FIG. 3.

MEK is constitutively activated in SFFV-infected cells. MEK was immunoprecipitated from uninfected and SFFV-infected HCD-57 cells that had been starved overnight in 1.5% FCS and either left unstimulated (−) or stimulated with Epo for 15 min (+). Immunoprecipitates were assayed for MEK kinase activity by immune complex kinase assays measuring incorporation of ³²P into GST-MAPK⁻. Phosphorylation of MAPK⁻ was detected by autoradiography, and the filter was then probed with anti-MEK antibody to visualize the amount of MEK present in the kinase reaction.