Actin self-organization in gliding parasitic cells

Following a playful introduction to the Toner-Tu flocking theory first inspired by the collective motion of animals, we will move down a million-fold in length scale to focus on “molecular flocks”: collectively moving actin filaments at the surface of the single-celled parasite Toxoplasma gondii. During host infection, apicomplexan parasites like Plasmodium and Toxoplasma use these myosin-powered surface actin movements to drive a form of cell locomotion called gliding, which differs fundamentally from the swim-or-crawl paradigm of eukaryotic cell motility. How does the collective motion of surface actin filaments emerge, and how does it drive the varied parasite gliding movements that we observe experimentally? I will present findings based on single-molecule imaging in live parasites and use continuum flocking theory to predict emergent filament flows in the unusual confines provided by parasite geometry. This molecular filament flocking model enables the exploration of distinct self-organized states tuned by filament lifetime, which can account for the diversity of observed Toxoplasma gliding motions. This theory-experiment interplay will illustrate how different forms of gliding motility can arise as an intrinsic consequence of emergent active filament flows on a complex surface.