The receptor for leukemia inhibitory factor (LIFR), in combination with the signal-transducing subunit for interleukin-6-type cytokine receptors, gp130, and LIF, activates transcription of acute-phase plasma protein genes in human and rat hepatoma cells and the vasoactive intestinal peptide gene in a human neuroblastoma cell line. To identify the regions within the cytoplasmic domain of LIFR that initiate signal transduction independently of gp130, we constructed a chimeric receptor by linking the extracellular domain of the granulocyte colony-stimulating factor receptor (G-CSFR) to the transmembrane and cytoplasmic domain of human LIFR. The function of the chimeric receptor protein in transcriptional activation was assessed by G-CSF-mediated stimulation of cotransfected cytokine-responsive reporter gene constructs in hepatoma and neuroblastoma cells. By using the full-length cytoplasmic domain and mutants with progressive carboxy-terminal deletions, internal deletions, or point mutations, we identified the first 150 amino acid residues of LIFR as the minimal region necessary for signaling. The signaling reaction appears to involve a cooperativity between the first 70-amino-acid region containing the two sequence motifs conserved among hematopoietin receptors (box 1 and box 2) and a critical sequence between residues 141 and 150 (box 3). Analogous analyses of the cytoplasmic domains of G-CSFR and gp130 indicated similar arrangements of functional domains in these receptor subunits and the requirement of a box 3-related motif for signaling.