Mediation of cell-substratum adhesion by RasG in Dictyostelium

Previous studies on the functions of the RasG gene in the cellular slims mo ld, 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 positi on 12 into wild type KAX-3 cells. This produced moderate but constitutive R asG(G12T) protein expression, which causes cells to become significantly mo re adherent to the substratum than are wild type cells. The RasG(G12T) tran sformants also grow slowly on bacterial plates, and engulf fewer bacteria o n filter surfaces, indicating a defect in phagocytosis when cells are adher ed. The expression of the activated RasG also dramatically reduces the numb er of filopodia on the cell surface. Tyrosine phosphorylation on a 43 kDa p rotein (most likely actin) of the RasG (G12T) transformants is highly eleva ted. Taken together, our observations suggest that RasG is crucial for Dict yostelium cell-substratum adhesion during growth and that RasG may play a r ole in adhesion-mediated phagocytosis. Our results also suggest that RasG i s important in filopodial formation and that RasG is involved in the signal pathway that is regulated by tyrosine phosphorylation. (C) 2000 Wiley-Liss , Inc.