RasGEFX triggers spontaneous Ras excitation with RasGEFB/M/U for random cell migration

Excitable systems of eukaryotic chemotaxis can generate asymmetric signals of Ras-GTP-enriched domains spontaneously to drive random cell migration without guidance cues. The molecules responsible for the spontaneous signal generation remain elusive. Here, we characterized RasGEFs encoded in Dictyostelium discoideum by the live-cell imaging of the Ras dynamics and hierarchical clustering, finding that RasGEFX triggers symmetry breaking to generate a Ras-GTP-enriched domain and is essential for random migration in combination with RasGEFB/M/U but dispensable for chemotaxis. Among RasGEFs, RasGEFX and RasGEFB co-localize with Ras-GTP on oscillatory waves propagating along the membrane and regulate the temporal periods and spatial sizes, respectively, of the waves and thus the cytoskeletal dynamics differently. RasGEFU localized uniformly on the membrane and regulated cell adhesion. These findings provide mechanistic insights into the internal signal generation that operates independently of external chemotaxis signaling and suggest a coordinated control of the cytoskeletal dynamics by multiple RasGEFs for spontaneous motility.