engleski

Oscillatory dynamics of Rac1 GTPases in Dictyostelium

Small GTPases from the Rho family are a group of signalling proteins that regulate and coordinate many aspects of cellular activities driven by the actin cytoskeleton. We are particularly interested in understanding their function in random cell migration. Here, we present results of experimental and modelling approaches regarding the dynamics of Rac1 GTPases and their effector molecules in Dictyostelium cells. It is well known that binding of active Rac to the Scar/Wave complex induces actin polymerization and formation of protrusions at the front end of the cell. Our results imply that Rac1 also plays a prominent role at the trailing end of the cell, where Rac1-GTP initiates assembly of a DGAP1/Cortexillin complex, which determines structural properties of the cell cortex. We recently formulated a hypothesis that the competition between two Rac1 effectors, the Scar/Wave and DGAP1/Cortexillin complexes, for the common pool of active Rac1, is responsible for observed self-sustained fluctuations in cell polarity. In order to test this hypothesis, the Rac1 activity in randomly migrating Dictyostelium cells was monitored over time by confocal microscopy using a specific fluorescent biosensor. Besides active Rac1, a component of an effector complex, DGAP1, was also tagged with a fluorescent protein. Dynamics of active Rac1 and DGAP1 were processed by QuimP software, visualized in the form of kymographs, and studied by wavelet-based cross-correlation analysis. We observed that active Rac1 and DGAP1 in the cell cortex exhibit anti-correlated oscillations in the form of standing and travelling waves. In order to gain insight into the molecular mechanisms underlying the observed dynamics, we formulated a reaction-diffusion model that incorporates interactions between Rac1, DGAP1 and a Rac1-inactivating protein, GAP. The model was able to reproduce the basic features of obtained experimental results.

Dictyostelium ; oscillators ; Rac1

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