The Listeria monocytogenes PASTA Kinase PrkA and Its Substrate YvcK Are Required for Cell Wall Homeostasis, Metabolism, and Virulence

Obstacles to bacterial survival and replication in the cytosol of host cells, and the mechanisms used by bacterial pathogens to adapt to this niche are not well understood. Listeria monocytogenes is a well-studied Gram-positive foodborne pathogen that has evolved to invade and replicate within the host cell cytosol; yet the mechanisms by which it senses and responds to stress to survive in the cytosol are largely unknown. To assess the role of the L. monocytogenes penicillin-binding-protein and serine/threonine associated (PASTA) kinase PrkA in stress responses, cytosolic survival and virulence, we constructed a ΔprkA deletion mutant. PrkA was required for resistance to cell wall stress, growth on cytosolic carbon sources, intracellular replication, cytosolic survival, inflammasome avoidance and ultimately virulence in a murine model of Listeriosis. In Bacillus subtilis and Mycobacterium tuberculosis, homologues of PrkA phosphorylate a highly conserved protein of unknown function, YvcK. We found that, similar to PrkA, YvcK is also required for cell wall stress responses, metabolism of glycerol, cytosolic survival, inflammasome avoidance and virulence. We further demonstrate that similar to other organisms, YvcK is directly phosphorylated by PrkA, although the specific site(s) of phosphorylation are not highly conserved. Finally, analysis of phosphoablative and phosphomimetic mutants of YvcK in vitro and in vivo demonstrate that while phosphorylation of YvcK is irrelevant to metabolism and cell wall stress responses, surprisingly, a phosphomimetic, nonreversible negative charge of YvcK is detrimental to cytosolic survival and virulence in vivo. Taken together our data identify two novel virulence factors essential for cytosolic survival and virulence of L. monocytogenes. Furthermore, our data demonstrate that regulation of YvcK phosphorylation is tightly controlled and is critical for virulence. Finally, our data suggest that yet to be identified substrates of PrkA are essential for cytosolic survival and virulence of L. monocytogenes and illustrate the importance of studying protein phosphorylation in the context of infection.
Author Summary Infection with intracellular pathogens causes a majority of the global infectious disease associated mortality. A number of intracellular pathogens must directly access the host cytosol in order to cause disease; however, non-cytosol adapted bacteria do not survive or replicate upon access to the cytosol. The mechanisms cytosolic pathogens use to adapt to this niche are largely unknown. The model cytosolic bacterial pathogen Listeria monocytogenes contains a single penicillin-binding-protein and serine/threonine associated (PASTA) kinase, PrkA. In other bacteria, PASTA kinases bind cell wall fragments and phosphorylate downstream effectors involved in cell wall synthesis, central metabolism, virulence, cell division, and biofilm formation. We demonstrate that in L. monocytogenes, PrkA is required for cell wall homeostasis, growth under nutrient limiting conditions, survival and replication in host cells, and virulence in vivo. Furthermore, we identify a highly conserved protein of unknown function, YvcK, as a PrkA substrate. We demonstrate that L. monocytogenes YvcK is similarly required for cell wall stress responses, growth on glycerol, cytosolic survival and virulence in vivo. Surprisingly, a phosphomimetic, nonreversible negative charge at the phosphorylation sites on YvcK inactivates functions of the protein related to intracellular survival and virulence, suggesting that the identification of PASTA kinase substrates phosphorylated during infection will be critical to our understanding of this central regulator metabolism, cell wall homeostasis and ultimately virulence.