The GPIbα–thrombin interaction: far from crystal clear

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.