Activation of the mitogen-activated protein kinase pathway by a G(q/11)-coupled muscarinic receptor is independent of receptor internalization

A number of recent studies have demonstrated an essential role for receptor endocytosis in the activation of the mitogen-activated protein (MAP) kinas es, Erk-1 and Erk-2 (extracellular activated protein kinases 1 and 2), by g rowth factor receptors and the G-protein coupled beta(2)-adrenergic recepto r, Because ligand-mediated receptor endocytosis and activation of the MAP k inase pathway are common phenomena among G-protein coupled receptors, it ha s been suggested that the essential role of endocytosis in MAP kinase activ ation identified for the beta(2)-adrenergic receptor may be universal for a ll G-protein coupled receptors (Daaka,Y,, Luttrell, L, M,, Ahn, S,, Della R occa, G, J., Ferguson, S, S, G,, Caron, M. G,, and Lefkowitz, R, J, (1998) J, Biol, Chem, 273, 685-688), We tested this hypothesis using the G(q/11)-c oupled m3-muscarinic receptor expressed in Chinese hamster ovary cells and an m3-muscarinic receptor mutant that does not undergo endocytosis. We demo nstrate that inhibition of endocytosis by concanavalin A and cytochalasin D does not affect the ability of the wild type m3-muscarinic receptor to act ivate Erk-1/2, Furthermore, the mutant m3-muscarinic receptor that is unabl e to undergo endocytosis, activates the MAP kinase pathway in an identical manner to the wild type receptor. We conclude that receptor endocytosis is not universally essential for MAP kinase activation by G-protein coupled re ceptors, We discuss the possibility that the differential roles played by e ndocytosis in MAP kinase activation between various receptor subtypes may b e linked to the mechanism of upstream activation of Raf-1.