Mediation of cell-substratum adhesion by RasG in Dictyostelium

Previous studies on the functions of the RasG gene in the cellular slime mold, Dictyostelium discoideum, have revealed that it is required for normal motility and cytokinesis. To further understand how the RasG gene regulates various cellular processes, we transformed an activated form of RasG, that is, RasG (G12T), a mutation from glycine to threonine at amino acid position 12 into wild type KAX-3 cells. This produced moderate but constitutive RasG(G12T) protein expression, which causes cells to become significantly more adherent to the substratum than are wild type cells. The RasG(G12T) transformants also grow slowly on bacterial plates, and engulf fewer bacteria on filter surfaces, indicating a defect in phagocytosis when cells are adhered. The expression of the activated RasG also dramatically reduces the number of filopodia on the cell surface. Tyrosine phosphorylation on a 43 kDa protein (most likely actin) of the RasG (G12T) transformants is highly elevated. Taken together, our observations suggest that RasG is crucial for Dictyostelium cell-substratum adhesion during growth and that RasG may play a role in adhesion-mediated phagocytosis. Our results also suggest that RasG is important in filopodial formation and that RasG is involved in the signal pathway that is regulated by tyrosine phosphorylation.