G Protein Function during Biomembrane Fusion in Dictyostelium: Presence and Importance of a Gαs Subunit during Fertilization and Phagocytosis

Previous work has shown that GTPase function is essential for fertilization and cannibalistic phagocytosis during the sexual development of Dictyostelium discoideum . In this work, the importance of heterotrimeric G proteins during these events was established further using aluminum fluoride which inhibited both fertilization and cannibalistic phagocytosis, as well as the phagocytosis of bacteria by vegetative amoebae. Using distinct immune sera directed against the amino terminus and the carboxy terminus of mammalian Gαs, we have provided unique evidence for a Gαs subunit of approximately 55 kDa in D. discoideum (referred to as dGαs). Furthermore, this protein localizes to the membranes of fusing cells as well as to both vegetative and zygote giant cells, indicating that it might function during fertilization as well as during both vegetative (i.e., bacterial) and cannibalistic (i.e., amoebal) phagocytosis. During its down-regulation in nonphagocytic cells new isozymes of dGαs appear, suggesting that it may be posttranslationally modified. Having identified a putitive Gαs homologue, this work has set the stage for further investigations into its function in Dictyostelium.