Receptor activation by the haematopoietic growth factor proteins interleukin 5 (IL-5) and granulocyte-macrophage colony-stimulating factor (GM-CSF) leads to phosphorylation of JAK2 as a key trigger of signal transduction. JAB has recently been identified as a regulator of JAK2 phosphorylation and activity by binding phosphorylated JAK2 and inducing its degradation. As part of our effort to define molecular recognition networks that lead to signalling, we investigated the effect of JAB on both JAK2 phosphorylation and JAK2 interaction state that ensue upon IL-5 stimulation in recombinant 293T cells cotransfected 293T cells with IL-5Rα, βc and hJAK2 either with or without JAB. Without JAB, stimulation with wild-type and re-engineered single chain (sc) IL-5 induced a time-dependent phosphorylation of JAK2. In the presence of JAB cotransfection, no phospho-JAK2 was observed, and JAB was observed co-immunoprecipitated with non-phosphorylated JAK2. The time dependence of JAB co-immunoprecipitation correlated with the time dependence of JAK2 phosphorylation when JAB was absent. Since JAB has already been shown to bind JAK2 via a phosphorylated tyrosine, the current data suggest that JAB binds to phosphorylated JAK2, enhances JAK2 dephosphorylation and remains associated in a complex, with dephosphorylated JAK2, that may be a precursor leading to irreversible JAK2 degradation. (C) 2000 Academic Press.
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