The GPIbα–thrombin interaction: far from crystal clear

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Abstract

The interaction of thrombin with platelet glycoprotein (GP)-Ibα has been well demonstrated. However, recent data have provided new insights into the GPIb–thrombin interaction. GPIb-clustering, which seems to be required for signal transduction, might be achieved by removal of GPV from the complex. In addition, the GPIbα subunits might need to be relatively mobile, as would occur in rafts or with GPIbα that has dissociated from the cytoskeleton. Finally, by resolving the crystal structures, two groups have identified different interaction sites in both thrombin and GPIbα that could be involved in cross-linking. Our direct comparison of the two structures reveals that, whereas one thrombin molecule binds to exactly the same site in GPIbα in both crystals, the other does not. Nevertheless, present biochemical and structural data complement each other well and help to clarify how GPIb might facilitate platelet activation by thrombin.

Section snippets

GPIbα as a thrombin receptor

Although GPIbα was the first thrombin receptor to be identified on the platelet surface [7], an understanding of its mechanism-of-action remains elusive.

Thrombin binds to human platelets with either high affinity (Kd=0.5 nmol l−1) or moderate affinity (Kd=50 nmol l−1) 23, 24, with the former corresponding to GPIbα binding 25, 26. In addition, GPV modulates the high-affinity thrombin-binding site on GPIbα [27]. However, it remains puzzling why only 105–1050 high-affinity thrombin-binding sites on

Interaction sites in GPIbα and thrombin

Many studies have identified sites in thrombin and GPIbα that could account for their interaction. However, several studies have identified different sites in both proteins. Furthermore, two groups recently resolved the crystal structure of the N-terminal fragment of GPIbα in complex with thrombin, each showing different interaction sites. A direct comparison of both crystal structures reveals a striking similarity (see later).

Two crystal structures of the GPIbα–thrombin complex

Crystal structures of a complex between thrombin and a GPIbα fragment were published in the same issue of Science by Celikel et al. [59] and Dumas et al. [60]. These groups of investigators used N-terminal GPIbα fragments of comparable length that included the anionic region, with sulfated tyrosine residues at their C terminus. These GPIbα fragments were mixed with thrombin and a 1:1 complex was isolated by gel filtration and crystallized.

Concluding remarks

Although the thrombin receptor function of GPIbα has long been recognized, the precise physiological role and the mechanism and contribution of GPIbα in thrombin-mediated platelet activation remained unclear. Recently, new data became available that shed light on both the signal transduction mechanisms and the molecular interactions involved.

The emerging picture is that GPIbα-clustering seems to be required to achieve signal transduction. This cross-linking can be achieved by removal of GPV

Acknowledgements

K.V. and H.U. are postdoctoral fellows of the Fonds voor Wetenschappelijk Onderzoek Vlaanderen Flanders and Katholieke Universiteit Leuven Research Councils, respectively.

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