TYROSINE PHOSPHORYLATION OF THE ERYTHROPOIETIN RECEPTOR - ROLE FOR DIFFERENTIATION AND MITOGENIC SIGNAL-TRANSDUCTION

The erythropoietin (Epo) receptor belongs to the cytokine receptor sup erfamily. Although the cytokine receptors do not possess a tyrosine ki nase consensus sequence in the intracellular domain, rapid stimulation of a tyrosine kinase activity occurs after activation by the ligand. We and others have shown that Epo induces the tyrosine phosphorylation of its cognate receptor as well as phosphorylation of other proteins. In this report, we examined the role of the receptor tyrosine residue s in signal transduction. Eight tyrosine residues are located within t he intracellular domain of the murine Epo receptor, A single tyrosine residue is present in the region previously shown to be sufficient for proliferative signal transduction, This tyrosine (Tyr 343) was mutate d to phenylalanine. Moreover, mutant receptors were also generated wit h either a tyrosine residue or a phenylalanine residue at position 343 and with a COOH terminal truncation that removed the 7 other tyrosine residues. Expression vectors carrying these mutated receptors were tr ansfected into the interleukin-3-dependent murine cell line Ba/F3. Epo -induced growth was sustained efficiently by all these receptors, alth ough receptors without any tyrosine residues conferred a significantly reduced mitogenic activity, Moreover, all receptors were able to medi ate Epo-dependant accumulation of beta-globin mRNA. The mutated recept ors all induced the tyrosine phosphorylation of several cellular prote ins after Epo stimulation. However, the truncated receptors induced th e phosphorylation of a reduced number of proteins, suggesting that pho sphorylated tyrosines of the receptor could have a role in the recruit ment either of a tyrosine kinase or of tyrosine kinase substrate prote ins, The receptors were all able to mediate Epo-induced activation of phosphatidylinositol 3-kinase, although truncated receptors no longer bound phosphatidylinositol 3-kinase. (C) 1995 by The American Society of Hematology.