PMID: 21555542

Authors:

Zarpellon A, Celikel R, Roberts JR, McClintock RA, Mendolicchio GL, Moore KL, Jing H, Varughese KI, Ruggeri ZM

Title:

Binding of {alpha}-thrombin to surface-anchored platelet glycoprotein Ib{alpha} sulfotyrosines through a two-site mechanism involving exosite I.

Journal:

Proc Natl Acad Sci U S A. 2011 May 24;108(21):8628-33. Epub 2011 May 9.

Abstract:

The involvement of exosite I in alpha-thrombin (FIIa) binding to platelet glycoprotein Ibalpha (GPIbalpha), which could influence interactions with other substrates, remains undefined. To address the problem, we generated the GPIbalpha amino terminal domain (GPIbalpha-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 GPIbalpha-N. Mutating Tyr(276), which mostly contacts exosite II residues, markedly reduced FIIa interaction with both soluble and immobilized GPIbalpha-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 GPIbalpha-N 2 to 6-fold, respectively. Moreover, three exosite I ligands-aptamer HD1, hirugen, and lepirudin-did not interfere with soluble FIIa complexing to GPIbalpha-N, excluding that their binding caused allosteric effects influencing the interaction; nonetheless, all impaired FIIa binding to immobilized GPIbalpha-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 GPIbalpha molecules. Through exosite engagement, GPIbalpha may influence FIIa-dependent processes relevant to hemostasis and thrombosis.