Activation of the Mitogen-activated Protein Kinase ERK2 by the Chemoattractant Folic Acid in Dictyostelium*

Abstract

The Dictyostelium MAP kinase ERK2 is activated by extracellular cAMP in aggregation-competent cells and is required for receptor activation of adenylyl cyclase (Maeda, M., Aubry, L., Insall, R., Gaskins, C., Devreotes, P. N., and Firtel, R. A. (1996) J. Biol. Chem. 271, 3351–3354; Segall, J., Kuspa, A., Shaulsky, G., Ecke, M., Maeda, M., Gaskins, C., Firtel, R., and Loomis, W. (1995) J. Cell Biol. 128, 405–413). This cAMP-dependent activation of ERK2 is mediated by the serpentine, G protein-coupled cAMP receptors. However, ERK2 activation by cAMP is at least partially heterotrimeric G protein-independent, with a level of activation in cells lacking the sole Gβ subunit or the G protein-coupled cAMP receptors-coupled Gα2 subunit that is ∼50% that of wild-type cells (Maeda, M., Aubry, L., Insall, R., Gaskins, C., Devreotes, P. N., and Firtel, R. A. (1996)J. Biol. Chem. 271, 3351–3354; Segall, J., Kuspa, A., Shaulsky, G., Ecke, M., Maeda, M., Gaskins, C., Firtel, R., and Loomis, W. (1995) J. Cell Biol. 128, 405–413). Folic acid, a chemoattractant in the vegetative cells that enables amoebae to find bacteria in the wild, also triggers the activation of adenylyl cyclase, which is impaired in the vegetative cells lacking the Gα protein subunit Gα4 (Hadwiger, J., Lee, S., and Firtel, R. (1994) Proc. Natl. Acad. Sci. U. S. A. 91, 10566–10570). In this study, we show that folic acid activates ERK2 in developmentally regulated manner and is required for ERK2 stimulation of adenylyl cyclase activity. Maximum levels of folate-stimulated ERK2 activity occur in cells from very early in development, prior to aggregation, and again at the tipped aggregate stages, corresponding to the stages in which folate receptors and the coupled Gα subunit Gα4 are maximally expressed. During the activation by folic acid, ERK2 is phosphorylated on tyrosine residue(s) and contemporaneously shows a mobility shift on SDS-PAGE. Interestingly, this activation is not elicited in the absence of Gβ subunits, in contrast to the response to cAMP. This response also requires the Gα4 subunit known to be required for other folic acid-mediated responses (Hadwiger, J., Lee, S., and Firtel, R. (1994)Proc. Natl. Acad. Sci. U. S. A. 91, 10566–10570). Furthermore, we show that the activation of ERK2 by cAMP is independent of the Gα4 subunit, while the activation of ERK2 by folate is independent of Gα2. Taken together, these data indicate that there are at least two pathways of ERK2 activation, heterotrimeric G protein-dependent and -independent pathways.