Critical proline residues of the cytoplasmic domain of the IL-5 receptor α chain and its function in IL-5-mediated activation of JAK kinase and STAT5.

The high-affinity receptor (R) for IL-5 consists of a unique a chain (IL-5Rcα) and a β chain (βc) thatis shared with the receptors for IL-3 and granulocyte macrophage colony stimulating factor (GM-CSF). We defined two regions of IL-5Rα for the IL-5-induced proliferative response, the expression of nuclear proto-oncogenes, and the tyrosine phosphorylation of cellular proteins including βc, SH2/SH3-containing proteins and JAK2 kinase. In the studies described here, we demonstrate that IL-5, IL-3 or GM-CSF stimulation induces the tyrosine phosphorylation of JAK2, and to a lesser extent JAK1, and of STAT5. Mutational analysis revealed that one of the proline residues, particularly Pro352and Pro355, in the membrane-proximal proline-rich sequence (Pro352-Pro353-X-Pro355) of the cytoplasmic domain of IL-5Rα is required for cell proliferation, and for both JAK1 and JAK2 activation. In addition, transfectants expressing chimeric receptors which consist of the extracellular domain of IL-5Rα and the cytoplasmic domain of βc respondedto IL-5 for proliferation and tyrosine phosphorylation of JAK1. Intriguingly, electrophoretic mobility shift assay analysis revealed that STAT5 was activated in cells showing either JAK1 or JAK2 tyrosine phosphorylation. These results indicate that activation of JAK1, JAK2 and STAT5 is critical to coupling IL-5-induced tyrosine phosphorylation and ultimately mitogenesis, and that Pro352 and Pro355 in the proline-rich sequence appear to play more essential roles in cell growth andin both JAK1/STAT5 and JAK2/STAT5 activation than Pro353 does. [ABSTRACT FROM AUTHOR]