Heparin modulates the conformation and signaling of platelet integrin ?IIb?3.

ABSTRACT: The glycosaminoglycan heparin has been shown to bind to platelet integrin ?IIb?3 and induce platelet activation and aggregation, although the relationship between binding and activation is unclear. We analyzed the interaction of heparin and ?IIb?3 in detail, to obtain a better understanding of the mechanism by which heparin acts on platelets.We assessed conformational changes in ?IIb?3 by flow cytometry of platelets exposed to unfractionated heparin. In human platelets and K562 cells engineered to express ?IIb?3, we assayed the effect of heparin on key steps in integrin signaling: phosphorylation of the ?3 chain cytoplasmic tail, and activation of src kinase. We measured the heparin binding affinity of purified ?IIb?3, and of recombinant fragments of ?IIb and ?3, by surface plasmon resonance.Heparin binding results in conformational changes in ?IIb?3, similar to those observed upon ligand binding. Heparin binding alone is not sufficient to induce tyrosine phosphorylation of the integrin ?3 cytoplasmic domain, but the presence of heparin increased both ?3 phosphorylation and src kinase activation in response to ligand binding. Specific recombinant fragments derived from ?IIb bound heparin, while recombinant ?3 did not bind. This pattern of heparin binding, compared to the crystal structure of ?IIb?3, suggests that heparin-binding sites are located in clusters of basic amino acids in the headpiece and/or leg domains of ?IIb. Binding of heparin to these clusters may stabilize the transition of ?IIb?3 to an open conformation with enhanced affinity for ligand, facilitating outside-in signaling and platelet activation.

SUBMITTER: Yagi M 

PROVIDER: S-EPMC3340540 | biostudies-literature | 2012 Jun

REPOSITORIES: biostudies-literature