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Blood, 15 January 2007, Vol. 109, No. 2, pp. 603-609.
Prepublished online as a Blood First Edition Paper on September 28, 2006; DOI 10.1182/blood-2006-05-024091.
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HEMOSTASIS, THROMBOSIS, AND VASCULAR BIOLOGY
Glycoprotein Ib forms disulfide bonds with 2 glycoprotein Ibß subunits in the resting platelet
Shi-Zhong Luo1,
Xi Mo1,
Vahid Afshar-Kharghan2,
Sankaranarayanan Srinivasan1,
José A. López3, and
Renhao Li1,
1 Center for Membrane Biology, Department of Biochemistry and Molecular Biology, The University of Texas Health Science Center at Houston;
2 Thrombosis Research Section, Department of Medicine, Baylor College of Medicine, Houston, TX;
3 Puget Sound Blood Center, Division of Hematology, Department of Medicine, University of Washington, Seattle
It is widely accepted that glycoprotein (GP) Ib contains one Ib and one Ibß subunit that are connected by a disulfide bond. It is unclear which Cys residue in Ib, C484 or C485, forms the disulfide bond with Ibß. Using mutagenesis studies in transfected Chinese hamster ovary (CHO) cells, we found that both C484 and C485 formed a disulfide bond with C122 in Ibß. In the context of isolated peptides containing the Ib or Ibß transmembrane domain and nearby Cys residue, C484 and C485 in the Ib peptide were both capable of forming a disulfide bond with the Ibß peptide. Furthermore, coimmunoprecipitation of epitope-tagged subunits showed that at least 2 Ibß subunits but only 1 Ib and 1 IX subunit were present in the GP Ib-IX complex. Finally, the size difference between GP Ib from transfected CHO cells and human platelets was attributed to a combination of sequence polymorphism and glycosylation difference in Ib, not the number of Ibß subunits therein. Overall, these results demonstrate that Ib is covalently connected to 2 Ibß subunits in the resting platelet, necessitating revision of the subunit stoichiometry of the GP Ib-IX-V complex. The ß2 composition in GP Ib may provide the basis for possible disulfide rearrangement in the receptor complex.
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