Fig. 4.

Filopodial retraction stall force is controlled by ligand clustering. (A) Signalling induced by a low density of Invasin (red dots) on the bead induces filopodial retraction that is stalled with a lower force (top panel) than by a high Invasin density (bottom panel). (B) When the filopodium interacts with the T3SS (red and blue clusters), a few clusters of ligands are sufficient to induce retraction with a higher stall force than at a high density of Invasin molecules. (C) Carboxylated beads interacting with different types of receptors, depicted by multi-coloured rectangles, induce filopodial retraction with a high stall force. The width of the green arrows represents the strength of the stall force; empty finger-like shapes represent filopodia; grey circles represent the beads. (D) Scanning electron micrographs of Shigella interacting with filopodia. The filopodial area contacting the bacterium does not significantly differ from that contacting the beads (Materials and Methods). Scale bar: 1 µm.