Physical and functional interaction between p72(syk) and erythropoietin receptor

Erythropoietin (Epo) regulates the proliferation and differentiation of ery throid cells through interaction with a cell surface receptor (EpoR) that b elongs to the cytokine receptor family. The Jak2 tyrosine kinase was previo usly shown to bind to the EpoR, to be activated upon Epo stimulation, and t o play a critical role ill Epo-induced proliferation. However, little is kn own about the role of other tyrosine kinases in Epo signaling. In this pape r, we examined whether Syk was involved in EpoR activation. Coimmunoprecipi tation experiments showed that the phosphorylated EpoR was associated with the Syk kinase in activated UT7 cells. The interaction of Epo with its rece ptor led to an increased kinase activity. The use of recombinant Syk Src ho mology 2 (SH2) domains expressed in tandem or individually revealed that bo th N- and C-SH2 domains of Syk participated in EpoR binding with a major co ntribution of the C-terminal SH2 domain. Far Western blotting further indic ated that Syk directly binds to the EpoR and that the interaction of Syk wi th EpoR only occurred after Epo activation. These data suggest that phospho rylation of EpoR on tyrosine residues may mediate Syk binding to the recept or through interaction between the two SH2 domains of Syk and tyrosines of the receptor. We propose that in addition to Jak2, Syk protein kinase may b e a component of EpoR signaling.