IL-4 phosphorylates c-Cbl on tyrosine residues. (A) Ba/F3 cells express endogenous IL-4R. Methionine-labeled Ba/F3 and 32D cells were lysed, immunoprecipitated with anti-α subunit of IL-4R. The immunoprecipitates were subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and visualized by autoradiography. An arrow indicates the 140-kD α subunit of IL-4R. (B) Tyrosine phosphorylation of c-Cbl by IL-4. Ba/F3 cells were serum-starved for 10 hours and stimulated with murine IL-4 (10 ng/mL) for 5 minutes at 37°C, lysed, and immunoprecipitated with anti–c-Cbl antibody. The immunoprecipitates were subjected to immunoblotting with antiphosphotyrosine antibody, 4G10 (upper panel) or anti–c-Cbl antibody (lower panel). An arrow indicates c-Cbl.