Practice question

Intracellular pathogenesis of Listeria monocytogenes illustrates actin based propulsion mechanism. After internalization via E cadherin or Met receptor mediated endocytosis, bacterium escapes vacuole via pore forming toxin listeriolysin O and phospholipases PlcA PlcB, enters cytosol and replicates. It then expresses ActA protein anchored via C terminal transmembrane domain, retained at old pole after secretion. ActA N terminal domain mimics eukaryotic nucleation promoting factor recruiting and activating host Arp2/3 complex via acidic motifs and binding Ena VASP via proline rich repeats accelerating elongation. Activation produces dense branched network forming actin tail up to 10 micron long comet behind bacterium with barbed ends oriented toward bacterial surface. Continuous polymerization generates compressive force propelling bacterium forward at approximately 0.1 to 1 micron per second, enabling protrusion into neighboring cell forming double membrane secondary vacuole. Isogenic actA deletion mutants avirulent immobile in cytosol. Capping protein regulates tail length but not initiation, tropomyosin stabilizes stress fibers, vinculin adhesion linking, none responsible for Listeria propulsion mechanism.