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Fig. 5. Serum activation of Erk1/2 but not K-ras p21 in E10 cells. After growth arrest of 60% confluent E10 cells by serum-starvation for 48 h, complete medium with serum was restored. Lysates at each time point were used for assay of total K-ras p21 (A), K-ras-GTP (B), total Erk1/2 (C), and activated phosphorylated Erk1P/2P (D). B, at the left, the p21 standard. Serum stimulation did not lead to an increase in K-ras-GTP (B), whereas marked activation of Erk1/2 was observed (D). In a separate experiment, similar cultures were followed for 12 h and no change in total K-ras or K-ras-GTP was observed (not shown).