Probing chemical and conformational differences in the resting and active conformers of platelet integrin...

Integrin alpha(IIb)beta(3) is the fibrinogen receptor that mediates platelet adhesion and aggregation. The ligand binding function of alpha(IIb)beta(3) is "activated" on the platelet surface by physiologic stimuli. Two forms of alpha(IIb)beta(3) can be purified from platelet lysates. These forms are facsimiles of the resting (Activation State-1 or AS-1) and the active (Activation State-2 or AS-2) conformations of the integrin found on the platelet surface. Here, the differences between purified AS-1 and AS-2 were examined to gain insight into the mechanism of activation. Four major findings are put forth. 1) The association rate (k(1)) between fibrinogen and the integrin is a key difference between AS-1 and AS-2. 2) Although the divalent ion Mn(2+) enhances the ligand binding function of AS-1, this ion is unable to convert AS-1 to AS-2. Therefore, its effect on integrin is unrelated to activation. 3) Peptide mass fingerprints indicate that the chemical structure of AS-1 and AS-2 are virtually identical, calling into question the idea that post-translational modifications are necessary for activation. 4) The two forms of alpha(IIb)beta(3) have significant conformational differences at three positions. These include the junction of the heavy and light chain of alpha(IIb), the divalent ion binding sites on alpha(IIb), and at a disulfide-bonded knot linking the amino terminus of beta(3) to the cysteine-rich domain. These observations indicate that integrin is activated by a series of specific conformational rearrangements in the ectodomain that increase the rate of ligand association.