Summary
The GpIb-IX complex constitutes the major receptor for von Wille-brand factor (vWF)
on the surface of blood platelets. The vWF-binding site has been mapped to one of
the three constituent chains, Gplba. Surface expression of Gplba depends on the. correct
intracellular assembly with the Gplb(3 and GpIX chains. We have now grafted a portion
of the extracellular domain of Gplba containing the vWF binding site onto the transmembrane/intracellular
domain of the single chain surface molecule ICAM-1. Transient transfection of this
chimeric protein in COS cells resulted in surface expression as assessed by immunostain-ing
of live cells. Similar results were obtained after stable transfection into BHK cells.
Purified vWF bound to the surface of transfected cells in the presence of ristocetin
and botrocetin with a Kd of 52 ng/ml, comparable to the Kd for fixed platelets (65.5
ng/ml). This study indicates that functional expression of the vWF-binding domain
of GpIbα on the surface of mammalian cells can be obtained in the absence of GpIbβ
and GpIX. Furthermore, this model system simplifies existing methods for the assessment
of the functional consequences of mutations in Gplba as found in pseudo-von Willebrand
disease and Bemard-Soulier syndrome.
