Synthesis, assembly, and intracellular transport of the platelet glycoprotein Ib-IX-V complex

The platelet glycoprotein lb-M-V complex plays critical roles in adhering platelets to sites of blood vessel injury and in platelet aggregation under high fluid shear stress. The complex is composed of four membrane-spanning polypeptides: glycoprotein (GP) Ib alpha, GP lb beta, GP IX, and GP V, Glycoprotein lb alpha contains a binding site for von Willebrand factor through which it mediates platelet adhesion; GP V is required for the complex to bind thrombin with high affinity; and both GP Ib beta and GP IX are necessary for efficient plasma membrane expression of the complex. To further define the roles of the individual polypeptide subunits in the biosynthesis and intracellular transport of the GP fb-M-V complex, we studied full and partial complexes expressed in heterologous mammalian cells. We found that the full complex was formed within minutes in the endoplasmic reticulum before being transported into the Golgi cisternae, Approximately 160 min were required for the complex to be fully processed and to appear on the plasma membrane. About 25% of GP Iba expressed as part of either a GP Ib-IX complex or a GP Ib-M-V complex was degraded through a nonlysosomal pathway, Over 60% of GP Ib alpha, however, was degraded when it was expressed in partial complexes with only GP Ib beta or GP IX, The increased degradation was blocked by treating cells either with brefeldin A to prevent the transport of proteins from the endoplasmic reticulum to the Golgi or with lysosomal inhibitors, indicating that GP Iba expressed in partial complexes was targeted to the lysosomes for degradation. These results indicate that the presence of both GP Ib beta and GP IX, but not the presence of GP V, is required for efficient processing and targeting of GP Iba to the plasma membrane. Absence of either GP Ib beta or GP IX increased the rate of GP Ib alpha degradation, providing an explanation for why mutation of their genes leads to deficient GP Iba expression and platelet adhesion in Bernard-Soulier syndrome, the deficiency disorder of the complex.