrude to contact the lipid bilayer. Distance constraints provided by disulfide bond patterns, epitope mapping, and ligand mimetic cross-linking studies rather suggest a somewhat more compact conformation for the α_IIbβ₃ complex. We have studied the shape of detergent-solubilized α_IIbβ₃ by employing a low-resolution modeling procedure in which each polypeptide has been represented as an array of interconnected, nonoverlapping spheres (beads) of various sizes. The number, size, and three-dimensional relationships among the beads were defined either solely by dimensions obtained from published EM images of integrin receptors (EM models, 21 beads), or solely by interdomain constraints derived from published biochemical data (biochemical model, 37 beads). Interestingly, although no EM data were employed in its construction, the resulting overall shape of the biochemical model was still compatible with the EM data. Both kinds of models were then evaluated for their calculated solution properties. The more elongated EM models have diffusion and sedimentation coefficients that differ, at best, by +2% and -18% from the experimental values, determined, respectively, in octyl glucoside and Triton X-100. On the other hand, the parameters calculated for the more compact biochemical model showed a more consistent agreement with experimental values, differing by -7% (octyl glucoside) to -6% (Triton X-100). Thus, it appears that using the biochemical constraints as a starting point has resulted in not only a more detailed model of the detergent-solubilized α_IIbβ₃ complex, where the relative spatial location of specific domains the size of 5–10 kDa can be tentatively mapped, but in a model that can also reconcile the electron microscopy with the biochemical and the solution data.