JAK2 and JAK1 constitutively associate with an interleukin-5 (IL-5) receptor α and βc subunit, respectively, and are activated upon IL-5 stimulation

Abstract

The human interleukin-5 receptor (hIL-5R) consists of a unique α subunit (hIL-SRα) and a common β subunit (βc) 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-SRα and βc by immunoprecipitation using anti- hIL-5Rα and anti-βc monoclonal antibodies. The binding of JAK1 and JAK2 to each hIL-5R subunit was also evaluated in the hIL-5-responsive cell line, TF- h5Rα. It was observed that IL-5 stimulation induced the recruitment of βc to hIL-5Rα, although in the absence of IL-5 the subunits remain independent. In the absence of IL-5, JAK2 and JAK1 were associated with hIL-5Rα and βc, respectively. IL-5 stimulation resulted in tyrosine phosphorylation of JAK2, JAK1, βc, and STAT5. Moreover, IL-5-induced dimerization of IL-5R subunits caused JAK2 activation and βc 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-SRα 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-5Rα-associated JAK2 is indispensable for the IL-5 signaling event.