CRITICAL CYTOPLASMIC DOMAINS OF THE COMMON BETA-SUBUNIT OF THE HUMAN GM-CSF, IL-3 AND IL-5 RECEPTORS FOR GROWTH SIGNAL TRANSDUCTION AND TYROSINE PHOSPHORYLATION

The high-affinity receptors for human granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin 3 (IL-3) and interleukin 5 (IL-5) are composed of two distinct subunits, alpha and beta(c). The alpha-subunits are specific for each cytokine, whereas the beta-subunit (beta(c)) is shared by the three receptors and is an essential component of signal transduction. We have made a series of mutant beta(c) cDNAs that delete various regions of the cytoplasmic domain and examined the function of these mutants by coexpressing them with the alpha-subunit of the human GM-CSF receptor (hGMR) in an IL-3-dependent mouse pro-B cell line BaF3. Two domains in the membrane-proximal portion of beta(c) were found to be important for transducing the hGM-CSF-mediated growth signals: one domain between Arg456 and Phe487 appears to be essential for proliferation, and the second domain between Val518 and Asp544 enhances the response to GM-CSF, but is not absolutely required for proliferation. The region between Val518 and Leu626 was responsible for major tyrosine phosphorylation of 95 and 60 kDa proteins. Thus, beta(c)-mediated major tyrosine phosphorylation of these proteins was apparently separated from proliferation. However, the beta-517 mutant lacking residues downstream of Val518 transmitted a herbimycin-sensitive proliferation signal, suggesting that beta-517 still activates a tyrosine kinase(s). We also evaluated the role of the cytoplasmic domain of the GMR alpha-subunit and the results suggest that it is involved in the hGM-CSF-mediated signal transduction, but is not essential. GM-CSF-induced tyrosine phosphorylation of beta(c) and the amino acid sequences surrounding the tyrosine residues as well as phosphorylation of deletion mutants suggest that Tyr750 is most likely the site for tyrosine phosphorylation.