EXTRACELLULAR TRUNCATIONS OF H-BETA-C, THE COMMON SIGNALING SUBUNIT FOR INTERLEUKIN-3 (IL-3), GRANULOCYTE-MACROPHAGE COLONY-STIMULATING FACTOR (GM-CSF), AND IL-5, LEAD TO LIGAND-INDEPENDENT ACTIVATION

The hypothesis that extracellular truncation of the common receptor su bunit for interleukin-3 (IL-3), granulocyte-macrophage colony-stimulat ing factor, and IL-5 (h beta c) can lead to ligand-independent activat ion was tested by infecting factor-dependent hematopoietic cell lines with retroviruses encoding truncated forms of h beta c. A truncation, resembling that in v-Mpl, and retaining 45 h beta c-derived extracellu lar residues, led to constitutive activation in the murine myeloid cel l line, FDC-P1, However, infection of cells with retrovirus encoding a more severely truncated receptor, retaining only 7 h beta c-derived e xtracellular residues, did not confer factor independence on these cel ls. These experiments show that truncation activates the receptor and define a 37-amino acid segment of h beta c (H395-A431) which contains two motifs conserved throughout the cytokine receptor superfamily (con sensus Y/H XX R/O VR and WSXWS), as essential for factor-independent s ignaling. The mechanism of activation was also investigated in less se vere truncations. A receptor that retains the entire membrane-proximal domain (domain 4) also conferred factor independent growth on FDC-P1 cells; however, a retrovirus encoding a truncated form of h beta c hav ing two intact membrane proximal domains did not have this ability, su ggesting that domain 3 may have an inhibitory role in h beta c. The ab ility of these receptors to confer factor independence was cell specif ic as demonstrated by their inability to confer factor-independent gro wth when introduced into the murine IL-3-dependent pro-B cell line BaF -B03. These results are consistent with a model in which activation re quires unmasking of an interactive receptor surface in domain 4 and as sociation with a myeloid-specific receptor or accessory component. We suggest that in the absence of ligand intramolecular interactions prev ent inappropriate signaling. (C) 1996 by The American Society of Hemat ology.