ERYTHROPOIETIN INDUCES ACTIVATION OF STAT5 THROUGH ASSOCIATION WITH SPECIFIC TYROSINES ON THE RECEPTOR THAT ARE NOT REQUIRED FOR A MITOGENIC RESPONSE

The cytoplasmic domain of the erythropoietin receptor (EpoR) contains a membrane-distal region that is dispensable for mitogenesis but is re quired for the recruitment and tyrosine phosphorylation of a variety o f signaling proteins. The membrane-proximal region of 96 amino acids i s necessary and sufficient for mitogenesis as well as Jak2 activation, induction of c-fos, c-myc, cis, the T-cell receptor gamma locus (TCR- gamma), and c-pim-l, The studies presented here demonstrate that this region is also necessary and sufficient for the activation of Stat5A a nd Stat5B, The membrane-proximal domain contains a single tyrosine, Y- 343, which when mutated eliminates the ability of the receptor to coup le Epo binding to the activation of Stat5. Furthermore, peptide compet itions demonstrate that this site, when phosphorylated, can disrupt St at5 DNA binding activity, consistent with a role of Y-343 as a site of recruitment to the receptor, Cells expressing the truncated, (YF)-F-3 43 mutant (a mutant with a Y-to-F alteration at position 343) prolifer ate in response to Epo in a manner comparable to that of the controls, However, in these cells, Epo stimulation does not induce the appearan ce of transcripts for cis, TCR-gamma, or c-fos, suggesting a role for Stat5 in their regulation.