Practice question

Arp2/3 mediated branch formation depends on nucleation promoting factors bearing C central A acidic region plus WH2 actin binding motifs. Endogenous factors include N WASP activated by Cdc42 and PIP2 in endocytosis and filopodia, WAVE activated by Rac at lamellipodia, WHAMM at Golgi. Bacterial pathogens mimic motif to hijack host cytoskeleton. ActA surface protein of Listeria monocytogenes 639 amino acids contains N terminal polycationic stretch, central proline rich repeats binding Ena VASP and acidic actin binding motifs functioning as VCA equivalent recruiting Arp2/3 with affinity 0.3 micromolar. By polar localization achieved through Sec translocation and proteolytic processing leaving ActA concentrated at old pole, bacterium creates sustained local activation generating polarized comet tail propulsion at 0.05 to 0.3 micron per second enabling cell to cell spread without extracellular exposure. Rho GTP activates formin ROCK but not directly Arp2/3 as NPF, gelsolin caps and severs, vinculin links adhesion plaques. Thus ActA exemplary bacterial NPF for Arp2/3 dependent motility and spread.