Analysis of inter-subunit contacts reveals the structural malleability of extracellular domains in platelet glycoprotein Ib-IX complex

Authors

  • L. Zhou,

    1. Department of Pediatrics, Aflac Cancer and Blood Disorders Center, Emory University School of Medicine, Atlanta, GA, USA
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  • W. Yang,

    1. Department of Biochemistry and Molecular Biology, Center for Membrane Biology, University of Texas Health Science Center at Houston, Houston, TX, USA
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  • R. Li

    Corresponding author
    1. Department of Pediatrics, Aflac Cancer and Blood Disorders Center, Emory University School of Medicine, Atlanta, GA, USA
    • Correspondence: Renhao Li, Department of Pediatrics, Emory University School of Medicine, 2015 Uppergate Drive NE, Room 440, Atlanta, GA 30322, USA.

      Tel.: +1 404 727 8217; fax: +1 404 727 4859.

      E-mail: renhao.li@emory.edu

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  • Manuscript handled by: S. Watson
  • Final Decision: P. H. Reitsma 21 Oct 2013

Summary

Background

The glycoprotein (GP)Ib-IX complex is critical to hemostasis and thrombosis. Its proper assembly is closely correlated with its surface expression level and requires cooperative interactions among extracellular and transmembrane domains of Ibα, Ibβ and IX subunits. Two interfaces have been previously identified between the extracellular domains of Ibβ and IX.

Objective

To understand how extracellular domains interact in GPIb-IX.

Methods

The Ibβ extracellular domain (IbβE) or the IX counterpart (IXE) in GPIb-IX was replaced with a well-folded IbβE/IXE chimera called IbβEabc, and the effect of domain replacement on assembly and expression of the receptor complex in transiently transfected Chinese hamster ovary cells was analyzed.

Results

Replacing IXE with IbβEabc in GPIb-IX retained interface 1 but not interface 2 between the extracellular domains. While this domain replacement preserved complex integrity, the expression levels of Ibβ and Ibα were significantly reduced. Additional domain replacement with IbβEabc or IbβE in GPIb-IX produced the complex at disparate expression levels that cannot be simply explained by two separate interfaces. In particular, when IbβE in GPIb-IX was replaced by IbβEabc, Ibα and IX were expressed at approximately 70% of the wild-type level. Their levels were not reduced when IXE was changed further to IbβE.

Conclusions

Our results demonstrate the importance of the association between Ibβ and IX extracellular domains for complex assembly and efficient expression, and provide evidence for the structural malleability of these domains that may accommodate and propagate conformational changes therein.

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