Chemotactic responses of eukaryotic cells require a signal processing system that translates an external gradient of attractant into directed motion. To challenge the response system to its limits, we increased the size of Dictyostelium cells using electric-pulse induced fusion. Large cells formed multiple protrusions at different sites along the gradient of chemoattractant, which independently turned into the gradient direction and competed with each other. Finally, these cells succeeded to re-establish polarity by coordinating front and tail activities. To analyze the responses, we combined two approaches, one aimed at local responses by visualizing the dynamics of Ras activation at the front regions of re-orientating cells, the other at global changes of polarity by monitoring front-to-tail directed actin flow. Asymmetric Ras activation in turning protrusions underscores that gradients can be sensed locally and translated into orientation. Different from normal-sized cells, the polarity of large cells is not linked to an increasing front-to-tail gradient of the PIP3-phosphatase PTEN. But even in large cells, the front communicates with the tail by an actin flow that may act as carrier of a protrusion inhibitor.
- Received April 18, 2016.
- Accepted August 1, 2016.
- © 2016. Published by The Company of Biologists Ltd