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© The Rockefeller University Press, 0021-9525/1998//767 $5.00
The Journal of Cell Biology, Volume 140, Number 4, , 1998 767-777

Article

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



Lilach Gilboa*, Rebecca G. Wells{ddagger},§, Harvey F. Lodish{ddagger},||, and Yoav I. Henis*

* Department of Neurobiochemistry, The George S. Wise Faculty of Life Sciences, Tel Aviv University, Tel Aviv 69978, Israel; {ddagger} 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 || Department of Biology, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139

Abstract. Transforming growth factor β (TGF-β) signaling involves interactions of at least two different receptors, types I (TβRI) and II (TβRII), which form ligand-mediated heteromeric complexes. Although we have shown in the past that TβRII in the absence of ligand is a homodimer on the cell surface, TβRI 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-β signaling and regulation, emphasizing the importance of a detailed understanding of the homooligomerization of TβRI or TβRII. 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 TβRI and TβRII 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 TβRI 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-β1 is able to promote dimerization of the type II receptor ectodomain. These findings may have important implications for the regulation of TGF-β signaling.

1. Abbreviations used in this paper: {alpha}-HA, mouse monoclonal antibody which recognizes a specific epitope of the HA protein; {alpha}-myc, mouse monoclonal antibody which recognizes a specific c-myc sequence; {alpha}-IIC, a polyclonal rabbit antiserum raised against the COOH-terminal 16 amino acids of TβRII; I-SF, soluble HA-tagged TβRI ectodomain; II-SF, soluble HA-tagged TβRII ectodomain; ECL, enhanced chemiluminescence; Endo H, endoglycosidase H; Epo, erythropoietin; G{alpha}M, goat IgG anti– mouse IgG; HA, influenza hemagglutinin; Octyl-POE, n-octyl-polyoxyethylene; TGF-β, transforming growth factor β; TβRI, TGF-β type I receptor; TβRII, TGF-β type II receptor.

We thank Professor Z. Kam (Weizmann Institute, Israel) for his invaluable help with image analysis, and Orit Gutman (Tel Aviv University) and Haya Yankelev (Whitehead Institute) for expert technical assistance.

This research was supported in part by a grant from the Israel Science Foundation administered by the Israel Academy of Sciences and Humanities (to Y.I. Henis), by a project grant from the Israel Cancer Research Fund (Y.I. Henis), and by National Institutes of Health Grants CA63260 (to H.F. Lodish) and DK02290 (to R.G. Wells).

Lilach Gilboa and Rebecca G. Wells contributed equally to this work.

Address all correspondence to Harvey F. Lodish, The Whitehead Institute, Nine Cambridge Center, Cambridge, MA 02142. Tel.: (617) 258-5216. Fax: (617) 258-6768. E-mail: lodish{at}wi.mit.edu


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