THE G-PROTEIN BETA-SUBUNIT IS ESSENTIAL FOR MULTIPLE RESPONSES TO CHEMOATTRACTANTS IN DICTYOSTELIUM

Increasing evidence suggests that the beta gamma-subunit dimers of heterotrimeric G proteins play a pivotal role in transducing extracellular signals. The recent construction of G beta null mutants (g beta(-)) in Dictyostelium provides a unique opportunity to study the role of beta gamma dimers in signaling processes mediated by chemoattractant receptors. We have shown previously that g beta(-) cells fail to aggregate; in this study, we report the detailed characterization of these cells. The g beta(-) cells display normal motility but do not move towards chemoattractants. The typical GTP-regulated high affinity chemoattractant-binding sites are lost in g beta(-) cells and membranes. The g beta(-) cells do not display chemoattractant-stimulated adenylyl cyclase or guanylyl cyclase activity. These results show that in vivo G beta links chemoattractant receptors to effecters and is therefore essential in many chemoattractant-mediated processes. In addition, we find that G beta is required for GTP gamma S stimulation of adenylyl cyclase activity, suggesting that the beta gamma-dimer activates the enzyme directly. Interestingly, the g beta(-) cells grow at the same rate as wild-type cells in axenic medium but grow more slowly on bacterial lawns and, therefore, may be defective in phagocytosis.