The protease GtgE from Salmonella exclusively targets inactive Rab GTPases

Modulation of host Rab GTPases by Salmonella: mechanisms of immune evasion and intracellular replication

Salmonella is one of the major pathogens responsible for foodborne illnesses worldwide, characterized by diverse serotypes and a broad host range. As an intracellular bacterium, Salmonella invades host cells and establishes a protected niche known as the Salmonella-containing vacuoles (SCVs), which provide a suitable environment for intracellular survival. Rab GTPases, key regulators of intracellular membrane trafficking, play a crucial role in the biogenesis and dynamics of SCVs. Through its type III secretion systems (T3SSs), Salmonella delivers a repertoire of effector proteins into host cells, which modulate the activity of Rab GTPases and alter membrane trafficking to facilitate SCVs formation and maintenance. This review summarizes recent advances in understanding how Salmonella effectors manipulate Rab GTPases to promote intracellular survival and evade host innate immune responses.

Identification of a RAB32-LRMDA-Commander membrane trafficking complex reveals the molecular mechanism of human oculocutaneous albinism type 7

Commander is an endosome associated sixteen protein assembly that associates with the sorting nexin 17 (SNX17) cargo adaptor to regulate cell surface recycling of internalised integral membrane proteins including integrins and lipoprotein receptors. Mutations in Commander are causative for Ritscher-Schinzel syndrome (RSS), a multiorgan developmental disease associated with a core triad of cerebellar-cardiac-craniofacial malformation. Here, using unbiased proteomics and computational modelling, we identify leucine rich melanocyte differentiation associated (LRMDA) as a novel Commander binding protein. Using recombinant protein reconstitution, we show that LRMDA simultaneously associates with Commander and active RAB32, and, by revealing that LRMDA and SNX17 share a common mechanism of Commander association, establish the mutually exclusive nature of RAB32-LRMDA-Commander and SNX17-Commander assemblies. From functional analysis in human melanocytes, we establish an essential role for RAB32-LRMDA-Commander in melanosome biogenesis and pigmentation and reveal a distinct functional role for SNX17-Commander in this organelle biogenesis pathway. We reveal how LRMDA mutations, causative for oculocutaneous albinism type 7 (OCA7), a hypopigmentation disorder accompanied by poor visual acuity, uncouple RAB32 and Commander binding thereby establishing the mechanistic basis of this disease. Our discovery and characterisation of this alternative Commander assembly establishes an unrecognised plasticity of Commander function within a highly complex organelle biogenesis pathway. This extends Commander function beyond the confines of SNX17-mediated cell surface recycling into RAB32-family mediated biogenesis of lysosome-related organelles and, potentially, other RAB32 regulated pathways including host-pathogen defence mechanisms. Our work also extends the breath of Commander pathway dysfunction for human disease.

Infection biology of Salmonella enterica

Salmonella enterica is the leading cause of bacterial foodborne illness in the USA, with an estimated 95% of salmonellosis cases due to the consumption of contaminated food products. Salmonella can cause several different disease syndromes, with the most common being gastroenteritis, followed by bacteremia and typhoid fever. Among the over 2,600 currently identified serotypes/serovars, some are mostly host-restricted and host-adapted, while the majority of serotypes can infect a broader range of host species and are associated with causing both livestock and human disease. Salmonella serotypes and strains within serovars can vary considerably in the severity of disease that may result from infection, with some serovars that are more highly associated with invasive disease in humans, while others predominantly cause mild gastroenteritis. These observed clinical differences may be caused by the genetic make-up and diversity of the serovars. Salmonella virulence systems are very complex containing several virulence-associated genes with different functions that contribute to its pathogenicity. The different clinical syndromes are associated with unique groups of virulence genes, and strains often differ in the array of virulence traits they display. On the chromosome, virulence genes are often clustered in regions known as Salmonella pathogenicity islands (SPIs), which are scattered throughout different Salmonella genomes and encode factors essential for adhesion, invasion, survival, and replication within the host. Plasmids can also carry various genes that contribute to Salmonella pathogenicity. For example, strains from several serovars associated with significant human disease, including Choleraesuis, Dublin, Enteritidis, Newport, and Typhimurium, can carry virulence plasmids with genes contributing to attachment, immune system evasion, and other roles. The goal of this comprehensive review is to provide key information on the Salmonella virulence, including the contributions of genes encoded in SPIs and plasmids during Salmonella pathogenesis.

Vibrio MARTX toxin processing and degradation of cellular Rab GTPases by the cytotoxic effector Makes Caterpillars Floppy

Vibrio vulnificus causes life-threatening wound and gastrointestinal infections, mediated primarily by the production of a Multifunctional-Autoprocessing Repeats-In-Toxin (MARTX) toxin. The most commonly present MARTX effector domain, the Makes Caterpillars Floppy-like (MCF) toxin, is a cysteine protease stimulated by host adenosine diphosphate (ADP) ribosylation factors (ARFs) to autoprocess. Here, we show processed MCF then binds and cleaves host Ras-related proteins in brain (Rab) guanosine triphosphatases within their C-terminal tails resulting in Rab degradation. We demonstrate MCF binds Rabs at the same interface occupied by ARFs. Moreover, we show MCF preferentially binds to ARF1 prior to autoprocessing and is active to cleave Rabs only subsequent to autoprocessing. We then use structure prediction algorithms to demonstrate that structural composition, rather than sequence, determines Rab target specificity. We further determine a crystal structure of aMCF as a swapped dimer, revealing an alternative conformation we suggest represents the open, activated state of MCF with reorganized active site residues. The cleavage of Rabs results in Rab1B dispersal within cells and loss of Rab1B density in the intestinal tissue of infected mice. Collectively, our work describes an extracellular bacterial mechanism whereby MCF is activated by ARFs and subsequently induces the degradation of another small host guanosine triphosphatase (GTPase), Rabs, to drive organelle damage, cell death, and promote pathogenesis of these rapidly fatal infections.

Functional Divergence of the Paralog Salmonella Effector Proteins SopD and SopD2 and Their Contributions to Infection

Salmonella enterica is a leading cause of bacterial food-borne illness in humans and is responsible for millions of cases annually. A critical strategy for the survival of this pathogen is the translocation of bacterial virulence factors termed effectors into host cells, which primarily function via protein-protein interactions with host proteins. The Salmonella genome encodes several paralogous effectors believed to have arisen from duplication events throughout the course of evolution. These paralogs can share structural similarities and enzymatic activities but have also demonstrated divergence in host cell targets or interaction partners and contributions to the intracellular lifecycle of Salmonella. The paralog effectors SopD and SopD2 share 63% amino acid sequence similarity and extensive structural homology yet have demonstrated divergence in secretion kinetics, intracellular localization, host targets, and roles in infection. SopD and SopD2 target host Rab GTPases, which represent critical regulators of intracellular trafficking that mediate diverse cellular functions. While SopD and SopD2 both manipulate Rab function, these paralogs display differences in Rab specificity, and the effectors have also evolved multiple mechanisms of action for GTPase manipulation. Here, we highlight this intriguing pair of paralog effectors in the context of host-pathogen interactions and discuss how this research has presented valuable insights into effector evolution.

Rab32, a novel Rab small GTPase from orange-spotted grouper, Epinephelus coioides involved in SGIV infection

Rab32 is a member of the Rab GTPase family that is involved in membrane trafficking and immune response, which are crucial for controlling pathogen infection. However, the role of Rab32 in virus infection is not well understood. In this study, we focused on the regulation of Rab32 on virus infection and the host immunity in orange-spotted grouper, Epinephelus coioides. EcRab32 encoded a 213-amino acid polypeptide, which shared a high sequence identity with other Rab32 proteins from fishes to mammals. In healthy orange-spotted grouper, the mRNA of EcRab32 was expressed in all the detected tissues, with the more expression levels in the head kidney, liver and gill. Upon SGIV infection, the expression of EcRab32 was significantly up-regulated in vitro, indicating its potential role in viral infection. EcRab32 was observed to be distributed in the cytoplasm as punctate and vesicle-like structures. EcRab32 overexpression was found to notably inhibit SGIV infection, while the interruption of EcRab32 significantly promoted SGIV infection. In addition, using single particle imaging analysis, we found that EcRab32 overexpression prominently reduced the attachment and internalization of SGIV particles. Furthermore, the results demonstrated that EcRab32 played a positive role in regulating the interferon immune and inflammatory responses. Taken together, these findings indicated that EcRab32 influenced SGIV infection by regulating the host immune response, providing an overall understanding of the interplay between the Rab32 and innate immunity.

How Bacterial Pathogens Coordinate Appetite with Virulence

Cells adjust growth and metabolism to nutrient availability. Having access to a variety of carbon sources during infection of their animal hosts, facultative intracellular pathogens must efficiently prioritize carbon utilization. Here, we discuss how carbon source controls bacterial virulence, with an emphasis on Salmonella enterica serovar Typhimurium, which causes gastroenteritis in immunocompetent humans and a typhoid-like disease in mice, and propose that virulence factors can regulate carbon source prioritization by modifying cellular physiology. On the one hand, bacterial regulators of carbon metabolism control virulence programs, indicating that pathogenic traits appear in response to carbon source availability. On the other hand, signals controlling virulence regulators may impact carbon source utilization, suggesting that stimuli that bacterial pathogens experience within the host can directly impinge on carbon source prioritization. In addition, pathogen-triggered intestinal inflammation can disrupt the gut microbiota and thus the availability of carbon sources. By coordinating virulence factors with carbon utilization determinants, pathogens adopt metabolic pathways that may not be the most energy efficient because such pathways promote resistance to antimicrobial agents and also because host-imposed deprivation of specific nutrients may hinder the operation of certain pathways. We propose that metabolic prioritization by bacteria underlies the pathogenic outcome of an infection.

From Eberthella typhi to Salmonella Typhi: The Fascinating Journey of the Virulence and Pathogenicity of Salmonella Typhi

Salmonella Typhi (S. Typhi), the invasive typhoidal serovar of Salmonella enterica that causes typhoid fever in humans, is a severe threat to global health. It is one of the major causes of high morbidity and mortality in developing countries. According to recent WHO estimates, approximately 11-21 million typhoid fever illnesses occur annually worldwide, accounting for 0.12-0.16 million deaths. Salmonella infection can spread to healthy individuals by the consumption of contaminated food and water. Typhoid fever in humans sometimes is accompanied by several other critical extraintestinal complications related to the central nervous system, cardiovascular system, pulmonary system, and hepatobiliary system. Salmonella Pathogenicity Island-1 and Salmonella Pathogenicity Island-2 are the two genomic segments containing genes encoding virulent factors that regulate its invasion and systemic pathogenesis. This Review aims to shed light on a comparative analysis of the virulence and pathogenesis of the typhoidal and nontyphoidal serovars of S. enterica.

Paired single-cell host profiling with multiplex-tagged bacterial mutants reveals intracellular virulence-immune networks

Encounters between host cells and intracellular bacterial pathogens lead to complex phenotypes that determine the outcome of infection. Single-cell RNA sequencing (scRNA-seq) is increasingly used to study the host factors underlying diverse cellular phenotypes but has limited capacity to analyze the role of bacterial factors. Here, we developed scPAIR-seq, a single-cell approach to analyze infection with a pooled library of multiplex-tagged, barcoded bacterial mutants. Infected host cells and barcodes of intracellular bacterial mutants are both captured by scRNA-seq to functionally analyze mutant-dependent changes in host transcriptomes. We applied scPAIR-seq to macrophages infected with a library of Salmonella Typhimurium secretion system effector mutants. We analyzed redundancy between effectors and mutant-specific unique fingerprints and mapped the global virulence network of each individual effector by its impact on host immune pathways. ScPAIR-seq is a powerful tool to untangle bacterial virulence strategies and their complex interplay with host defense strategies that drive infection outcome.

Speaking the host language: how Salmonella effector proteins manipulate the host

Salmonella injects over 40 virulence factors, termed effectors, into host cells to subvert diverse host cellular processes. Of these 40 Salmonella effectors, at least 25 have been described as mediating eukaryotic-like, biochemical post-translational modifications (PTMs) of host proteins, altering the outcome of infection. The downstream changes mediated by an effector's enzymatic activity range from highly specific to multifunctional, and altogether their combined action impacts the function of an impressive array of host cellular processes, including signal transduction, membrane trafficking, and both innate and adaptive immune responses. Salmonella and related Gram-negative pathogens have been a rich resource for the discovery of unique enzymatic activities, expanding our understanding of host signalling networks, bacterial pathogenesis as well as basic biochemistry. In this review, we provide an up-to-date assessment of host manipulation mediated by the Salmonella type III secretion system injectosome, exploring the cellular effects of diverse effector activities with a particular focus on PTMs and the implications for infection outcomes. We also highlight activities and functions of numerous effectors that remain poorly characterized.

Vibrio MARTX toxin processing and degradation of cellular Rab GTPases by the cytotoxic effector Makes Caterpillars Floppy

Vibrio vulnificus causes life threatening infections dependent upon the effectors released from the Multifunctional-Autoprocessing Repeats-In-Toxin (MARTX) toxin. The Makes Caterpillars Floppy-like (MCF) cysteine protease effector is activated by host ADP ribosylation factors (ARFs), although the targets of processing activity were unknown. In this study we show MCF binds Ras-related proteins in brain (Rab) GTPases at the same interface occupied by ARFs and then cleaves and/or degrades 24 distinct members of the Rab GTPases family. The cleavage occurs in the C-terminal tails of Rabs. We determine the crystal structure of MCF as a swapped dimer revealing the open, activated state of MCF and then use structure prediction algorithms to show that structural composition, rather than sequence or localization, determine Rabs selected as MCF proteolytic targets. Once cleaved, Rabs become dispersed in cells to drive organelle damage and cell death to promote pathogenesis of these rapidly fatal infections.

Identification and Classification of Papain-like Cysteine Proteinases

Papain-like cysteine peptidases form a big and highly diverse superfamily of proteins involved in many important biological functions, such as protein turnover, deubiquitination, tissue remodeling, blood clotting, virulence, defense, and cell wall remodeling. High sequence and structure diversity observed within these proteins hinders their comprehensive classification as well as the identification of new representatives. Moreover, in general protein databases, many families already classified as papain-like lack details regarding their mechanism of action or biological function. Here, we use transitive remote homology searches and 3D modeling to newly classify 21 families to the papain-like cysteine peptidase superfamily. We attempt to predict their biological function, and provide structural chacterization of 89 protein clusters defined based on sequence similarity altogether spanning 106 papain-like families. Moreover, we systematically discuss observed diversity in sequences, structures, and catalytic sites. Eventually, we expand the list of human papain-related proteins by seven representatives, including dopamine receptor-interacting protein (DRIP1) as potential deubiquitinase, and centriole duplication regulating CEP76 as retaining catalytically active peptidase-like domain. The presented results not only provide structure-based rationales to already existing peptidase databases but also may inspire further experimental research focused on peptidase-related biological processes.

Determination of the Salmonella intracellular lifestyle by the diversified interaction of Type III secretion system effectors and host GTPases

Intracellular bacteria have developed sophisticated strategies to subvert the host endomembrane system to establish a stable replication niche. Small GTPases are critical players in regulating each step of membrane trafficking events, such as vesicle biogenesis, cargo transport, tethering, and fusion events. Salmonella is a widely studied facultative intracellular bacteria. Salmonella delivers several virulence proteins, termed effectors, to regulate GTPase dynamics and subvert host trafficking for their benefit. In this review, we summarize an updated and systematic understanding of the interactions between bacterial effectors and host GTPases in determining the intracellular lifestyle of Salmonella. This article is categorized under: Infectious Diseases > Molecular and Cellular Physiology.

Interictal Functional Connectivity in Focal Refractory Epilepsies Investigated by Intracranial EEG

Introduction: Focal epilepsies are diseases of neuronal excitability affecting macroscopic networks of cortical and subcortical neural structures. These networks ("epileptogenic networks") can generate pathological electrophysiological activities during seizures, and also between seizures (interictal period). Many works attempt to describe these networks by using quantification methods, particularly based on the estimation of statistical relationships between signals produced by brain regions, namely functional connectivity (FC). Results: FC has been shown to be greatly altered during seizures and in the immediate peri-ictal period. An increasing number of studies have shown that FC is also altered during the interictal period depending on the degree of epileptogenicity of the structures. Furthermore, connectivity values could be correlated with other clinical variables including surgical outcome. Significance: This leads to a conceptual change and to consider epileptic areas as both hyperexcitable and abnormally connected. These data open the door to the use of interictal FC as a marker of epileptogenicity and as a complementary tool for predicting the effect of surgery. Aim: In this article, we review the available data concerning interictal FC estimated from intracranial electroencephalograhy (EEG) in focal epilepsies and discuss it in the light of data obtained from other modalities (EEG imaging) and modeling studies.

Control of host PTMs by intracellular bacteria: An opportunity toward novel anti-infective agents

Intracellular bacteria have developed a multitude of mechanisms to influence the post-translational modifications (PTMs) of host proteins to pathogen advantages. The recent explosion of insights into the diversity and sophistication of host PTMs and their manipulation by infectious agents challenges us to formulate a comprehensive vision of this complex and dynamic facet of the host-pathogen interaction landscape. As new discoveries continue to shed light on the central roles of PTMs in infectious diseases, technological advances foster our capacity to detect old and new PTMs and investigate their control and impact during pathogenesis, opening new possibilities for chemical intervention and infection treatment. Here, we present a comprehensive overview of these pathogenic mechanisms and offer perspectives on how these insights may contribute to the development of a new class of therapeutics that are urgently needed to face rising antibiotic resistances.

Host manipulation by bacterial type III and type IV secretion system effector proteases

Proteases are powerful enzymes, which cleave peptide bonds, leading most of the time to irreversible fragmentation or degradation of their substrates. Therefore they control many critical cell fate decisions in eukaryotes. Bacterial pathogens exploit this power and deliver protease effectors through specialised secretion systems into host cells. Research over the past years revealed that the functions of protease effectors during infection are diverse, reflecting the lifestyles and adaptations to specific hosts; however, only a small number of peptidase families seem to have given rise to most of these protease virulence factors by the evolution of different substrate-binding specificities, intracellular activation and subcellular targeting mechanisms. Here, we review our current knowledge about the enzymology and function of protease effectors, which Gram-negative bacterial pathogens translocate via type III and IV secretion systems to irreversibly manipulate host processes. We highlight emerging concepts such as signalling by protease cleavage products and effector-triggered immunity, which host cells employ to detect and defend themselves against a protease attack. TAKE AWAY: Proteases irreversibly cleave proteins to control critical cell fate decisions. Gram-negative bacteria use type III and IV secretion systems to inject effectors. Protease effectors are integral weapons for the manipulation of host processes. Effectors evolved from few peptidase families to target diverse substrates. Effector-triggered immunity upon proteolytic attack emerges as host defence.

SopD from Salmonella specifically inactivates Rab8

Salmonella outer protein D (SopD) is secreted into a host during the first stages of the Salmonella infection and contributes to the systemic virulence of the bacterium. SopD2 is a SopD homolog and possesses GTPase activating protein (GAP) activity towards Rab32. Here, we identified Rab-proteins as putative SopD-targets using a yeast two-hybrid approach. In vitro investigations subsequently revealed Rab8a as an exclusive SopD substrate in contrast to SopD2, which has a broader specificity targeting Rab29, Rab32 and Rab38 in vitro. Additionally, we determined the catalytic efficiencies of SopD and SopD2 towards their physiologically relevant substrates. Moreover, mutagenesis studies provided insights into possible key residues of the Rab-protein and the GAP involved in the conversion of active to inactive GTPase. In conclusion, we demonstrate that Salmonella SopD and SopD2 act as RabGAPs and can inactivate Rab signaling.

Structural and functional study of Legionella pneumophila effector RavA

Legionella pneumophila is an intracellular pathogen that causes Legionnaire's disease in humans. This bacterium can be found in freshwater environments as a free-living organism, but it is also an intracellular parasite of protozoa. Human infection occurs when inhaled aerosolized pathogen comes into contact with the alveolar mucosa and replicates in alveolar macrophages. Legionella enters the host cell by phagocytosis and redirects the Legionella-containing phagosomes from the phagocytic maturation pathway. These nascent phagosomes fuse with ER-derived secretory vesicles and membranes forming the Legionella-containing vacuole. Legionella subverts many host cellular processes by secreting over 300 effector proteins into the host cell via the Dot/Icm type IV secretion system. The cellular function for many Dot/Icm effectors is still unknown. Here, we present a structural and functional study of L. pneumophila effector RavA (Lpg0008). Structural analysis revealed that the RavA consists of four ~85 residue long α-helical domains with similar folds, which show only a low level of structural similarity to other protein domains. The ~90 residues long C-terminal segment is predicted to be natively unfolded. We show that during L. pneumophila infection of human cells, RavA localizes to the Golgi apparatus and to the plasma membrane. The same localization is observed when RavA is expressed in human cells. The localization signal resides within the C-terminal sequence C⁴⁰⁹ WTSFCGLF⁴¹⁷ . Yeast-two-hybrid screen using RavA as bait identified RAB11A as a potential binding partner. RavA is present in L. pneumophila strains but only distant homologs are found in other Legionella species, where the number of repeats varies.

Structural biology of the invasion arsenal of Gram-negative bacterial pathogens

In the last several years, there has been a tremendous progress in the understanding of host-pathogen interactions and the mechanisms by which bacterial pathogens modulate behavior of the host cell. Pathogens use secretion systems to inject a set of proteins, called effectors, into the cytosol of the host cell. These effectors are secreted in a highly regulated, temporal manner and interact with host proteins to modify a multitude of cellular processes. The number of effectors varies between pathogens from ~ 30 to as many as ~ 350. The functional redundancy of effectors encoded by each pathogen makes it difficult to determine the cellular effects or function of individual effectors, since their individual knockouts frequently produce no easily detectable phenotypes. Structural biology of effector proteins and their interactions with host proteins, in conjunction with cell biology approaches, has provided invaluable information about the cellular function of effectors and underlying molecular mechanisms of their modes of action. Many bacterial effectors are functionally equivalent to host proteins while being structurally divergent from them. Other effector proteins display new, previously unobserved functionalities. Here, we summarize the contribution of the structural characterization of effectors and effector-host protein complexes to our understanding of host subversion mechanisms used by the most commonly investigated Gram-negative bacterial pathogens. We describe in some detail the enzymatic activities discovered among effector proteins and how they affect various cellular processes.

LRRK2 binds to the Rab32 subfamily in a GTP-dependent manner via its armadillo domain

LRRK2 is a multi-domain Ser/Thr kinase that is associated with inherited and sporadic cases of Parkinson's disease. Many mutations linked to disease are associated within a central ROC-COR regulatory region and the subsequent kinase domain, leading to enhanced catalytic activity. The N-terminus of human LRRK2 consists of armadillo repeat motifs (ARMs) followed by ankyrin repeats (ANKs). Recently, Rab GTPases have emerged as key players in LRRK2 function, both as substrates of the kinase, and as regulators of the catalytic activity. Rabs recruit effector proteins via their GTP-dependent switch 1 and 2 regions to distinct sub-cellular compartments to regulate membrane trafficking. LRRK2 phosphorylates Rab8, Rab10 and Rab12 in switch 2, and this activity is regulated via interactions with Rab29. Furthermore, the related Rab32-subfamily GTPases, Rab32 and Rab38, have also been shown to interact with LRRK2. Here, we have mapped the interactions of the Rab32-subfamily to the ARM domain of LRRK2. The complexes are dependent on the GTP state of the Rabs in vitro, implying that LRRK2 may be an effector of the Rab32-subfamily of small GTPases. X-ray crystal structures of the Rab32-family GTPases and subsequent mutational studies reveal that a positively charged residue in switch 1 is critical for binding of Rab32/38 to LRRK2. Homology modelling and mutational analyses of the ARM domain point to a patch of negatively charged residues that contribute to complex formation. These structural and biochemical studies provide a framework for understanding the molecular basis for Rab regulation of LRRK2 and its role in Parkinson's disease.

Proteolysis of Rab32 by Salmonella GtgE induces an inactive GTPase conformation

Rab GTPases are central regulators of intracellular vesicular trafficking. They are frequently targeted by bacterial pathogens through post-translational modifications. Salmonella typhimurium secretes the cysteine protease GtgE during infection, leading to a regioselective proteolytic cleavage of the regulatory switch I loop in the small GTPases of the Rab32 subfamily. Here, using a combination of biochemical methods, molecular dynamics simulations, NMR spectroscopy, and single-pair Förster resonance energy transfer, we demonstrate that the cleavage of Rab32 causes a local increase of conformational flexibility in both switch regions. Cleaved Rab32 maintains its ability to interact with the GDP dissociation inhibitor (GDI). Interestingly, the Rab32 cleavage enables GDI binding also with an active GTP-bound Rab32 in vitro. Furthermore, the Rab32 proteolysis provokes disturbance in the interaction with its downstream effector VARP. Thus, the proteolysis of Rab32 is not a globally degradative mechanism but affects various biochemical and structural properties of the GTPase in a diverse manner.

The Enemy of My Enemy: New Insights Regarding Bacteriophage-Mammalian Cell Interactions

Bacteriophages (phages) are the most abundant biological entity in the human body, but until recently the role that phages play in human health was not well characterized. Although phages do not cause infections in human cells, phages can alter the severity of bacterial infections by the dissemination of virulence factors amongst bacterial hosts. Recent studies, made possible with advances in genome engineering and microscopy, have uncovered a novel role for phages in the human body - the ability to modulate the physiology of the mammalian cells that can harbor intracellular bacteria. In this review, we synthesize key results on how phages traverse through mammalian cells - including uptake, distribution, and interaction with intracellular receptors - highlighting how these steps in turn influence host cell killing of bacteria. We discuss the implications of the growing field of phage-mammalian cell interactions for phage therapy.

Salmonella secretion systems: Differential roles in pathogen-host interactions

The bacterial genus Salmonella includes a large group of food-borne pathogens that cause a variety of gastrointestinal or systemic diseases in hosts. Salmonella use several secretion devices to inject various effectors targeting eukaryotic hosts, or bacteria. In the past few years, considerable progress has been made towards understanding the structural features and molecular mechanisms of the secretion systems of Salmonella, particularly regarding their roles in host-pathogen interactions. In this review, we summarize the current advances about the main characteristics of the Salmonella secretion systems. Clarifying the roles of the secretion systems in the process of infecting various hosts will broaden our understanding of the importance of microbial interactions in maintaining human health and will provide information for developing novel therapeutic approaches.

The Salmonella Effector SseK3 Targets Small Rab GTPases

During infection, Salmonella species inject multiple type III secretion system (T3SS) effector proteins into host cells that mediate invasion and subsequent intracellular replication. At early stages of infection, Salmonella exploits key regulators of host intracellular vesicle transport, including the small GTPases Rab5 and Rab7, to subvert host endocytic vesicle trafficking and establish the Salmonella-containing vacuole (SCV). At later stages of intracellular replication, interactions of the SCV with Rab GTPases are less well defined. Here we report that Rab1, Rab5, and Rab11 are modified at later stages of Salmonella infection by SseK3, an arginine N-acetylglucosamine (GlcNAc) transferase effector translocated via the Salmonella pathogenicity island 2 (SPI-2) type III secretion system. SseK3 modified arginines at positions 74, 82, and 111 within Rab1 and this modification occurred independently of Rab1 nucleotide binding. SseK3 exhibited Golgi localization that was independent of its glycosyltransferase activity but Arg-GlcNAc transferase activity was required for inhibition of alkaline phosphatase secretion in transfected cells. While SseK3 had a modest effect on SEAP secretion during infection of HeLa229 cells, inhibition of IL-1 and GM-CSF cytokine secretion was only observed upon over-expression of SseK3 during infection of RAW264.7 cells. Our results suggest that, in addition to targeting death receptor signaling, SseK3 may contribute to Salmonella infection by interfering with the activity of key Rab GTPases.

The Role of the Type III Secretion System in the Intracellular Lifestyle of Enteric Pathogens

Several pathogens have evolved to infect host cells from within, which requires subversion of many host intracellular processes. In the case of Gram-negative pathogenic bacteria, adaptation to an intracellular life cycle relies largely on the activity of type III secretion systems (T3SSs), an apparatus used to deliver effector proteins into the host cell, from where these effectors regulate important cellular functions such as vesicular trafficking, cytoskeleton reorganization, and the innate immune response. Each bacterium is equipped with a unique suite of these T3SS effectors, which aid in the development of an individual intracellular lifestyle for their respective pathogens. Some bacteria adapt to reside and propagate within a customized vacuole, while others establish a replicative niche in the host cytosol. In this article, we review the mechanisms by which T3SS effectors contribute to these different lifestyles. To illustrate the formation of a vacuolar and a cytosolic lifestyle, we discuss the intracellular habitats of the enteric pathogens Salmonella enterica serovar Typhimurium and Shigella flexneri, respectively. These represent well-characterized systems that function as informative models to contribute to our understanding of T3SS-dependent subversion of intracellular processes. Additionally, we present Vibrio parahaemolyticus, another enteric Gram-negative pathogen, as an emerging model for future studies of the cytosolic lifestyle.

A SUMOylation-dependent switch of RAB7 governs intracellular life and pathogenesis of Salmonella Typhimurium

Salmonella Typhimurium is an intracellular pathogen that causes gastroenteritis in humans. Aided by a battery of effector proteins, S. Typhimurium resides intracellularly in a specialized vesicle, called the Salmonella-containing vacuole (SCV) that utilizes the host endocytic vesicular transport pathway (VTP). Here, we probed the possible role of SUMOylation, a post-translation modification pathway, in SCV biology. Proteome analysis by complex mass-spectrometry (MS/MS) revealed a dramatically altered SUMO-proteome (SUMOylome) in S. Typhimurium-infected cells. RAB7, a component of VTP, was key among several crucial proteins identified in our study. Detailed MS/MS assays, in vitro SUMOylation assays and structural docking analysis revealed SUMOylation of RAB7 (RAB7A) specifically at lysine 175. A SUMOylation-deficient RAB7 mutant (RAB7^K175R) displayed longer half-life, was beneficial to SCV dynamics and functionally deficient. Collectively, the data revealed that RAB7 SUMOylation blockade by S. Typhimurium ensures availability of long-lived but functionally compromised RAB7, which was beneficial to the pathogen. Overall, this SUMOylation-dependent switch of RAB7 controlled by S. Typhimurium is an unexpected mode of VTP pathway regulation, and unveils a mechanism of broad interest well beyond Salmonella-host crosstalk. This article has an associated First Person interview with the first author of the paper.