The small GTPase Rac1 shows spontaneous oscillations and rotations in amoeboid cells of the protist Dictyostelium discoideum

GTPases of the Rho family are small signaling proteins that play a central role in the regulation of cell motility by controlling actin cytoskeleton remodeling. Intracellular oscillations of Rho GTPases have been identified in a variety of different cell types and species, including yeast, plants and mammals. In the free-living amoeboid cells of the protist Dictyostelium discoideum, the active form of the Rho GTPase Rac1 regulates actin polymerases at the leading edge and actin filament bundling proteins at the posterior cortex of polarized cells. We monitored the spatio-temporal dynamics of Rac1 and its effector DGAP1 in vegetative amoebae using specific fluorescent probes. We observed that plasma membrane domains enriched in active Rac1 not only exhibited stable polarization, but also showed oscillating and rotating intracellular patterns with a typical period between 2 and 5 minutes, while DGAP1 was depleted from Rac1-enriched regions. To simulate the observed dynamics of the two proteins, we developed a mass-conserving reaction-diffusion model based on the circulation of Rac1 between the membrane and the cytoplasm in conjunction with its activation by guanine nucleotide exchange factors (GEFs), deactivation by GTPase activating proteins (GAPs) and interaction with DGAP1. Our theoretical model accurately reproduced the experimentally observed dynamic patterns, including the predominant anti-correlation between active Rac1 and DGAP1. Significantly, the model predicted a new colocalization regime of these two proteins in polarized cells, which we confirmed experimentally. In summary, our results improve the understanding of Rac1 dynamics and reveal how the occurrence and transitions between different regimes depend on biochemical reaction rates, protein levels and cell size. This study not only expands our knowledge of Rac1 behavior in Dictyostelium discoideum amoebae, but also shows how specific types of interaction between a small GTPase and its effector lead to their counterintuitive anticorrelated dynamics.