JAK2 AND JAK1 CONSTITUTIVELY ASSOCIATE WITH AN INTERLEUKIN-5 (IL-5) RECEPTOR-ALPHA AND BETA-C SUBUNIT, RESPECTIVELY, AND ARE ACTIVATED UPONIL-5 STIMULATION

The human interleukin-5 receptor (hIL-5R) consists of a unique of subu nit (hIL-5R alpha) and a common beta subunit (beta c) that activate tw o Janus kinases (JAK1 and JAK2) and a signal transducer and activator of transcription (STAT5), The precise stoichiometry of the hIL-5R subu nits and the role of JAK kinases used in IL-5 signaling were investiga ted, We analyzed the interaction between hIL-5R alpha and beta c by im munoprecipitation using anti-hIL-5R alpha and anti-beta c monoclonal a ntibodies. The binding of JAK1 and JAK2 to each hIL-5R subunit was als o evaluated in the hIL-5-responsive cell line, TF-h5R alpha. It was ob served that IL-5 stimulation induced the recruitment of beta c to hIL- 5R alpha, although in the absence of IL-5 the subunits remain independ ent. In the absence of IL-5, JAK2 and JAK1 were associated with hIL-5R alpha and beta c, respectively, IL-5 stimulation resulted in tyrosine phosphorylation of JAK2, JAK1, beta c, and STAT5, Moreover, IL-5-indu ced dimerization of IL-5R subunits caused JAK2 activation and beta c p hosphorylation even in the absence of JAK1 activation, Furthermore, ty rosine phosphorylation of JAK1 was dependent on the activation of JAK2 , Detailed study of the C-terminal truncated cytoplasmic domain of hIL -5R alpha revealed that the cytoplasmic stretch at position 346-387, c ontaining the proline-rich region, is necessary for JAK2 binding, Thes e observations suggest that activation of hIL-5R alpha-associated JAK2 is indispensable for the IL-5 signaling event. (C) 1998 by The Americ an Society of Hematology.