During the late stages of infection,
Salmonella secretes numerous effectors through a type III secretion system that is encoded within
Salmonella pathogenicity island 2 (SPI2). Despite the importance of SPI2 as a major virulence factor leading to the systemic spread of the bacteria and diseases, a global view of its effects on host responses is still lacking. Here, we measured global impacts of SPI2 effectors on the host phosphorylation and protein expression levels in RAW264.7 and in HeLa cells, as macrophage and nonphagocytic models of infection. We observe that SPI2 effectors differentially modulate the host phosphoproteome and cellular processes (
e.g. protein trafficking, cytoskeletal regulation, and immune signaling) in a host cell-dependent manner. Our unbiased approach reveals the involvement of many previously unrecognized proteins, including E3 ligases (HERC4, RanBP2, and RAD18), kinases (CDK, SIK3, and WNK1), and histones (H2B1F, H4, and H15), in late stages of
Salmonella infection. Furthermore, from this phosphoproteome analysis and other quantitative screens, we identified HSP27 as a direct
in vitro and
in vivo molecular target of the only type III secreted kinase, SteC. Using biochemical and cell biological assays, we demonstrate that SteC phosphorylates multiple sites in HSP27 and induces actin rearrangement through this protein. Together, these results provide a broader landscape of host players contributing to specific processes/pathways mediated by SPI2 effectors than was previously appreciated.Type III secretion systems (T3SSs)
1 are specialized virulence factors in Gram-negative pathogens that play an important role in delivering effector proteins to host cells.
Salmonella enterica employs two distinct T3SSs encoded in
Salmonella pathogenicity islands 1 and 2 (SPI1 and SPI2), with numerous effectors encoded around the genome, including a small number in SPI1 and SPI2 (
1). SPI1 T3SS effectors are required for the bacterial internalization by intestinal epithelial cells at early stages of infection after oral ingestion. Although
Salmonella is subsequently taken up by intestinal macrophages via phagocytosis, SPI2 T3SS effectors function to promote intracellular replication. Part of the role of SPI2 effectors is to control the maturation of the membrane-enclosed,
Salmonella-containing vacuole (SCV) where
Salmonella survives and replicates, eventually leading to a systemic infection known as typhoid fever (
2,
3).Approximately 30 effectors are known to be translocated by the SPI2 T3SS but the actions and targets of most of these effectors are largely unknown (
1,
3,
4). A recent systematic study using a single mutant collection of SPI2 genes showed particular virulence factors (
e.g. SpvB, SifA, and SteC) play a dominant role in replication within macrophages (
5). It is known that SpvB induces cytotoxicity through its ADP-ribosyltransferase activity (
6), and SifA is required for maturation of the SCV and the formation of
Salmonella-induced filaments (
7). SteC has been identified as the sole serine/threonine protein kinase encoded in the
Salmonella genome (
8), but the target substrates of this kinase within the host are not fully understood, although it has been demonstrated that SteC partially targets the MAP kinase MEK (
9). Interestingly, SteC is capable of promoting assembly of an F-actin meshwork around the SCV; this is dependent on its kinase activity but does not require activation of signaling pathways through Rho-associated protein kinase (
8), Cdc42, Rac, N-WASP, Scar/WAVE, and Arp2/3 (
10). These host signaling proteins are the main targets of T3SS-secreted effectors from many pathogens, including the SPI1 system in
Salmonella (
11) and
Shigella (
12). Therefore, SteC is thought to manipulate actin in a unique way through phosphorylation of host protein target(s).Recent advances in high throughput measurements allow us to characterize host gene expression profiles (
13) and host phosphoproteme dynamics (
14) dependent on the presence of SPI1 effectors in an unbiased, comprehensive manner. However, although it is clear that SPI2 T3SS is a major virulence factor contributing to systemic infection, our knowledge of its effects on host responses is limited. In this study, we used a mass spectrometry (MS)-based quantitative proteomics approach and measured global host phosphorylation changes as well as proteome abundance altered by SPI2 effectors. Furthermore, we explore a molecular target of SPI2 effector kinase SteC by integrating the phosphoproteomics data and other quantitative proteomics screens.
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