GP-Ib alpha/CD42b

Glycoprotein Ib alpha (CD42b), the product of GP1BA, is the ligand-capturing extracellular giant of the GPIb-IX-V complex, extended by a mucin macroglycopeptide stalk. Its N-terminal flanking domain binds the A1 domain of von Willebrand factor, enabling platelet tethering at arterial shear, and the same region also accommodates thrombin, thrombospondin-1, P-selectin and coagulation factors XI and XII. Gain-of-function mutations in this domain cause platelet-type von Willebrand disease, while biallelic loss of GP1BA yields Bernard-Soulier syndrome. The shed ectodomain, glycocalicin, circulates in plasma and is measured as a marker of platelet turnover. CD42b is thus the principal receptor through which platelets sense vascular injury before integrin-mediated aggregation.