GP-Ib, a surface membrane protein of platelets, participates in the formation of platelet plugs by binding to the A1 domain of vWF, which is already bound to the subendothelium. Two GP-Ib beta are disulfide-linked to one GP-Ib alpha. GP-IX is complexed with the GP-Ib heterodimer via a non covalent linkage. Interacts with FLNB. Interacts with FLNA (via filamin repeats 4, 9, 12, 17, 19, 21, and 23). O-glycosylated. Glycocalicin is the product of a proteolytic cleavage/shedding, catalyzed by ADAM17, which releases most of the extracellular domain. Binding sites for vWF and thrombin are in this part of the protein. Platelet activation apparently involves disruption of the macromolecular complex of GP-Ib with the platelet glycoprotein IX (GP-IX) and dissociation of GP-Ib from the actin-binding protein.