Kit Receptor Dimerization Is Driven by Bivalent Binding of Stem Cell Factor

Abstract

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...

Oscillations and patterns in spatially discrete models for developmental intercellular signalling

“…It was shown that, given sufficiently strong feedback, (18,19) can generate patterns with a length scale of two cells (for a string) or three cells (for hexagons), in which cells with a high level of Delta expression (and low Notch) are surrounded by cells with low Delta (and high Notch) [4]. This is consistent with some fine-grained patterns observed in early development.…”

“…This reduced model (21,22) and Collier et al's model (18,19) have the same homogeneous steady states (so that u = u), provided k i = 0 and…”

“…Another important co-operative behaviour is when the ligand is multivalent, such as platelet derived growth factor (PDGF) which can cross-link and bind to two or more receptors [9,11]. Tumour necrosis factor-β is a trimeric ligand that binds simultaneously to three receptor molecules [19]. In other systems, monovalent ligands that are incapable of crosslinking two or more receptors can still induce receptor clustering.…”

“…Again, if we assume that the binding and dissociation rates k a , k d , k c and k −c are small, so that the dominant mechanisms in the a equation are ligand production P a (c) and decay −d a a, then (35) reduces to a system that is similar to (18,19). The level of Delta activity is again represented by a, and the level of Notch activity by c. However, because of the term α 3 a in (36), there is no straightforward formal identification between f (·) and R 2 (·).…”

“…Furthermore, the requirement for small binding and dissociation rates is not compatible with the quasi-steady state assumption on b, which in contrast requires fast binding and dissociation. If we are only concerned with steady states, then there is no need for a quasi-steady state argument, but the α 3 a term still means there is no direct equivalence between the steady states of (32) and the Collier et al model (18,19). Multivalent reactions are often characterised by the Hill function, f (x) = Ax k /(B + x k ), as used by Collier et al [4].…”