The dermatonecrotic toxin produced by Pasteurella multocida is one of the most potent mitogenic substances known for fibroblasts in vitro. Exposure to recombinant P. multocida toxin (rPMT) causes phospholipase C-mediated hydrolysis of inositol phospholipids, calcium mobilization, and activation of protein kinase C via a poorly characterized mechanism involving G q/11 family heterotrimeric G proteins. To determine whether the regulation of G protein pathways contributes to the mitogenic effects of rPMT, we have examined the mechanism whereby rPMT stimulates the Erk mitogen-activated protein kinase cascade in cultured HEK-293 cells. Treatment with rPMT resulted in a dose and time-dependent increase in Erk 1/2 phosphorylation that paralleled its stimulation of inositol phospholipid hydrolysis. Both rPMT-and ␣-thrombin receptor-stimulated Erk phosphorylation were selectively blocked by cellular expression of two peptide inhibitors of G q/11 signaling, the dominant negative mutant G protein-coupled receptor kinase, GRK2(K220R), and the G␣ q carboxyl-terminal peptide, G␣ q -(305-359). Like ␣-thrombin receptor-mediated Erk activation, the effect of rPMT was insensitive to the protein kinase C inhibitor GF109203X, but was blocked by the epidermal growth factor receptor-specific tyrphostin, AG1478 and by dominant negative mutants of mSos1 and Ha-Ras. These data indicate that rPMT employs G q/11 family heterotrimeric G proteins to induce Ras-dependent Erk activation via protein kinase C-independent "transactivation" of the epidermal growth factor receptor.Pasteurella multocida is a wide ranging bacterium found in the respiratory tracts of over 60 avian and 40 mammalian species (1). It is a significant veterinary, and occasional human, pathogen and the cause of swine atrophic rhinitis, a condition characterized by osteoclastic bone resorption and severe progressive turbinate damage. The pathogenicity of P. multocida is related to the production of a 146-kDa toxin (2, 3) that exhibits little functional homology to other known toxins or proteins. Both native and recombinant PMT 1 are potently mitogenic for several cell types in vitro (4). In Rat1 fibroblasts, rPMT produces anchorage-independent DNA synthesis and growth in soft agar (5). In primary osteoblastic cells in culture, rPMT induces cell proliferation and down-regulation of markers of osteoblast differentiation (6). The mechanisms whereby rPMT exerts its mitogenic effects are poorly understood. The toxin interacts with a gangliosidetype cell surface receptor and is internalized via both coated and noncoated endocytic structures (7,8). The mitogenic effects of rPMT require its internalization, since exposure of cells to rPMT at 4°C or incubation with the weak base methylamine prevents the response (4). Exposure to rPMT stimulates the hydrolysis of inositol phospholipids, calcium mobilization, and phosphorylation of protein kinase C (PKC) substrates, including the myristoylated alanine-rich C kinase substrate protein (4, 5, 9). In addition, rPMT has been shown to stimu...