The G protein β subunit is essential for multiple responses to chemoattractants in Dictyostelium

Increasing evidence suggests that the βγ-subunit dimers of heterotrimeric G proteins play a pivotal role in transducing extracellular signals. The recent construction of Gβ null mutants (gβ^-) in Dictyostelium provides a unique opportunity to study the role of dimers in signaling processes mediated by chemoattractant receptors. We have shown previously that gβ^- cells fail to aggregate; in this study, we report the detailed characterization of these cells. The gβ^- cells display normal motility but do not move towards chemoattractants. The typical GTP-regulated high affinity chemoattractant-binding sites are lost in gβ^- cells and membranes. The gβ^- cells do not display chemoattractant-stimulated adenylyl cyclase or guanylyl cyclase activity. These results show that in vivo Gβ links chemoattractant receptors to effectors and is therefore essential in many chemoattractant-mediated processes. In addition, we find that Gβ is required for GTPγS stimulation of adenylyl cyclase activity, suggesting that the βγ-dimer activates the enzyme directly. Interestingly, the gβ^- 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.