By exploiting cellular pathways of its host cell, the bacterial pathogen, Listeria monocytogenes generates F-actin “tails” that propel it from one host cell to the next. Its membrane protein, ActA, stimulates host proteins involved in both lamellipodia and filopodia protrusion. Using high-resolution laser-tracking, we discovered nanometer-scale stepping during motility, both in living cells and in tissue extracts. For newer studies of mutant ActA molecules unable to interact with VASP, nanometer-level motility is more erratic, both in direction and in apparent step size. To slow Listeria in brain extracts, we used thickening agents and discovered a highly biphasic force-velocity relationship. Newer experiments using purified proteins suggest much higher forces that require different thickening agents. We suspect that higher forces are due to higher concentrations of key proteins. Collectively, these discoveries have prompted new or altered molecular models for force-generation by actin polymerization.
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