An oncogenic epidermal growth factor receptor signals via a p21-activated kinase-caldesmon-myosin phosphotyrosine complex

Many ligand-independent receptor tyrosine kinases are tumorigenic. The biochemical signals that mediate ligand-independent transformation of cells by these transmembrane receptors are poorly defined. In this report, we demonstrate that a constitutively activated mutant epidermal growth factor receptor (v-ErbB) induces the formation of a transformation-specific signaling module that complexes with myosin II. The components of this signaling complex include the signal adapter proteins Shc, Grb2, and Nck, and tyrosine-phosphorylated forms of p21-activated kinase (Pak), caldesmon, and myosin light chain kinase. Transformation-specific, tyrosine phosphorylation of Pak enhances the catalytic activity of this serine/threonine kinase. Furthermore, the tyrosine phosphorylation of Pak is Rho-, but not Ras-, Rac-, or Cdc42-dependent. These results demonstrate that a ligand-independent epidermal growth factor receptor mutant can transduce oncogenic signals that are distinct from ligand-dependent, mitogenic signals. In addition, these data provide evidence for the coupling of oncogenic receptor tyrosine kinases with the actomyosin molecular motor. This myosin-associated signaling module may mediate some of the biochemical changes of myosin found in v-ErbB-transformed fibroblasts, thereby contributing to the regulation of the mechanical forces governing cellular adhesion, cytoskeletal tension, and, hence, anchorage-independent cell growth.