Upper panel: Epo induces tyrosine phosphorylation of thec-kit receptor in UT-7/Epo cells. UT-7/Epo cells were exposed to no added growth factor (lane 1), SCF (150 ng/mL, lane 2), Epo (2.5 U/mL, lane 3), or GM-CSF (5 ng/mL, lane 4) for 10 minutes at 37°C. UT-7/Tpo cells were exposed to no added growth factor (lane 5), SCF (lane 6), Tpo (25 ng/mL, lane 7), or GM-CSF (lane 8). Cell lysates were immunoprecipitated with SR-1 anti–c-kit receptor monoclonal antibody, subjected to SDS-PAGE, and blotted with the 4G10 antiphosphotyrosine monoclonal antibody. Molecular weight markers are indicated. The arrow identifies the c-kit receptor. Two additional experiments gave similar results. Lower panel: tyrosine phosphorylation of the c-kit receptor in UT-7/Epo cells is detectable within 5 minutes of Epo stimulation. UT-7/Epo cells were exposed to no added growth factor (lane 1), SCF (150 ng/mL, lane 2) for 5 minutes, SCF (lane 3) for 10 minutes, Epo (2.5 U/mL, lane 4) for 5 minutes, Epo (lane 5) for 10 minutes, or GM-CSF (2.5 ng/mL) for 5 minutes at 37°C.