G(I2)-MEDIATED ACTIVATION OF THE MAP KINASE CASCADE

The G(i) class of heterotrimeric G proteins has been implicated in transmitting mitogenic signals from a variety of seven-transmembrane domain receptors. In addition, the alpha subunit of G(i2) (alpha(i2)) is oncogenic when mutated to a constitutively active form (gip2). The mechanism by which G(i2) stimulates cellular proliferation is unknown, but is believed to involve activation of the mitogen-activated protein kinase (MAPK) signaling cascade. To study G(i2) activation of the cascade, we transiently expressed a mutant, pertussis toxin (PTX)-resistant alpha(i2) in Chinese hamster ovary cells. After PTX treatment of these cells, G(i)-coupled receptors specifically activated PTX-resistant G(i2) without activating other G(i) proteins. Receptor-mediated activation of G(i2) led to activation of MAPK and its upstream activator, MAPK/ERK-activating kinase (MEK). Activation of MAPK and MEK by G(i2) was blocked by expression of a dominant-negative mutant of Ras. G(i2) activation did not, however, detectably increase the proportion of Ras protein in the GTP-bound form. Additional experiments suggest that G(i2) stimulates the MAPK pathway, at least in part, by mechanisms that involve release of its beta gamma subunit, as well as activation of phosphatidylinositol-3 kinase.