Characterization of 85Kd protein tyrosine phosphorylated by erythropoietin stimulation

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Erythropoietin (Epo) is the major regulator of the proliferation and differentiation of erythroid precursors through interaction with its receptor (Epo-R). Although Epo-R lacks a tyrosine kinase consensus sequence within its intracellular domain, the addition of its ligand to Epo-responsive cells, UT-7/Epo, induces the rapid and transient tyrosine phosphorylation of 145,130,80-85 and 40 Kd cellular proteins. Tyrosine phosphorylation of these proteins occurred dose- and time-dependently. We showed that the tyrosine phosphorylated 145 Kd protein is identical to PLC-_<gamma>1. Tyrosine phosphorylation of this protein is detectable within 30 seconds and almost reaches the maximum at 1 minute. This can last up to 10 minutes and declines thereafter. Additionally, in Epo-stimulated cells, PLC-_<gamma>1 become physically associated with 80-85 and 40 Kd proteins which have been tyrosine phosphorylated in response to Epo. We tried to identified the 80-85 Kd protein and found that this protein is not identical to P13K,Epo-R,nor Raf-1. We are now trying to identify this protein.