Platelet-derived growth factor (PDGF)-induced actin rearrangement is deregulated in cells expressing a mutant Y778F PDGF beta-receptor

Platelet-derived growth factor-stimulated actin rearrangement and edge ruffle formation have previously been shown to be dependent on activation of phosphatidylinositol 3'-kinase, the activity of which also is important for directed migration of cells. This lipid kinase binds to phosphorylated tyrosine residues Y740 and Y751 in the kinase insert of the human platelet-derived growth factor ss-receptor. We examined the role of two other tyrosine residues in the kinase insert of this receptor, Y775 and Y778, for ligand-induced actin rearrangement. Both were shown to be phosphorylation sites; Y775 was only marginally phosphorylated in cells expressing the wild-type ss-receptor, whereas Y778 was phosphorylated at higher stoichiometry. Mutant receptors Y775F, Y778F and Y775/778F were active kinases and mediated proliferative responses when expressed in porcine aortic endothelial cells. Fluorescence staining of actin in platelet-derived growth factor-stimulated PAE cells revealed that Y778 is involved in regulation of the actin cytoskeleton since the cells contained, apart from edge ruffles and circular ruffles, a novel type of giant ruffle on the dorsal side of the cell, which consisted of irregular multilayered actin structures. Mutation at Y778 had no effect on activation of phosphatidylinositol 3'-kinase, nor on the GTPase activating protein of Ras and phospholipase C(gamma), and the extent of directed migration towards platelet-derived growth factor of these cells was not changed. We conclude that actin rearrangement is regulated in part by Y778 in the platelet-derived growth factor ss-receptor, potentially through binding of a novel signaling molecule to this site.