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Characterization of critical residues in the cytoplasmic domain of the human interleukin‐5 receptor α chain required for growth signal transduction

Interleukin (IL)‐5 binds to a cell surface receptor composed of two polypeptide chains, α and β, both belonging to the hemopoietic cytokine receptor family. Mouse cells expressing common mouse β chain (AIC2B) that were transfected with human IL‐5 receptor (R)α cDNA proliferated in response to picomolar concentrations of human IL‐5, indicating that a functional receptor was reconstituted. We show that in these cells, human (h)IL‐5 as well as mouse (m)IL‐3 induce tyrosine phosphorylation of β chain and JAK 2 kinase. Phosphorylated β receptor was co‐precipitated with anti‐JAK 2 antibodies, suggesting that both molecules were physically associated. IL‐5 and IL‐3 also induce cytosolic DNA binding activity as measured by an electrophoretic mobility shift assay using the interferon‐γ responsive region of human Fc γ1 gene DNA element. A deletion mutant of hIL‐5Rα lacking the cytoplasmic part could bind hIL‐5 normally in association with the β chain, but was unable to transmit a biological signal. The cytoplasmic domain was also indispensable for tyrosine phosphorylation and activation of DNA binding proteins. A membrane‐proximal proline‐rich element of the hIL‐5Rα cytoplasmic domain that is conserved among different members of the hemopoietic cytokine receptor family was essential for biological activity. Point mutations in this motif also knocked out IL‐5‐inducible JAK 2 phosphorylation.