Signal transduction by the high-affinity GM-CSF receptor: two distinct cytoplasmic regions of the common beta subunit responsible for different signaling.

The high-affinity receptors for granulocyte-macrophage colony stimulating factor (GM-CSF), interleukin 3 (IL-3) and IL-5 consist of two subunits, alpha and beta. The alpha subunits are specific to each cytokine and the same beta subunit (beta c) is shared by these three receptors. Although none of these receptor subunits has intrinsic kinase activity, these cytokines induce protein tyrosine phosphorylation, activation of Ras, Raf-1 and MAP kinase, and transcriptional activation of nuclear proto-oncogenes such as c-myc, c-fos and c-jun. In this paper, we describe a detailed analysis of the signaling potential of the beta c subunit by using a series of cytoplasmic deletion mutants. The human beta c consists of 881 amino acid residues. A C-terminal deletion mutant of beta c at amino acid 763 (beta 763) induced phosphorylation of Shc and activation of Ras, Raf-1, MAP kinase and p70 S6 kinase, whereas a deletion at amino acid 626 (beta 626) induced none of these effects. The beta 763 mutant, as well as the full-length beta c, induced transcription of c-myc, c-fos and c-jun. Deletions at amino acid 517 (beta 517) and 626 (beta 626) induced c-myc and pim-1, but no induction of c-fos and c-jun was observed. GM-CSF increased phosphatidylinositol 3 kinase (PI3-K) activity in anti-phosphotyrosine immunoprecipitates from cells expressing beta 763 as well as beta c, whereas it was only marginally increased from cells expressing beta 517 or beta 626. Thus, there are at least two distinct regions within the cytoplasmic domain of beta c that are responsible for different signals, i.e. a membrane proximal region of approximately 60 amino acid residues upstream of Glu517 is essential for induction of c-myc and pim-1, and a distal region of approximately 140 amino acid residues (between Leu626 and Ser763) is required for activation of Ras, Raf-1, MAP kinase and p70 S6 kinase, as well as induction of c-fos and c-jun.