Localization and characterization of an α-thrombin-binding site on platelet glycoprotein Ibα

Glycoprotein (GP) Ibα is required for expression of the highest affinity α-thrombin-binding site on platelets, possibly contributing to platelet activation through a pathway involving cleavage of a specific receptor. This function may be important for the initiation of hemostasis and may also play a role in the development of pathological vascular occlusion. We have now identified a discrete sequence in the extracytoplasmic domain of GP Ibα, including residues 271-284 of the mature protein, which appears to be part of the high affinity α-thrombin-binding site. Synthetic peptidyl mimetics of this sequence inhibit α-thrombin binding to GP Ib as well as platelet activation and aggregation induced by subnanomolar concentrations of the agonist; they also inhibit α-thrombin binding to purified glycocalicin, the isolated extracytoplasmic portion of GP Ibα. The inhibitory peptides interfere with the clotting of fibrinogen by α-thrombin but not with the amidolytic activity of the enzyme on a small synthetic substrate, a finding compatible with the concept that the identified GP Ibα sequence interacts with the anion-binding exosite of α-thrombin but not with its active proteolytic site. The crucial structural elements of this sequence necessary for thrombin binding appear to be a cluster of negatively charged residues as well as three tyrosine residues that, in the native protein, may be sulfated. GP Ibα has no significant overall sequence homology with the thrombin inhibitor, hirudin, nor with the specific thrombin receptor on platelets; all three molecules, however, possess a distinct region rich in negatively charged residues that appear to be involved in thrombin binding. This may represent a case of convergent evolution of unrelated proteins for high affinity interaction with the same ligand.