The transmembrane domain of glycoprotein Ibβ is critical to efficient expression of glycoprotein Ib-IX Complex in the plasma membrane

Abstract

Lack of expression of glycoprotein (GP) Ib-IX-V complex in platelets often results from mutations in its three subunits: GP Ibα, GP Ibβ, or GP IX. The requirement of all three subunits in the efficient surface expression of the receptor complex has been reproduced in Chinese hamster ovary cells. Here, we probed the role of the transmembrane domains in expression of the GP Ib-IX complex and potential interactions between these domains. Replacing the transmembrane domains of either GP Ibα or GP Ibβ, but not that of GP IX, with unrelated sequences markedly diminished surface expression of the GP Ib-IX complex in transiently transfected Chinese hamster ovary cells. Replacement of the Ibβ transmembrane domain produced the largest effect. Furthermore, several single-site mutations in the Ibβ transmembrane domain were found to significantly decrease overall expression as well as surface expression of GP Ibα, probably by perturbing the interaction between the Ibα and Ibβ transmembrane domains and in turn reducing the stability of GP Ibα in the cell. Mutations S503V and S503L in the Ibα transmembrane domain partly reversed the expression-decreasing effect of mutation H139L, but not the others, in the Ibβ transmembrane domain, suggesting a specific interaction between these two polar residues. Together, our results have demonstrated the importance of the Ibβ transmembrane domain, through its interaction with the Ibα counterpart, to the proper assembly and efficient surface expression of the GP Ib-IX complex. © 2006 by The American Society for Biochemistry and Molecular Biology, Inc.