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 activa tion 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 beta-receptor, We examined t he role of two other tyrosine residues in the kinase insert of this re ceptor, Y775 and Y778, for ligand-induced actin rearrangement. Both we re shown to be phosphorylation sites; Y775 was only marginally phospho rylated in cells expressing the wild-type beta-receptor, whereas Y778 was phosphorylated at higher stoichiometry. Mutant receptors Y775F, Y7 78F and Y775/778F were active kinases and mediated proliferative respo nses when expressed in porcine aortic endothelial cells. Fluorescence staining of actin in platelet-derived growth factor-stimulated PAE cel ls revealed that Y778 is involved in regulation of the actin cytoskele ton since the cells contained, apart from edge ruffles and circular ru ffles, a novel type of giant ruffle on the dorsal side of the cell, wh ich consisted of irregular multilayered actin structures, Mutation at Y778 had no effect on activation of phosphatidylinositol 3'-kinase, no r on the GTPase activating protein of Ras and phospholipase C gamma an d the extent of directed migration towards platelet-derived growth fac tor of these cells was not changed, We conclude that actin rearrangeme nt is regulated in part by Y778 in the platelet-derived growth factor beta-receptor, potentially through binding of a novel signaling molecu le to this site.