FIG. 4.

N-terminally truncated Bα is phosphorylated upon coexpression of TβRI and TβRII. Myc-epitope-tagged Bα, lacking amino acids 1 to 49, was coexpressed in the presence (+) or absence (−) of TβRI/R4 and TβRII (TβRI/II) in transfected COS cells. The in vivo phosphorylation of this deletion mutant of Bα was assessed by in vivo ³²P labeling followed by immunoprecipitation with anti-Myc antibody 9E10 and detection by SDS-PAGE and autoradiography (upper lanes). The lower lanes illustrate equal expression levels of the Myc-tagged Bα mutant as assessed by anti-Myc Western blotting of anti-Myc immunoprecipitated protein. Left lanes, without TβRI/TβRII expression; right lanes, with TβRI/TβRII expression.