Overexpression of ERK1 attenuates Ras-dependent cell growth in NIH 3T3 cells. (a) NIH 3T3 cells were stably transfected with different plasmids bearing hemagglutinin (HA) epitope-tagged ERK1, ERK1K72R, ERK2 or p38 or Myc epitope-tagged RasQ61L, all in the vector pMEX. Stable transfectants were generated and expression of the transgene monitored by western blotting. Clones were also serum starved and stimulated with 20% serum for 10 min and extracts were probed with either anti-ERK or anti-phospho-ERK antibodies. (b) Three independent NIH 3T3 clones per plasmid from (a) were plated in 10% serum and their growth was monitored for 5 days, as in Figure 1d. The data are the mean ± SEM of three independent experiments. (c) Expression of double transfectants was determined as in (a). (d) Clones from (c) were monitored for cell growth as in (b). Data are expressed as mean ± SEM of three independent experiments.