No abstract

Receptor dimerization is generally considered to be the primary signaling event upon binding of a growth factor to its receptor at the cell surface. Little, however, is known about the precise molecular details of ligand-induced receptor dimerization, except for studies of the human growth hormone (hGH) receptor. We have analyzed the binding of epidermal growth factor (EGF) to the extracellular domain of its receptor (sEGFR) using titration calorimetry, and the resulting dimerization of sEGFR using small-angle X-ray scattering. EGF induces the quantitative formation of sEGFR dimers that contain two EGF molecules. The data obtained from the two approaches suggest a model in which one EGF monomer binds to one sEGFR monomer, and that receptor dimerization involves subsequent association of two monomeric (1:1) EGF-sEGFR complexes. Dimerization may result from bivalent binding of both EGF molecules in the dimer and/or receptor-receptor interactions. The requirement for two (possibly bivalent) EGF monomers distinguishes EGF-induced sEGFR dimerization from the hGH and interferon-gamma receptors, where multivalent binding of a single ligand species (either monomeric or dimeric) drives receptor oligomerization. The proposed model of EGF-induced sEGFR dimerization suggests possible mechanisms for both ligand-induced homo- and heterodimerization of the EGFR (or erbB) family of receptors.

Most growth factors and cytokines activate their receptors by inducing dimerization upon binding. We have studied binding of the dimeric cytokine stem cell factor (SCF) to the extracellular domain of its receptor Kit, which is a receptor tyrosine kinase similar to the receptors for platelet-derived growth factor and colonystimulating factor-1. Calorimetric studies show that one SCF dimer binds simultaneously to two molecules of the Kit extracellular domain. Ligand-induced receptor dimerization provides the mechanism for transmembrane signaling by many receptors with a single transmembrane domain (1-3). Crystallographic and other biophysical studies have shown that this results from bivalent binding of the cognate ligand to the receptors for human growth hormone (hGH) 1 (4, 5) and interferon-␥ (IFN-␥) (6). hGH is a monomeric cytokine that has two receptor binding sites. Each site binds to a separate receptor molecule, forming a 1:2 (ligand:receptor) complex (4), with a 1:1 monomeric receptor complex occurring as an intermediate (4, 7). An x-ray crystal structure of the hGH⅐hGH-R complex shows that, in addition to ligand-mediated interactions, receptor-receptor interactions also contribute to stabilization of the receptor dimer (5). IFN-␥ is a dimeric cytokine, and one IFN-␥ dimer binds simultaneously to two molecules of the ␣-chain of its receptor. An x-ray crystal structure of the complex between IFN-␥ and the extracellular domain of the IFN-␥ receptor ␣-chain (IFN-␥R␣) shows that the dimer is stabilized solely through ligandmediated interactions (6). In another case of ligand-induced receptor oligomerization for which high resolution structural information is available, that of the tumor necrosis factor receptor, the ligand (tumor necrosis factor-␤) is a trimer that binds simultaneously to three receptor molecules. Each receptor extracellular domain contacts two protomers of the ligand trimer, and no direct receptor-receptor contact is seen in the trimeric complex (8).Although no detailed structural information exists for activated receptor tyrosine kinases in complex with their ligands, many reports indicate that bivalent binding of growth factors is key for inducing receptor dimerization (reviewed in Refs. 2 and 3). For example, platelet-derived growth factor (PDGF) is a disulfide-linked dimer that binds to two PDGF-receptor molecules (9, 10). The ligands for the Trk receptors also form dimers to which two receptor molecules can bind, resulting in receptor dimerization (11). Finally, acidic fibroblast growth factor (aFGF), although itself monomeric and incapable of inducing dimerization of its receptor, forms a multivalent complex with heparan sulfate proteoglycans that can in turn bind to two or more receptors and thus stabilize active FGF receptor dimers (12).In the process of our investigations of the mechanisms of growth factor-induced dimerization of receptor tyrosine kinases, we have studied the binding of stem cell factor (SCF) to the extracellular domain of its receptor, Kit, a class III receptor tyro...