Inhibition of Cdc42-dependent signalling in Saccharomyces cerevisiae by phosphatase-dead SigD/SopB from Salmonella typhimurium

Salmonella exploits membrane reservoirs for invasion of host cells

Salmonella utilizes a type 3 secretion system to translocate virulence proteins (effectors) into host cells during infection1. The effectors modulate host cell machinery to drive uptake of the bacteria into vacuoles, where they can establish an intracellular replicative niche. A remarkable feature of Salmonella invasion is the formation of actin-rich protuberances (ruffles) on the host cell surface that contribute to bacterial uptake. However, the membrane source for ruffle formation and how these bacteria regulate membrane mobilization within host cells remains unclear. Here, we show that Salmonella exploits membrane reservoirs for the generation of invasion ruffles. The reservoirs are pre-existing tubular compartments associated with the plasma membrane (PM) and are formed through the activity of RAB10 GTPase. Under normal growth conditions, membrane reservoirs contribute to PM homeostasis and are preloaded with the exocyst subunit EXOC2. During Salmonella invasion, the bacterial effectors SipC, SopE2, and SopB recruit exocyst subunits from membrane reservoirs and other cellular compartments, thereby allowing exocyst complex assembly and membrane delivery required for bacterial uptake. Our findings reveal an important role for RAB10 in the establishment of membrane reservoirs and the mechanisms by which Salmonella can exploit these compartments during host cell invasion.

Coordinated Regulation of Protoperithecium Development by MAP Kinases MAK-1 and MAK-2 in Neurospora crassa

Mitogen-activated protein (MAP) kinase pathways function as signaling hubs that are integral for many essential cellular processes, including sexual development. The molecular mechanisms and cross-talk between PR and CWI MAP kinase pathways have been extensively studied during asexual development. However, if these can be extended to sexual development remains elusive. By analyzing genome-wide transcriptional responses to deletion of each of two MAP kinase coding genes mak-2 (PR-MAP kinase pathway) and mak-1 (CWI-MAP kinase pathway) in Neurospora crassa during protoperithecium formation, 430 genes co-regulated by the MAK-1 and MAK-2 proteins were found, functionally enriched at integral components of membrane and oxidoreductase. These genes include 13 functionally known genes participating in sexual development (app, poi-2, stk-17, fsd-1, vsd-8, and NCU03863) and melanin synthesis (per-1, pkh-1, pkh-2, mld-1, scy-1, trn-2, and trn-1), as well as a set of functionally unknown genes. Phenotypic analysis of deletion mutants for the functionally unknown genes revealed that 12 genes were essential for female fertility. Among them, single-gene deletion mutants for NCU07743 (named as pfd-1), NCU02250 (oli), and NCU05948 (named as pfd-2) displayed similar protoperithecium development defects as the Δmak-1 and Δmak-2 mutants, failing to form protoperithecium. Western blotting analysis showed that both phosphorylated and total MAK-1 proteins were virtually abolished in the Δnrc-1, Δmek-2, and Δmak-2 mutants, suggesting that the posttranscriptional regulation of MAK-1 is dependent on the PR-MAP kinase pathway during the protoperithecium development. Taken together, this study revealed the regulatory roles and cross-talk between PR and CWI-MAP kinase pathways during protoperithecium development.

Blockade of endoplasmic reticulum stress-induced cell death by Ureaplasma parvum vacuolating factor

Previously, we found that Ureaplasma parvum internalised into HeLa cells and cytosolic accumulation of galectin-3. U. parvum induced the host cellular membrane damage and survived there. Here, we conducted vesicular trafficking inhibitory screening in yeast to identify U. parvum vacuolating factor (UpVF). U. parvum triggered endoplasmic reticulum (ER) stress and upregulated the unfolded protein response-related factors, including BiP, P-eIF2 and IRE1 in the host cells, but it blocked the induction of the downstream apoptotic factors. MicroRNA library screening of U. parvum-infected cells and UpVF-transfected cells identified miR-211 and miR-214 as the negative regulators of the apoptotic cascade under ER stress. Transient expression of UpVF induced HeLa cell death with intracellular vacuolization; however, some stable UpVF transformant survived. U. parvum-infected cervical cell lines showed resistance to actinomycin D, and UpVF stable transformant cell lines exhibited resistance to X-ray irradiation, as well as cisplatin and paclitaxel. UpVF expressing cervical cancer xenografts in nude mice also acquired resistance to cisplatin and paclitaxel. A mycoplasma expression vector based on Mycoplasma mycoides, Syn-MBA (multiple banded antigen)-UpVF, reduced HeLa cell survival compared with that of Syn-MBA after 72 hr of infection. These findings together suggest novel mechanisms for Ureaplasma infection and the possible implications for cervical cancer malignancy. TAKE AWAYS: • Ureaplasmal novel virulence factor, UpVF, was identified. • UpVF triggered ER stress but suppressed apoptotic cascade via miR-211 and -214. • UpVF conferred resistance to anticancer treatments both in vivo and in vitro. • Dual expression of MBA and UpVF in JCVI-syn3B showed host cell damage.

The TIR-domain containing effectors BtpA and BtpB from Brucella abortus impact NAD metabolism

Brucella species are facultative intracellular Gram-negative bacteria relevant to animal and human health. Their ability to establish an intracellular niche and subvert host cell pathways to their advantage depends on the delivery of bacterial effector proteins through a type IV secretion system. Brucella Toll/Interleukin-1 Receptor (TIR)-domain-containing proteins BtpA (also known as TcpB) and BtpB are among such effectors. Although divergent in primary sequence, they interfere with Toll-like receptor (TLR) signaling to inhibit the innate immune responses. However, the molecular mechanisms implicated still remain unclear. To gain insight into the functions of BtpA and BtpB, we expressed them in the budding yeast Saccharomyces cerevisiae as a eukaryotic cell model. We found that both effectors were cytotoxic and that their respective TIR domains were necessary and sufficient for yeast growth inhibition. Growth arrest was concomitant with actin depolymerization, endocytic block and a general decrease in kinase activity in the cell, suggesting a failure in energetic metabolism. Indeed, levels of ATP and NAD⁺ were low in yeast cells expressing BtpA and BtpB TIR domains, consistent with the recently described enzymatic activity of some TIR domains as NAD⁺ hydrolases. In human epithelial cells, both BtpA and BtpB expression reduced intracellular total NAD levels. In infected cells, both BtpA and BtpB contributed to reduction of total NAD, indicating that their NAD⁺ hydrolase functions are active intracellularly during infection. Overall, combining the yeast model together with mammalian cells and infection studies our results show that BtpA and BtpB modulate energy metabolism in host cells through NAD⁺ hydrolysis, assigning a novel role for these TIR domain-containing effectors in Brucella pathogenesis.

Brucella is a genus of zoonotic bacteria that cause severe disease in a variety of mammals, ranging from farm animals (as bovines, swine and ovine) to marine mammals. Transmission to humans, often by ingestion of non-treated dairy products, leads to serious systemic infection. Brucella abortus invades host cells and replicates intracellularly. Such behavior relies on the injection of bacterial proteins into the host cytoplasm via specialized secretion systems. Our work focuses on the study of two of these factors, BtpA and BtpB, previously described to contain Toll/Interleukin-1 Receptor (TIR)-domains that modulate innate immunity. We use here two biological models: the yeast Saccharomyces cerevisiae and human cell lines. We found that the TIR domains of both Brucella proteins were necessary and sufficient to collapse energy metabolism in yeast by depleting ATP and NAD⁺. This result was translatable to higher cells and consistent with the recently described NADase activity of some TIR domains both in mammalian and bacterial proteins. Importantly, we demonstrate that Brucella down-regulates total NAD levels in host cells by using both BtpA and BtpB effectors. Our results show that NAD⁺ is targeted by Brucella during infection, which may constitute a novel mechanism for its pathogenicity.

Yeast as a Heterologous Model System to Uncover Type III Effector Function

Type III effectors (T3E) are key virulence proteins that are injected by bacterial pathogens inside the cells of their host to subvert cellular processes and contribute to disease. The budding yeast Saccharomyces cerevisiae represents an important heterologous system for the functional characterisation of T3E proteins in a eukaryotic environment. Importantly, yeast contains eukaryotic processes with low redundancy and are devoid of immunity mechanisms that counteract T3Es and mask their function. Expression in yeast of effectors from both plant and animal pathogens that perturb conserved cellular processes often resulted in robust phenotypes that were exploited to elucidate effector functions, biochemical properties, and host targets. The genetic tractability of yeast and its amenability for high-throughput functional studies contributed to the success of this system that, in recent years, has been used to study over 100 effectors. Here, we provide a critical view on this body of work and describe advantages and limitations inherent to the use of yeast in T3E research. “Favourite” targets of T3Es in yeast are cytoskeleton components and small GTPases of the Rho family. We describe how mitogen-activated protein kinase (MAPK) signalling, vesicle trafficking, membrane structures, and programmed cell death are also often altered by T3Es in yeast and how this reflects their function in the natural host. We describe how effector structure–function studies and analysis of candidate targeted processes or pathways can be carried out in yeast. We critically analyse technologies that have been used in yeast to assign biochemical functions to T3Es, including transcriptomics and proteomics, as well as suppressor, gain-of-function, or synthetic lethality screens. We also describe how yeast can be used to select for molecules that block T3E function in search of new antibacterial drugs with medical applications. Finally, we provide our opinion on the limitations of S. cerevisiae as a model system and its most promising future applications.

Studying Coxiella burnetii Type IV Substrates in the Yeast Saccharomyces cerevisiae: Focus on Subcellular Localization and Protein Aggregation

Coxiella burnetii is a Gram-negative obligate parasitic bacterium that causes the disease Q-fever in humans. To establish its intracellular niche, it utilizes the Icm/Dot type IVB secretion system (T4BSS) to inject protein effectors into the host cell cytoplasm. The host targets of most cognate and candidate T4BSS-translocated effectors remain obscure. We used the yeast Saccharomyces cerevisiae as a model to express and study six C. burnetii effectors, namely AnkA, AnkB, AnkF, CBU0077, CaeA and CaeB, in search for clues about their role in C. burnetii virulence. When ectopically expressed in HeLa cells, these effectors displayed distinct subcellular localizations. Accordingly, GFP fusions of these proteins produced in yeast also decorated distinct compartments, and most of them altered cell growth. CaeA was ubiquitinated both in yeast and mammalian cells and, in S. cerevisiae, accumulated at juxtanuclear quality-control compartments (JUNQs) and insoluble protein deposits (IPODs), characteristic of aggregative or misfolded proteins. AnkA, which was not ubiquitinated, accumulated exclusively at the IPOD. CaeA, but not AnkA or the other effectors, caused oxidative damage in yeast. We discuss that CaeA and AnkA behavior in yeast may rather reflect misfolding than recognition of conserved targets in the heterologous system. In contrast, CBU0077 accumulated at vacuolar membranes and abnormal ER extensions, suggesting that it interferes with vesicular traffic, whereas AnkB associated with the yeast nucleolus. Both effectors shared common localization features in HeLa and yeast cells. Our results support the idea that C. burnetii T4BSS effectors manipulate multiple host cell targets, which can be conserved in higher and lower eukaryotic cells. However, the behavior of CaeA and AnkA prompt us to conclude that heterologous protein aggregation and proteostatic stress can be a limitation to be considered when using the yeast model to assess the function of bacterial effectors.

Wide-Ranging Effects of the Yeast Ptc1 Protein Phosphatase Acting Through the MAPK Kinase Mkk1

The Saccharomyces cerevisiae type 2C protein phosphatase Ptc1 is required for a wide variety of cellular functions, although only a few cellular targets have been identified. A genetic screen in search of mutations in protein kinase-encoding genes able to suppress multiple phenotypic traits caused by the ptc1 deletion yielded a single gene, MKK1, coding for a MAPK kinase (MAPKK) known to activate the cell-wall integrity (CWI) Slt2 MAPK. In contrast, mutation of the MKK1 paralog, MKK2, had a less significant effect. Deletion of MKK1 abolished the increased phosphorylation of Slt2 induced by the absence of Ptc1 both under basal and CWI pathway stimulatory conditions. We demonstrate that Ptc1 acts at the level of the MAPKKs of the CWI pathway, but only the Mkk1 kinase activity is essential for ptc1 mutants to display high Slt2 activation. We also show that Ptc1 is able to dephosphorylate Mkk1 in vitro. Our results reveal the preeminent role of Mkk1 in signaling through the CWI pathway and strongly suggest that hyperactivation of Slt2 caused by upregulation of Mkk1 is at the basis of most of the phenotypic defects associated with lack of Ptc1 function.

The structural organization of the N-terminus domain of SopB, a virulence factor of Salmonella, depends on the nature of its protein partners

The TTSS is used by Salmonella and many bacterial pathogens to inject virulence factors directly into the cytoplasm of target eukaryotic cells. Once translocated these so-called effector proteins hijack a vast array of crucial cellular functions to the benefit of the bacteria. In the bacterial cytoplasm, some effectors are stabilized and maintained in a secretion competent state by interaction with specific type III chaperones. In this work we studied the conformation of the Chaperone Binding Domain of the effector named Salmonella Outer protein B (SopB) alone and in complex with its cognate chaperone SigE by a combination of biochemical, biophysical and structural approaches. Our results show that the N-terminus part of SopB is mainly composed by α-helices and unfolded regions whose organization/stabilization depends on their interaction with the different partners. This suggests that the partially unfolded state of this N-terminal region, which confers the adaptability of the effector to bind very different partners during the infection cycle, allows the bacteria to modulate numerous host cells functions limiting the number of translocated effectors.

Impact of Salmonella enterica Type III Secretion System Effectors on the Eukaryotic Host Cell

Type III secretion systems are molecular machines used by many Gram-negative bacterial pathogens to inject proteins, known as effectors, directly into eukaryotic host cells. These proteins manipulate host signal transduction pathways and cellular processes to the pathogen’s advantage. Salmonella enterica possesses two virulence-related type III secretion systems that deliver more than forty effectors. This paper reviews our current knowledge about the functions, biochemical activities, host targets, and impact on host cells of these effectors. First, the concerted action of effectors at the cellular level in relevant aspects of the interaction between Salmonella and its hosts is analyzed. Then, particular issues that will drive research in the field in the near future are discussed. Finally, detailed information about each individual effector is provided.

The Salmonella Typhimurium effector SteC inhibits Cdc42-mediated signaling through binding to the exchange factor Cdc24 in Saccharomyces cerevisiae

Expression of the Salmonella effector SteC in yeast leads to down-regulation of the mating and HOG pathways by Cdc42 inhibition. This is mediated by the SteC N-terminal domain through binding to the GEF Cdc24. SteC alters Cdc24 localization and also interacts with human GEF Vav1, suggesting that SteC could target Cdc42 function in host cells.

Intracellular survival of Salmonella relies on the activity of proteins translocated into the host cell by type III secretion systems (T3SS). The protein kinase activity of the T3SS effector SteC is required for F-actin remodeling in host cells, although no SteC target has been identified so far. Here we show that expression of the N-terminal non-kinase domain of SteC down-regulates the mating and HOG pathways in Saccharomyces cerevisiae. Epistasis analyses using constitutively active components of these pathways indicate that SteC inhibits signaling at the level of the GTPase Cdc42. We demonstrate that SteC interacts through its N-terminal domain with the catalytic domain of Cdc24, the sole S. cerevisiae Cdc42 guanine nucleotide exchange factor (GEF). SteC also binds to the human Cdc24-like GEF protein Vav1. Moreover, expression of human Cdc42 suppresses growth inhibition caused by SteC. Of interest, the N-terminal SteC domain alters Cdc24 cellular localization, preventing its nuclear accumulation. These data reveal a novel functional domain within SteC, raising the possibility that this effector could also target GTPase function in mammalian cells. Our results also highlight the key role of the Cdc42 switch in yeast mating and HOG pathways and provide a new tool to study the functional consequences of Cdc24 localization.

A Yeast-Based In Vivo Bioassay to Screen for Class I Phosphatidylinositol 3-Kinase Specific Inhibitors

The phosphatidylinositol 3-kinase (PI3K) pathway couples receptor-mediated signaling to essential cellular functions by generating the lipid second messenger phosphatidylinositol-3,4,5- trisphosphate. This pathway is implicated in multiple aspects of oncogenesis. A low-cost bioassay that readily measures PI3K inhibition in vivo would serve as a valuable tool for research in this field. Using heterologous expression, we have previously reconstituted the PI3K pathway in the model organism Saccharomyces cerevisiae. On the basis of the fact that the overproduction of PI3K is toxic in yeast, we tested the ability of commercial PI3K inhibitors to rescue cell growth. All compounds tested counteracted the PI3K-induced toxicity. Among them, 15e and PI-103 were the most active. Strategies to raise the intracellular drug concentration, specifically the use of 0.003% sodium dodecyl sulfate and the elimination of the Snq2 detoxification pump, optimized the bioassay by enhancing its sensitivity. The humanized yeast-based assay was then tested on a pilot scale for high-throughput screening (HTS) purposes using a collection of natural products of microbial origin. From 9600 extracts tested, 0.6% led to a recovery of yeast growth reproducibly, selectively, and in a dose-dependent manner. Cumulatively, we show that the developed PI3K inhibition bioassay is robust and applicable to large-scale HTS.

Structure of Salmonella effector protein SopB N-terminal domain in complex with host Rho GTPase Cdc42

SopB is a type III secreted Salmonella effector protein with phosphoinositide phosphatase activity and a distinct GTPase binding domain. The latter interacts with host Cdc42, an essential Rho GTPase that regulates critical events in eukaryotic cytoskeleton organization and membrane trafficking. Structural and biochemical analysis of the SopB GTPase binding domain in complex with Cdc42 shows for the first time that SopB structurally and functionally mimics a host guanine nucleotide dissociation inhibitor (GDI) by contacting key residues in the regulatory switch regions of Cdc42 and slowing Cdc42 nucleotide exchange.

The current Salmonella-host interactome

Salmonella bacteria cause millions of infections and thousands of deaths every year. This pathogen has an unusually broad host range including humans, animals, and even plants. During infection, Salmonella expresses a variety of virulence factors and effectors that are delivered into the host cell triggering cellular responses through protein-protein interactions (PPI) with host cell proteins which make the pathogen's invasion and replication possible. To speed up proteomic efforts in elucidating Salmonella-host interactomes, we carried out a survey of the currently published Salmonella-host PPI. Such a list can serve as the gold standard for computational models aimed at predicting Salmonella-host interactomes through integration of large-scale biological data sources. Manual literature and database search of >2200 journal articles and >100 databases resulted in a gold standard list of currently 62 PPI, including primarily interactions of Salmonella proteins with human and mouse proteins. Only six of these interactions were directly retrievable from PPI databases and 16 were highlighted in databases featuring literature extracts. Thus, the literature survey resulted in the most complete interactome available to date for Salmonella. Pathway analysis using Ingenuity and Broad Gene Set Enrichment Analysis (GSEA) software revealed among general pathways such as MAPK signaling in particular those related to cell death as well as cell morphology, turnover, and interactions, in addition to response to not only Salmonella but also other pathogenic - viral and bacterial - infections. The list of interactions is available at http://www.shiprec.org/indicationslist.htm.

Quantitative mass spectrometry catalogues Salmonella pathogenicity island-2 effectors and identifies their cognate host binding partners

Gram-negative bacterial pathogens have developed specialized secretion systems to transfer bacterial proteins directly into host cells. These bacterial effectors are central to virulence and reprogram host cell processes to favor bacterial survival, colonization, and proliferation. Knowing the complete set of effectors encoded by a particular pathogen is the key to understanding bacterial disease. In addition, the identification of the molecular assemblies that these effectors engage once inside the host cell is critical to determining the mechanism of action of each effector. In this work we used stable isotope labeling of amino acids in cell culture (SILAC), a powerful quantitative proteomics technique, to identify the proteins secreted by the Salmonella pathogenicity island-2 type three secretion system (SPI-2 T3SS) and to characterize the host interaction partners of SPI-2 effectors. We confirmed many of the known SPI-2 effectors and were able to identify several novel substrate candidates of this secretion system. We verified previously published host protein-effector binding pairs and obtained 11 novel interactions, three of which were investigated further and confirmed by reciprocal co-immunoprecipitation. The host cell interaction partners identified here suggest that Salmonella SPI-2 effectors target, in a concerted fashion, cellular processes such as cell attachment and cell cycle control that are underappreciated in the context of infection. The technology outlined in this study is specific and sensitive and serves as a robust tool for the identification of effectors and their host targets that is readily amenable to the study of other bacterial pathogens.

Killing of Candida albicans filaments by Salmonella enterica serovar Typhimurium is mediated by sopB effectors, parts of a type III secretion system

Although bacterial-fungal interactions shape microbial virulence during polymicrobial infections, only a limited number of studies have evaluated this interaction on a genetic level. We report here that one interaction is mediated by sopB, an effector of a type III secretion system (TTSS) of Salmonella enterica serovar Typhimurium. In these studies, we screened 10 TTSS effector-related mutants and determined their role in the killing of C. albicans filaments in vitro during coinfection in planktonic environments. We found that deleting the sopB gene (which encodes inositol phosphatase) was associated with a significant decrease in C. albicans killing at 25°C after 5 days, similar to that caused by the deletion of sipB (which encodes TTSS translocation machinery components). The sopB deletion dramatically influenced the killing of C. albicans filaments. It was associated with repressed filamentation in the Caenorhabditis elegans model of C. albicans-S. Typhimurium coinfection, as well as with biofilm formation by C. albicans. We confirmed that SopB translocated to fungal filaments through SipB during coinfection. Using quantitative real-time PCR assays, we found that the Candida supernatant upregulated the S. Typhimurium genes associated with C. albicans killing (sopB and sipB). Interestingly, the sopB effector negatively regulated the transcription of CDC42, which is involved in fungal viability. Taken together, these results indicate that specific TTSS effectors, including SopB, play a critical role in bacterial-fungal interactions and are important to S. Typhimurium in order to selectively compete with fungal pathogens. These findings highlight a new role for TTSS of S. Typhimurium in the intestinal tract and may further explain the evolution and maintenance of these traits.

Reverse protein arrays applied to host-pathogen interaction studies

Infection of cells and tissues by pathogenic microorganisms often involves severe reprogramming of host cell signaling. Typically, invasive microorganisms manipulate host cellular pathways seeking advantage for replication and survival within the host, or to evade the immune response. Understanding such subversion of the host cell by intracellular pathogens at a molecular level is the key to possible preventive and therapeutic interventions on infectious diseases. Reverse Protein Arrays (RPAs) have been exploited in other fields, especially in molecular oncology. However, this technology has not been applied yet to the study of infectious diseases. Coupling classic in vitro infection techniques used by cellular microbiologists to proteomic approaches such as RPA analysis should provide a wealth of information about how host cell pathways are manipulated by pathogens. The increasing availability of antibodies specific for phosphorylated epitopes in signaling proteins allows monitoring global changes in phosphorylation through the infection process by utilizing RPA analyses. In our lab, we have shown the potential of RPA technology in this field by devising a microarray consisting of lysates from cell cultures infected by Salmonella typhimurium. The protocols used are described here.

Addressing the effects of Salmonella internalization in host cell signaling on a reverse-phase protein array

Through acute enteric infection, Salmonella invades host enterocytes and reproduces intracellularly into specialized vacuolae. This involves changes in host cell signaling elicited by bacterial proteins delivered via type III secretion systems (TTSS). One of the two TTSSs of Salmonella enterica serovar Typhimurium encoded by the Salmonella pathogenicity island-1, triggers bacterial internalization. Among the effector proteins translocated by this TTSS, the GTPase modulator SopE/E2 and the phosphoinositide phosphatase SigD are known to play key roles in these processes. To better understand their contribution to re-programming host cell pathways, we used ZeptoMARK reverse-phase protein array technology, which allows printing 32-sample lysate arrays that can be analyzed with phospho-specific antibodies to evaluate the phosphorylation of signaling proteins. Lysates were obtained at different times after infection of HeLa cells with WT, TTSS-deficient, sopE/E2 and sigD single and double deletants, as well as different sigD Salmonella mutants. Our analysis detected activation of p38, JNK and ERK mitogen-activated protein kinases, mainly dependent on SopE/E2, as well as SigD-dependent phosphorylation of PKB/Akt and its targets GSK-3beta and FKHR/FoxO. This is the first time that reverse-phase protein array technology is used in the cellular microbiology field, demonstrating its value to screen for host signaling events through bacterial infection.

Identification of cognate host targets and specific ubiquitylation sites on the Salmonella SPI-1 effector SopB/SigD

Salmonella enterica is a bacterial pathogen responsible for enteritis and typhoid fever. Virulence is linked to two Salmonella pathogenicity islands (SPI-1 and SPI-2) on the bacterial chromosome, each of which encodes a type III secretion system. While both the SPI-1 and SPI-2 systems secrete an array of effectors into the host, relatively few host proteins have been identified as targets for their effects. Here we use stable isotope labeling with amino acids in cell culture (SILAC) and quantitative mass spectrometry-based proteomics to identify the host targets of the SPI-1 effector, SopB/SigD. The only host protein found to bind immunoprecipitated SopB was the small G-protein Cdc42. The interaction was confirmed by reciprocal immunoprecipitation, and Cdc42 also bound glutathione S-transferase-fused SopB and SopB delivered through infection by the bacteria, confirming the interaction by an orthogonal method and in a more physiological context. The region of SopB responsible for the interaction was mapped to residues 117-168, and SopB is ubiquitylated at both K19 and K541, likely as monoubiquitylation. SopB colocalizes with activated Cdc42 near the plasmalemma, but we found no evidence that SopB alone can alter Cdc42 activity. This approach is also widely applicable to identify binding partners to other bacterial effectors.