We analysed the interaction of gp130, the common signaltransducing receptor chain of interleukin (IL)-6 type cytokines, with Jak1, the Janus family kinase which is crucial for signal transduction of this group of cytokines. With a truncated chimaeric IL-5Rβ-gp130 receptor expressed in COS-7 cells, we show that the membrane-proximal 69 amino acids are sufficient to mediate Jak1 binding and activation. Deletion of box2 drastically reduced binding of endogenous, but not of overexpressed, Jak1. Several point mutations in the membraneproximal region of gp130 (W652A, P671\P672A, F676A, Y683F, where W, A, P, F and Y are tryptophan, alanine, proline, phenylalanine and tyrosine) did not affect Jak1 association. However, stimulation of chimaeric receptors with the mutations P671\P672A and F676A in the interbox1\box2 region resulted in a reduced activation of STAT (signal transducer and activator of transcription) transcription factors. Most importantly, signal-

gp130 is the common signal-transducing receptor chain of interleukin (IL)-6-type cytokines. Here we describe, for the first time, a single amino acid substitution (Trp666 → Ala) in the membrane-proximal interbox1/2 region that abrogates activation of STAT (signal transducer and activator of transcription) transcription factors and the proliferative response of pro-B-cell transfectants. Moreover, association of the Janus kinase JAK1 is prevented. No signalling of heterodimeric IL-5 receptor (IL-5R)/gp130 chimaeras occurs in COS-7 cells, even when only a single cytoplasmic chain of a gp130 dimer contains the Trp666Ala mutation, indicating that it acts dominantly.