Effect of FGFR on ephrin-induced activation of EphB2 and MAPK. Immunoprecipitation and Western blots analyses were performed to determine the effect of prior iFGFR activation on the time course of ephrinB1-induced phosphorylation of EphB2 and activation of the MAPK pathway. (a) After iFGFR activation, there is a lower maximal phosphorylation of EphB2 (left) after incubation with ephrinB1 cells compared with in the absence of iFGFR (right); quantitation is shown in the graph on the right. (b) Analogous experiments were performed by activating EphB2 with soluble ephrinB1-Fc; in controls, Fc alone does not activate EphB2. Similar results are obtained as for activation by ephrinB1 cells, arguing against the possibility that the effect of iFGFR on EphB2 activation is secondary to altered cell–cell contacts. (c) Detection of ERK phosphorylation reveals that EphB2 activates the MAPK pathway (black line), but after iFGFR activation, EphB2 instead inhibits the MAPK pathway (red line). For Western blots, see Fig. 7. Error bars indicate range (n = 3). PiTyr, phosphorylated tyrosine.