Actin polymerization localizes to the activated epidermal growth factor receptor in the plasma membrane, independent of the cytosolic free calcium transient.

Epidermal growth factor (EGF) induces rapid actin filament assembly in the membrane skeleton of A431 cells, leading to a approximately 30% rise in cellular filamentous actin levels. EGF-induced actin polymerization depends upon EGF receptor (EGFR) tyrosine kinase activity, since the selective tyrosine kinase inhibitor AG213 abolishes EGF-induced actin polymerization. In accordance, confocal laser scanning microscopy shows that newly assembled actin filaments localize selectively to the tyrosine-phosphorylated EGFR in the plasma membrane, since actin polymerization is not observed at the internalized tyrosine-phosphorylated EGFR. Actin binding proteins (ABP's) are generally believed to regulate actin filament assembly. Ca2+ is known as one of the important regulatory factors for the activity of ABP's in vitro [15]. Therefore, we investigated the importance of the EGF-induced transient rise in [Ca2+]i for the regulation of actin polymerization in vivo. Continuous high [Ca2+]i in the millimolar range induces a prominent rise in cellular filamentous actin levels to approximately 50% over control cells. However, actin polymerization is unimpaired under conditions which effectively block the EGF-induced [Ca2+]i transient. These data demonstrate that EGF-induced actin polymerization localizes to the activated EGFR in the membrane skeleton, independent of the cytosolic free calcium transient.