A secondary disruption of the dmpA gene encoding a large membrane protein allows aggregation defective Dictyostelium rasC− cells to form multicellular structures

The disruption of the gene encoding the Dictyostelium Ras subfamily protein, RasC, results in a strain that does not aggregate and has defects in both cAMP signal relay and cAMP chemotaxis. Disruption of a second gene in the rasC− strain by Restriction Enzyme Mediated Integration produced cells that were capable of forming multicellular structures in plaques on bacterial lawns. The disrupted gene (dmpA) encoded a novel membrane protein that was designated Dmp1. Although the rasC−/dmpA− cells progressed through early development, they did not form aggregation streams on a plastic surface under submerged starvation conditions. Phosphorylation of PKB in response to cAMP, which is significantly reduced in rasC− cells, remained low in the rasC−/dmpA− cells. However, in spite of this low PKB phosphorylation, the rasC−/dmpA− cells underwent efficient chemotaxis to cAMP in a spatial gradient. Cyclic AMP accumulation, which was greatly reduced in the rasC− cells, was restored in the rasC−/dmpA− strain, but cAMP relay in these cells was not apparent. These data indicate that although the rasC−/dmpA− cells were capable of associating to form multicellular structures, normal aggregative cell signaling was clearly not restored. Disruption of the dmpA gene in a wild-type background resulted in cells that exhibited a slight defect in aggregation and a more substantial defect in late development. These results indicate that, in addition to the role played by Dmp1 in aggregation, it is also involved in late development.