Palladin Functionally Replaces the Arp2/3 Complex and Organizes Actin‐Rich Structures Generated during Listeria monocytogenes Infections

Abstract

Listeria monocytogenes (Listeria) co‐opts the actin cytoskeleton of host cells for their entry, intracellular motility and dissemination. These bacteria generate Arp2/3‐based, branched actin‐containing structures called comet tails that propel the bacteria within and amongst host cells. The host protein palladin is a crucial component of actin‐rich structures. Whether this protein can promote actin based motility events remains unclear. Here we tested the hypothesis that palladin is a crucial organizer of actin structures generated during Listeria infections and simultaneously used Listeria as a model to establish if palladin could compensate for processes requiring the Arp2/3 complex. Using immunofluorescence microscopy we identified palladin at bacterial invasion sites and motile comet tails. Strikingly, depletion of palladin via RNAi results in shorter and severely misshapen comet tails. When palladin mutants defective for actin or VASP binding are expressed in cells, comet tails disintegrate or become progressively thinner as they move. Through transmission electron microscopy, we demonstrate that these thin comet tails result from a switch in the actin network from branched arrays to parallel F‐actin bundles. To determine whether palladin could compensate for the Arp2/3 complex during comet tail motility, we overexpressed palladin in cells treated with the potent Arp2/3 inhibitor CK‐666. In these cells, Listeria can initiate and maintain its actin‐based motility. Additionally, we utilized a cell line depleted of multiple Arp2/3 complex subunits. Within these cells Listeria fail to generate comet tails. However, when palladin is overexpressed in this Arp2/3 functionally null cell line, the ability of Listeria to generate actin clouds and comet tails is restored. Taken together, we show that palladin is essential for the organization of F‐actin structures generated by Listeria and, importantly, can compensate for Arp2/3 complex functional defects without hindrance during bacterial actin‐based motility.Support or Funding InformationGrant Funding Source: NSERC (grant no. 355316 to J.A.G and grant no. 155397 to A.W.V), NIH (grant no. R15 GM120670 to M.R.B) and, SFU departmental fundsThis abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.