The involvement of exosite I in α-
thrombin (
FIIa) binding to
platelet glycoprotein Ibα (GPIbα), which could influence interactions with other substrates, remains undefined. To address the problem, we generated the GPIbα amino terminal domain (GPIbα-N) fully sulfated on three
tyrosine residues and solved the structure of its complex with
FIIa. We found that
sulfotyrosine (Tys) 278 enhances the interaction mainly by establishing contacts with exosite I. We then evaluated how substituting
tyrosine with
phenylalanine, which cannot be sulfated, affects
FIIa binding to soluble or surface-immobilized GPIbα-N. Mutating Tyr(276), which mostly contacts exosite II residues, markedly reduced
FIIa interaction with both soluble and immobilized GPIbα-N; mutating Tyr(278) or Tyr(279), which mostly contact exosite I residues, reduced
FIIa complexing in
solution by 0-20% but affinity for immobilized GPIbα-N 2 to 6-fold, respectively. Moreover, three exosite I
ligands--aptamer HD1,
hirugen, and
lepirudin--did not interfere with soluble
FIIa complexing to GPIbα-N, excluding that their binding caused allosteric effects influencing the interaction; nonetheless, all impaired
FIIa binding to immobilized GPIbα-N and platelet GPIb nearly as much as aptamer HD22 and
heparin, both exosite II
ligands. Bound HD1 and
hirugen alter Trp(148) orientation in a loop near exosite I preventing contacts with the
sulfate oxygen atoms of Tys(279). These results support a mechanism in which binding occurs when the two exosites of one
FIIa molecule independently interact with two immobilized GPIbα molecules. Through exosite engagement, GPIbα may influence
FIIa-dependent processes relevant to hemostasis and
thrombosis.