Localization of host endocytic and actin‐associated proteins during Shigella flexneri intracellular motility and intercellular spreading

Abstract Shigella flexneri ( S. flexneri ), the causative agent of bacillary dysentery, uses an effector‐mediated strategy to hijack host cells and cause disease. To propagate and spread within human tissues, S. flexneri bacteria commandeer the host actin cytoskeleton to generate slender actin‐rich comet tails to move intracellularly, and later, plasma membrane actin‐based protrusions to move directly between adjacent host cells. To facilitate intercellular bacterial spreading, large micron‐sized endocytic‐like membrane invaginations form at the periphery of neighboring host cells that come into contact with S. flexneri‐ containing membrane protrusions. While S. flexneri comet tails and membrane protrusions consist primarily of host actin cytoskeletal proteins, S. flexneri membrane invaginations remain poorly understood with only clathrin and the clathrin adapter epsin‐1 localized to the structures. Tangentially, we recently reported that Listeria monocytogenes, another actin‐hijacking pathogen, exploits an assortment of caveolar and actin‐bundling proteins at their micron‐sized membrane invaginations formed during their cell‐to‐cell movement. Thus, to further characterize the S. flexneri disease process, we set out to catalog the distribution of a variety of actin‐associated and caveolar proteins during S. flexneri actin‐based motility and cell‐to‐cell spreading. Here we show that actin‐associated proteins found at L. monocytogenes comet tails and membrane protrusions mimic those present at S. flexneri comet tails with the exception of α‐actinins 1 and 4, which were shed from S. flexneri membrane protrusions. We also demonstrate that all known host endocytic components found at L. monocytogenes membrane invaginations are also present at those formed during S. flexneri infections.