Depolarization-induced tyrosine phosphorylation of paxillin in PC12h cells

Treatment of PC12h cells with a high concentration of KCI induces depolarization of the plasma membrane and Ca2+ influx into the cells. We have previously shown that KCI induced tyrosine phosphorylation of cellular proteins of 120, 110, 68, 44 and 42 kDa. In the present study, we found that the 68-kDa protein is paxillin, a tyrosine kinase substrate associated with the actin cytoskeleton. A calcium ionophore, A23187. also induced tyrosine phosphorylation of the 68-kDa protein, while KCl did not in the presence of EGTA or nifedipine, indicating that the effect of KCI was due to the Ca2+ influx into the cells. Tyrosine phosphorylation of paxillin was also induced by nerve growth factor and epidermal growth factor, but its migration patterns on an SDS/polyacrylamide gel were different? that is, nerve growth factor and epidermal growth factor caused upward shifts of the bands, while KCl did not. However, both forms could associate with Csk and Crk. The effect of KCl was blocked by cytochalasin D, indicating that tyrosine phosphorylation required the integrity of actin filaments. These results suggest that tyrosine phosphorylation of paxillin may be involved in Ca2+-dependent events in neuronal and neuroendocrine cells.