Human platelet glycoprotein V: Its role in enhancing expression of the glycoprotein Ib receptor

Platelet adhesion to an injured blood vessel well is a critical initiating step in hemostasis mediated by a four member receptor complex (glycoprotein Ib/V/IX) interacting with plasma von Willebrand factor (vWF). The function of the GPV subunit within this complex is presently undefined. To study the role of glycoprotein (GP) V within the GPIb receptor complex, we transfected the GPV subunit gene into a hematopoietic cell line that constitutively expresses the other three subunits (human erythroleukemia [HEL] cells). Using flow cytometry, we found transfected GPV was surface expressed in HEL cells; this, in turn, led to increased surface expression of the ligand-binding GPIbα and GPIX subunits. Radioligand binding assays showed that GPV-transfected HEL cells bound more vWF than their non- or mock-transfected counterparts. We employed confocal microscopy of GPV-transfected HEL cells to show that GPV colocalizes with GPIbα on the cell surface. These findings suggest that the GPV subunit plays a role within the GPIb receptor complex by enhancing Ibα surface expression.