FIG. 1.

C/EBPβ phosphorylation in L cells is stimulated by coexpression of Ras^V12. (A) L cells were transfected with a vector encoding the p35 form of rat C/EBPβ (9, 37) alone (−) or with a Ras^V12 expression vector (+) and were labeled for 10 h in serum-free medium containing ³²P_i. C/EBPβ immunoprecipitates were prepared, resolved by SDS-PAGE, transferred to a polyvinylidene difluoride membrane, and visualized by autoradiography. (B) Labeled C/EBPβ proteins from panel A were released from the membrane by trypsin digestion, and the products were separated by reverse-phase HPLC. Phosphopeptide peaks are numbered 1 to 5.