Coordinate expression of the alpha and beta chains of human granulocyte-macrophage colony-stimulating factor receptor confers ligand-induced morphological transformation in mouse fibroblasts

Crucial role of the residue R280 at the F'-G' loop of the human granulocyte/macrophage colony-stimulating factor receptor alpha chain for ligand recognition

The receptor for granulocyte/macrophage colony-stimulating factor (GM-CSF) is composed of two chains, alpha and betac. Both chains belong to the superfamily of cytokine receptors characterized by a common structural feature, i.e., the presence of at least two fibronectin-like folds in the extracellular domain, which was first identified in the growth hormone receptor. The GM-CSF receptor (GMR)-alpha chain confers low affinity binding only (5-10 nM), whereas the other chain, betac, does not bind GM-CSF by itself but confers high affinity binding when associated with GMR-alpha (25-100 pM). The present study was designed to define the assembly of the GMR complex at the molecular level through site-directed mutagenesis guided by homology modeling with the growth hormone receptor complex. In our three-dimensional model, R280 of GMR-alpha, located in the F'-G' loop and close to the WSSWS motif, is in the vicinity of the ligand Asp112, suggesting the possibility of electrostatic interaction between these two residues. Through site directed mutagenesis, we provide several lines of evidence indicating the importance of electrostatic interaction in ligand-receptor recognition. First, mutagenesis of GMR-alphaR280 strikingly ablated ligand binding in the absence of beta common (betac); ligand binding was restored in the presence of betac with, nonetheless, a significant shift from high (26 pM) toward low affinity (from 2 to 13 nM). The rank order of the dissociation constant for the different GMR-alphaR280 mutations where Lys > Gln > Met > Asp, suggesting the importance of the charge at this position. Second, a mutant GM-CSF with charge reversal mutation at position Asp112 exhibited a 1,000-fold decrease in affinity in receptor binding, whereas charge ablation or conservative mutations were the least affected (10-20-fold). Third, removal of the charge at position R280 of GMR-alpha introduced a 10-fold decrease in the association rate constant and only a 2-fold change in the dissociation rate constant, suggesting that R280 is implicated in ligand recognition, possibly through interaction with Asp112 of GM-CSF. For all R280 mutants, the half-efficient concentrations of GM-CSF required for membrane (receptor binding) to nuclear events (c-fos promoter activation) and cell proliferation (thymidine incorporation) were in the same range, indicating that the threshold for biologic activity is governed mainly by the affinity of ligand-receptor interaction. Furthermore, mutation of other residues in the immediate vicinity of R280 was less drastic. Sequence alignment and modeling of interleukin (IL)-3R and IL-5R identified an arginine residue at the tip of a beta turn in a highly divergent context at the F'-G' loop, close to a conserved structural element, the WSXWS motif, suggesting the possibility of a ligand association mechanism similar to the one described herein for GMR.

A dominant negative granulocyte-macrophage colony-stimulating factor receptor alpha chain reveals the multimeric structure of the receptor complex

The receptor for the hemopoietic growth factor granulocyte-macrophage colony-stimulating factor (GM-CSF) is composed of two chains, both of which belong to the superfamily of cytokine receptors. The alpha chain confers low affinity binding only, whereas the beta chain (betac) confers high affinity binding when associated with alpha. Ectopic expression of both chains of the receptor in murine NIH-3T3 fibroblasts results in signal transduction, mitogenesis, and morphologic transformation. The cytoplasmic domain of the GM-CSF receptor alpha subunit (GMR-alpha) comprises 54 amino acids that have been shown to be important for signal transduction through the beta chain. The present study was designed to address the possibility of receptor oligomerization and its functional implication. Cross-linking studies with 125I-GM-CSF on NIH-3T3 transfectants is consistent with the presence of alpha and betac dimers and of receptor oligomers. We have, therefore, generated an inert alpha chain through polymerase chain reaction-mediated truncation of 47 amino acids of the COOH-terminal domain of alpha (alphat), and coexpressed alphat, alpha, and betac in NIH-3T3. In cells in which alphat and alpha are present in stoichiometric proportion within the GM-CSF-binding complex, we provide evidence that alphat is dominant negative over wild type alpha on the basis of two different functional assays: cell proliferation and foci formation. Hence, our results suggest the requirement for at least two functional alpha chains for signal transduction. Together with the cross-linking studies, our data indicate that the functional GMR is an oligomer that contains at least two alpha chains.

Roles of the cytoplasmic domains of the alpha and beta subunits of human granulocyte-macrophage colony-stimulating factor receptor

The high-affinity and functional granulocyte-macrophage colony-stimulating factor receptor (GMR) is composed of two distinct subunits, alpha and beta; and the cytoplasmic domain of the beta subunit is essential to transduce growth-promoting signals. In contrast to the beta subunit, the role of the alpha subunit is not well characterized. We examined the requirement of the cytoplasmic domain of the alpha subunit and its functional region by deletion analyses. We demonstrated that the cytoplasmic domain of the alpha subunit, especially 29 amino acids residues near the transmembrane domain, was absolutely required for various signaling events including activation of immediate early genes, induction of tyrosine phosphorylation of cellular proteins, and cell growth. We further analyzed the role of the cytoplasmic domain of each subunit by constructing chimeric subunits, designated alpha/beta and beta/alpha, by exchanging cytoplasmic domains of the alpha and beta subunits of human (h) GMR. Reconstituted high-affinity chimeric hGMRs, hGMR(alpha/beta,beta/alpha) and hGMR(alpha/beta,beta), transduced signals at levels similar to the wild type hGMR(alpha,beta) in Ba/F3 cells and in NIH3T3 cells. These observations indicate that the original configuration between the extracellular and the cytoplasmic domains of the hGMR(alpha,beta) subunits is not required and that hGMR(alpha/beta,beta) transduced signals through the cytoplasmic domain of the beta subunit in an oligomeric form, without involvement of the cytoplasmic domain of the alpha subunit. Therefore human granulocyte-macrophage colony-stimulating factor signals are mainly transduced through the beta subunit, and the cytoplasmic domain of the alpha subunit is likely to activate the beta subunit in the normal hGMR.

c-Cbl is inducibly tyrosine-phosphorylated by epidermal growth factor stimulation in fibroblasts, and constitutively tyrosine-phosphorylated and associated with v-Src in v-src-transformed fibroblasts

The c-cbl gene was cloned as the cellular homolog of the v-cbl oncogene that is the transforming component of a murine tumorigenic retrovirus, CAS NS-1, though the biological roles of c-Cbl remain to be elucidated. We have previously reported that c-Cbl is implicated in the signal transduction triggered by granulocyte-macrophage colony-stimulating factor or erythropoietin in hematopoietic cells. Here, we observed tyrosine phosphorylation of C-cbl in cells expressing epidermal growth factor receptor depending on EGF stimulation and in v-src transformed cells. Furthermore, c-Cbl was revealed to associate with v-Src in vivo. By means of binding experiments using glutathione S-transferase fusion proteins, we have found that the SH2 and SH3 domains of many proteins bind to c-Cbl. These findings strongly suggest that c-Cbl is implicated in a wide variety of signal transduction pathways, including those of EGF receptor and Src protein, as well as in the signaling pathways of hematopoietic cells.

Cytokine receptors and signal transduction

The past few years have seen an explosion in the identification, cloning and characterization of cytokines and their receptors. The pleiotropic effects of many of the growth factors and the considerable redundancy in the actions of growth factors have contributed to a mass of descriptive literature that often seems to defy summary. Only recently have common concepts begun to emerge. First, cytokines mediate their effects through a large family of receptors that have evolved from a common progenitor and retain structural and functional similarities. Within the haematopoietic system, the cytokines are not usually instructive in differentiation, but rather supportive, and may contribute to some differentiation-specific responses. The patterns of expression of cytokine receptors are therefore a product of differentiation and provide for changes in physiological regulation. The second important concept that is emerging is that the cytokines mediate their mitogenic effects through a common signal-transducing pathway involving tyrosine phosphorylation. Thus, although the cytokine receptor superfamily members do not have intrinsic protein tyrosine kinase activity, by coupling to activation of tyrosine phosphorylation they may affect cell growth by pathways that are common with the large family of growth factor receptors that contain intrinsic protein tyrosine kinase activity. The coupling of cytokine binding to tyrosine phosphorylation and mitogenesis requires a relatively small membrane-proximal domain of the receptors. This region has limited sequence similarity which may be required for the association of individual receptors with an appropriate kinase. Activation of kinase activity results from the dimerization or oligomerization of receptor homodimers or heterodimers. Again this requirement is similar to that seen with the growth factor receptors which have intrinsic protein tyrosine kinase activity. The protein tyrosine kinases that couple cytokine binding to tyrosine phosphorylation are members of the Jak family of kinases. The ubiquitous expression of these kinases provides a common cellular background on which the cytokine receptors can function and on which unique functionally distinct receptors have evolved. In particular, tyk2 is required for the responses initiated by IFN-alpha while Jak2 has been implicated in the responses to G-CSF, IL-3, EPO, growth hormone, prolactin and IFN-gamma.(ABSTRACT TRUNCATED AT 400 WORDS)