Ligand-specific utilization of the extracellular membrane-proximal region of the gp130-related signalling receptors

The receptor gp130 is used by the interleukin-6 (IL-6)-type cytokines, whic h include IL-6 and leukaemia-inhibitory factor (LIF). To investigate the ro le of the three extracellular membrane-proximal fibronectin-type-III-like ( FNIII) modules of gp130 and the related receptor for granulocyte colony-sti mulating factor (G-CSFR) in cytokine signal transduction we have transfecte d into murine myeloid MI-UR21 cells the chimaera (GR-FNIII)gp130, which con tains the membrane-proximal FNIII modules of the G-CSFR on a gp130 backbone , and its complement, the chimaera (gp130-FNIII)GR. Whereas the binding aff inities of I-125-labelled IL-6 to (GR-FNIII)gp130, or of I-125-Tyr1, 3-G-CS F to (gp130-FNIII)GR, were similar to wild-type gp130 and wild-type G-CSFR, respectively, I-125-LIF failed to bind with high affinity to (GR-FNIII)gp1 30. In assays measuring differentiation the (gp130-FNIII)GR cells were full y responsive to G-CSF, whereas the (GR-FNIII)gp130 cells responded fully to the agonistic anti-gp130 monoclonal antibody (mAb) B-S12, but not to IL-6 or LIF. Neutralizing mAbs that recognize the membrane-proximal FNIII module s of gp130 or the G-CSFR differentially interfered with signalling by B-S12 , LIF and G-CSF. The data suggest that B-S12 and G-CSF induce the correct o rientation or conformation for signalling by the wild-type and chimaeric ho modimeric receptors, that the membrane-proximal region of gp130 is importan t for the correct formation of the signalling IL-6-IL-6 receptor-gp130 comp lex and that this region is also involved in LIF-dependent receptor heterod imerization and signalling.