Requirement of leucine-rich repeats of glycoprotein (GP) Ibα for shear- dependent and static binding of von willebrand factor to the platelet membrane GP Ib-IX-V complex

The platelet glycoprotein (GP) Ib-IX-V complex mediates adhesion to von Willebrand factor (vWf) in (patho)physiologic thrombus formation. The vWf- binding site on GP Ib-IX-V is within the N-terminal 282 residues of GP Ibα, which consist of an N-terminal flanking sequence (His-1-IIe-35), 7 leucine- rich repeats (Leu-36-Ala200), a C-terminal flank (Phe-201-Gly268), and a sulfated tyrosine sequence (Asp-269-Glu-282). We have used mammalian cell expression of canine-human chimeras of GP Ibα, corresponding to precise structural boundaries, to demonstrate the first specific requirement for individual leucine-rich repeats for binding of vWf either induced by a modulator, ristocetin, or under hydrodynamic flow. Implicit in this approach was that the GP Ibα chimeras retained a functional conformation, a supposition confirmed by analyzing restoration of function to reversed human- canine chimeras and demonstrating that all chimeras bound vWf activated by botrocetin, a modulator that is indiscriminate between species. Leucine-rich repeats 2, 3, and 4 of GP Ibα were identified as being critical for vWf adhesion to GP Ib-IX-V.