JAK 2 and JAK 1 Constitutively Associate With an Interleukin - 5 ( IL - 5 ) Receptor a and b c Subunit , Respectively , and Are Activated Upon IL - 5 Stimulation

The human interleukin-5 receptor (hIL-5R) consists of a unique a subunit (hIL-5Ra) and a common b subunit (bc) that activate two Janus kinases (JAK1 and JAK2) and a signal transducer and activator of transcription (STAT5). The precise stoichiometry of the hIL-5R subunits and the role of JAK kinases used in IL-5 signaling were investigated. We analyzed the interaction between hIL-5Ra and bc by immunoprecipitation using anti–hIL-5Ra and anti-bc monoclonal antibodies. The binding of JAK1 and JAK2 to each hIL-5R subunit was also evaluated in the hIL-5–responsive cell line, TF-h5Ra. It was observed that IL-5 stimulation induced the recruitment of bc to hIL-5Ra, although in the absence of IL-5 the subunits remain independent. In the absence of IL-5, JAK2 and JAK1 were associated with hIL-5Ra and bc, respectively. IL-5 stimulation resulted in tyrosine phosphorylation of JAK2, JAK1, bc, and STAT5. Moreover, IL-5–induced dimerization of IL-5R subunits caused JAK2 activation and bc phosphorylation even in the absence of JAK1 activation. Furthermore, tyrosine phosphorylation of JAK1 was dependent on the activation of JAK2. Detailed study of the C-terminal truncated cytoplasmic domain of hIL-5Ra revealed that the cytoplasmic stretch at position 346-387, containing the proline-rich region, is necessary for JAK2 binding. These observations suggest that activation of hIL-5Ra–associated JAK2 is indispensable for the IL-5 signaling event. r 1998 by The American Society of Hematology.