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J. Cell Biol., Volume 140, Number 4, February 23, 1998 767-777

Oligomeric Structure of Type I and Type II Transforming Growth Factor beta  Receptors: Homodimers Form in the ER and Persist at the Plasma Membrane

Lilach Gilboa,* Rebecca G. Wells,Dagger § Harvey F. Lodish,Dagger par and Yoav I. Henis*

* Department of Neurobiochemistry, The George S. Wise Faculty of Life Sciences, Tel Aviv University, Tel Aviv 69978, Israel; Dagger  The Whitehead Institute for Biomedical Research, Cambridge, Massachusetts 02142; § Department of Medicine, Brigham and Women's Hospital and Harvard Medical School, Boston, Massachusetts 02115; and par  Department of Biology, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139

Abstract. Transforming growth factor beta  (TGF-beta ) signaling involves interactions of at least two different receptors, types I (Tbeta RI) and II (Tbeta RII), which form ligand-mediated heteromeric complexes. Although we have shown in the past that Tbeta RII in the absence of ligand is a homodimer on the cell surface, Tbeta RI has not been similarly investigated, and the site of complex formation is not known for either receptor. Several studies have indicated that homomeric interactions are involved in TGF-beta signaling and regulation, emphasizing the importance of a detailed understanding of the homooligomerization of Tbeta RI or Tbeta RII. Here we have combined complementary approaches to study these homomeric interactions in both naturally expressing cell lines and cells cotransfected with various combinations of epitope-tagged type I or type II receptors. We used sedimentation velocity of metabolically labeled receptors on sucrose gradients to show that both Tbeta RI and Tbeta RII form homodimer-sized complexes in the endoplasmic reticulum, and we used coimmunoprecipitation studies to demonstrate the existence of type I homooligomers. Using a technique based on antibody-mediated immunofluorescence copatching of receptors carrying different epitope tags, we have demonstrated ligand-independent homodimers of Tbeta RI on the surface of live cells. Soluble forms of both receptors are secreted as monomers, indicating that the ectodomains are not sufficient to mediate homodimerization, although TGF-beta 1 is able to promote dimerization of the type II receptor ectodomain. These findings may have important implications for the regulation of TGF-beta signaling.


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