Introductory Paragraph Several pathogenic bacteria, including Listeria monocytogenes, use an F-actin motility process to spread between mammalian cells1. Actin ‘comet tails’ propel Listeria through the cytoplasm, resulting in bacteria-containing membrane protrusions that are internalized by neighboring cells. The mechanism by which Listeria overcomes cortical tension to generate protrusions is unknown. Here, we identify bacterial and host proteins that directly regulate protrusions. We show that efficient spreading between polarized epithelial cells requires the secreted Listeria virulence protein InlC. We next identify the mammalian adaptor protein Tuba as a ligand of InlC. InlC binds to a C-terminal SH3 domain in Tuba, which normally engages the human actin regulatory protein N-WASP2. InlC promotes protrusion formation by inhibiting Tuba and N-WASP, most likely by impairing binding of N-WASP to the Tuba SH3 domain. Tuba and N-WASP are known to control the structure of apical junctions in epithelial cells3. We demonstrate that, by inhibiting Tuba and N-WASP, InlC makes taut apical junctions become slack. Experiments with myosin II inhibitors indicate that InlC-mediated perturbation of junctions accounts for the role of this bacterial protein in protrusion formation. Collectively, our results suggest that InlC promotes bacterial dissemination by relieving cortical tension, thereby enhancing the ability of motile bacteria to deform the plasma membrane into protrusions.

SummaryThe intracellular bacterial pathogen Listeria monocytogenes causes food-borne illnesses leading to gastroenteritis, meningitis or abortion. Listeria induces its internalization into some mammalian cells through binding of the bacterial surface protein InlB to its host receptor, the Met Receptor Tyrosine Kinase. InlBinduced activation of Met stimulates host signal transduction pathways that culminate in cell surface changes driving pathogen engulfment. One mammalian protein with the potential to couple Met to downstream signalling is the adaptor CrkII. CrkII contains an unusual carboxyl-terminal SH3 domain (SH3C) that promotes entry of Listeria. However, binding partners or downstream effectors of SH3C remain unknown. Here, we use RNA interference and overexpression studies to demonstrate that SH3C affects bacterial uptake, at least in part, through stimulation of host phosphatidylinositide (PI) 3-kinase. Experiments with latex beads coated with InlB protein indicated that one potential role of SH3C and PI 3 kinase is to promote changes in the F-actin cytoskeleton necessary for particle engulfment. Taken together, our results indicate that the CrkII SH3C domain engages a cellular ligand that regulates PI 3 kinase activity and host cell surface rearrangements.

The bacterial pathogen Listeria monocytogenes causes food-borne illnesses resulting in gastroenteritis, meningitis, or abortion. Listeria promotes its internalization into some human cells through binding of the bacterial surface protein InlB to the host receptor tyrosine kinase Met. The interaction of InlB with the Met receptor stimulates host signaling pathways that promote cell surface changes driving bacterial uptake. One human signaling protein that plays a critical role in Listeria entry is type IA phosphoinositide 3-kinase (PI 3-kinase). The molecular mechanism by which PI 3-kinase promotes bacterial internalization is not understood. Here we perform an RNA interference (RNAi)-based screen to identify components of the type IA PI 3-kinase pathway that control the entry of Listeria into the human cell line HeLa. The 64 genes targeted encode known upstream regulators or downstream effectors of type IA PI 3-kinase. The results of this screen indicate that at least 9 members of the PI 3-kinase pathway play important roles in Listeria uptake. These 9 human proteins include a Rab5 GTPase, several regulators of Arf or Rac1 GTPases, and the serine/threonine kinases phosphoinositide-dependent kinase 1 (PDK1), mammalian target of rapamycin (mTor), and protein kinase C-. These findings represent a key first step toward understanding the mechanism by which type IA PI 3-kinase controls bacterial internalization.

Listeria monocytogenes is a Gram-positive, food-borne bacterial pathogen capable of causing gastroenteritis, meningitis, or abortions (39, 32). Listeria induces its own internalization (entry) into nonphagocytic mammalian cells, a process that likely plays an important role in traversal of the intestinal, placental, and blood-brain barriers (7,14,16,23). One of the pathways of Listeria entry is mediated by interaction of the bacterial surface protein InlB with its host receptor, the Met receptor tyrosine kinase (16, 37).InlB-Met interaction triggers activation (tyrosine phosphorylation) of the Met receptor and subsequent rearrangements in the F-actin cytoskeleton of the mammalian cell (16,29). These cytoskeletal changes remodel the host cell surface, resulting in engulfment of adherent Listeria. One of the host mammalian proteins that acts downstream of Met to promote F-actin rearrangements and bacterial entry is type IA phosphoinositide (PI) 3-kinase (8,17,18). This PI 3-kinase is a heterodimeric enzyme comprised of an 85-kDa regulatory subunit and a 110-kDa catalytic subunit (4, 11). p85-p110 generates phosphatidylinositol 4,5-bisphosphate [PI(4,5)P 2 )] and phosphatidylinositol 3,4,5-trisphosphate [PI(3,4,5)P 3 ], which are lipid second messengers that regulate a variety of biological processes, including growth, survival, and motility of mammalian cells. A plethora of downstream "target" proteins that bind PI(4,5)P 2 and/or PI(3,4,5)P 3 and mediate the biological effects of p85-p110 have been identified (4, 24). However, mammalian proteins that act downstream of type IA PI 3-kinase to control Listeria uptake have yet to found.In this work, we demonstrate that the human GTPase-activating protein (GAP) ARAP2 is required for InlB-mediated cytoskeletal changes and entry of Listeria. ARAP2 is known to bind PI(3,4,5)P 3 , resulting in upregulation of a GAP domain that inactivates the mammalian GTPase Arf6 (42). We provide genetic evidence indicating that the ArfGAP domain of ARAP2 stimulates Listeria entry by antagonizing Arf6. ARAP2 has several functional domains in addition to its ArfGAP domain. One of these domains in ARAP2 interacts with the mammalian GTPase RhoA (42). Our genetic data demonstrate that this "RhoA binding" (RB) domain also plays a critical role in bacterial entry. Surprisingly, pharmacological experiments indicate that the RB domain controls Listeria uptake through an unknown mechanism that does not involve Rho proteins. Our work indicates a key role for host ARAP2 in InlB-mediated entry of Listeria. One of the likely ways that type IA PI 3-kinase controls entry of Listeria is through regulation of ARAP2. MATERIALS AND METHODSBacterial strains, mammalian cell lines, and media. The wild-type Listeria monocytogenes strain EGD and the isogenic ⌬inlB mutant strain containing an in-frame deletion of the inlB gene were previously described (10). These strains were grown in brain heart infusion (BHI) broth (Difco) and prepared for infection as described previously (10,18).The human epithelial cell lin...