Activation of Gα_i and subsequent uncoupling of receptor-Gα_i signaling by Pasteurella multocida toxin

Bacterial protein toxins are powerful tools for elucidating signaling mechanisms in eukaryotic cells. A number of bacterial protein toxins, e.g. cholera toxin, pertussis toxin (PTx), or Pasteurella multocida toxin (PMT), target heterotrimeric G proteins and have been used to stimulate or block specific signaling pathways or to demonstrate the contribution of their target proteins in cellular effects. PMT is a major virulence factor of P. multocida causing pasteurellosis in man and animals and is responsible for atrophic rhinitis in pigs. PMT modulates various signaling pathways, including phospholipase Cβ and RhoA, by acting on the heterotrimeric G proteins Gα_q and Gα_12/13, respectively. Here we report that PMT is a powerful activator of G_i protein. We show that PMT decreases basal isoproterenol and forskolin-stimulated cAMP accumulation in intact Swiss 3T3 cells, inhibits adenylyl cyclase activity in cell membrane preparations, and enhances the inhibition of cAMP accumulation caused by lysophosphatidic acid via endothelial differentiation gene receptors. PMT-mediated inhibition of cAMP production is independent of toxin activation of Gα_q and/or Gα_12/13. Although the effects of PMT are not inhibited by PTx, PMT blocks PTx-catalyzed ADP-ribosylation of G_i. PMT also inhibits steady-state GTPase activity and GTP binding of G_i in Swiss 3T3 cell membranes stimulated by lysophosphatidic acid. The data indicate that PMT is a novel activator of G_i, modulating its GTPase activity and converting it into a PTx-insensitive state.