Modeling the α_IIbβ₃ integrin solution conformation

Abstract

The (~l I b P 3 platelet integrin is the prototypical member of a widely distributed class of transmembrane receptors formed by the noncovalent association of a and fi subunits. Electron microscopic (EM) images of the a[& 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 a r [ b & complex.We have studied the shape of detergent-solubilized a l I b & 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 ~111bP3 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.

The Disintegrin Echistatin Stabilizes Integrin αIIbβ3's Open Conformation and Promotes Its Oligomerization

“…For example, If echistatin binding to aIIbb3's ectodomain were to induce a conformational change that propagates "downward" leading to either release the "deadbolt" proposed to lock an integrin into its bent conformation, 2 or to disrupt interactions between its cytoplasmic domains, 27 the resultant increase in hydrodynamic volume could account for the slower-sedimenting conformers. 57 The magnitude of these echistatininduced conformational perturbations are comparable to those we have obtained with linear RGDX peptides, 10 but are two-to threefold greater than the effects of tirofiban, 32 a non-peptide integrin antagonist designed to replicate the RGD spatial charge. Tirofiban's smaller size and higher affinity for the aIIbb3 integrin 32 could explain why its effects on aIIbb3 conformation were smaller than echistatin's.…”

Integrin-mediated signal transduction

“…Disulphide bond arrangements and intersubunit contacts of proteolytic fragments of hIIbi3 have been proposed [11]. These models relate to biophysical and electron microscopy data [12]. Integrins are subject to disulphide bond rearrangements and alternative splicing [10].…”

Expression and Function of Calcium Binding Domain Chimeras of the Integrins αIIb and α5

“…Previous studies have implicated multiple ligand interaction areas on both subunits, leading to models in which the ligand binding pocket is proposed to be formed by amino acids contributed by both subunits (12,13). The ␣ IIb ␤ 3 complex must undergo a conformational change before it is capable of high affinity binding of fibrinogen, but the resting complex is still capable of binding small peptide ligands.…”