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Department of Microbiology, University of Pennsylvania, Philadelphia, Pennsylvania 19104
Listeria monocytogenes is a facultative intracellular bacterial pathogen that spreads cell to cell without exposure to the extracellular environment. Bacterial cell-to-cell spread is mediated in part by two
secreted bacterial phospholipases C (PLC), a broad spectrum PLC (PC-PLC) and a phosphatidylinositolspecific PLC (PI-PLC). PI-PLC is secreted in an active
state, whereas PC-PLC is secreted as an inactive proenzyme (proPC-PLC) whose activation is mediated in
vitro by an L. monocytogenes metalloprotease (Mpl).
Analysis of PI-PLC, PC-PLC, and Mpl single and double mutants revealed that Mpl also plays a role in the
spread of an infection, but suggested that proPC-PLC
has an Mpl-independent activation pathway. Using biochemical and microscopic approaches, we describe
three intracellular proteolytic pathways regulating PCPLC activity. Initially, proPC-PLC secreted in the cytosol of infected cells was rapidly degraded in a proteasome-dependent manner. Later during infection, PCPLC colocalized with bacteria in lysosome-associated membrane protein 1-positive vacuoles. Activation of
proPC-PLC in vacuoles was mediated by Mpl and an
Mpl-independent pathway, the latter being sensitive to
inhibitors of cysteine proteases. Lastly, proPC-PLC activation by either pathway was sensitive to bafilomycin
A1, a specific inhibitor of vacuolar ATPase, suggesting that activation was dependent on acidification of the
vacuolar compartment. These results are consistent
with a model in which proPC-PLC activation is compartment specific and controlled by a combination of
bacterial and host factors.
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