Fig. 1. Both mitogenic and non-mitogenic agonists trigger the rapid entry of activated p42/p44 MAPKs into the nucleus. Quiescent CCL39 fibroblast cells were stimulated for the indicated times with either 100 µM thrombin receptor peptide agonist (TRP, A) or 10% serum (B). 0 minutes corresponds to non-stimulated cells. Indirect immunofluorescence detection was performed with the monoclonal antibody anti-activated p42/p44 MAPKs from Sigma. Identical results were obtained with the polyclonal antibody anti-activated p42/p44 MAPKs V6671 from Promega (data not shown). The specificity of the antibody from Sigma has been published (Yung et al., 1997) and was verified in Raf-1:ER cells in which the p42/p44 MAPKs can selectively be activated (see Fig. 2B for immunofluorescence and Fig. 5, middle blot, for western blot).