The Arp2/3 complex is essential for the actin-based motility of Listeria monocytogenes

Actin polymerisation is thought to drive the movement of eukaryotic cells a nd some intracellular pathogens such as Listeria monocytogenes, The Listeri a surface protein ActA synergises with recruited host proteins to induce ac tin polymerisation, propelling the bacterium through the host cytoplasm [1] . The Arp2/3 complex is one recruited host factor [2,3]; it is also believe d to regulate actin dynamics in lamellipodia [4,5], The Arp2/3 complex prom otes actin filament nucleation in vitro, which is further enhanced by ActA [6,7]. The Arp2/3 complex also interacts with members of the Wiskott-Aldric h syndrome protein (WASP) [8] family Scar1 [9,10] and WASP itself [11], We interfered with the targeting of the Arp2/3 complex to Listeria by using ca rboxy-terminal fragments of Scar1 that bind the Arp2/3 complex [11]. These fragments completely blocked actin tail formation and motility of Listeria, both in mouse brain extract and in Ptk2 cells overexpressing Scar1 constru cts. In both systems, Listeria could initiate actin cloud formation, but ta il formation was blocked, Full motility in vitro was restored by adding pur ified Arp2/3 complex. We conclude that the Arp2/3 complex is a host-cell fa ctor essential for the actin-based motility of L. monocytogenes, suggesting that it plays a pivotal role in regulating the actin cytoskeleton.