Palladin is needed for proper actin‐rich comet tail formation and can functionally replace the Arp2/3 complex during intracellular motility of Listeria bacteria

Abstract

Listeria monocytogenes (Listeria) bacteria are well‐known for their ability to hijack the eukaryotic actin cytoskeleton during infections. To propagate disease, Listeria generate Arp2/3‐based actin‐rich structures called comet tails that move the bacteria within and amongst host cells. The host protein palladin is a key component of actin‐rich structures normally generated during eukaryotic cell motility, however the precise function(s) of palladin during motility remains unclear. Here we tested the hypothesis that palladin is a crucial factor for Listeria motility and simultaneously used Listeria as a model to determine if palladin itself could functionally replace the Arp2/3 complex. Using palladin‐targeting antibodies, we identified palladin at Listeria invasion sites and comet tails. Strikingly, when we depleted cells of palladin, comet tails became shorter and severely misshapen. In cells expressing palladin mutants defective for actin or VASP binding, comet tails began to disintegrate or became progressively thinner as they moved. Ultrastructural examination of these thin comet tails revealed a switch in the comet tail actin network from highly branched arrays to parallel bundles. To test whether palladin could compensate for the Arp2/3 complex during bacterial motility, we overexpressed palladin in cells treated with the potent Arp2/3 inhibitor CK‐666. In these cells Listeria motility was unperturbed. Next we used a cell line depleted of several Arp2/3 complex subunits. As expected, Listeria were non‐motile during infections of these cells, however palladin overexpression in this Arp2/3 functionally null cell line restored the ability of Listeria to generate the actin‐rich structures formed by the bacteria. To definitively demonstrate palladin's ability to compensate for a lack of functional Arp2/3, we used purified protein components in conjunction with Listeria bacteria to show that actin‐rich structures formed by Listeria are generated in a cell‐free system containing palladin in place of the Arp2/3 complex. In conclusion, we show that palladin structurally organizes bacterial actin‐rich structures and importantly, compensates for the Arp2/3 complex 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), SFU departmental funds and SFU Multi‐Year Funding (A.S.D)This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.