A repeat unit of Vibrio diarrheal T3S effector subverts cytoskeletal actin homeostasis via binding to interstrand region of actin filaments

Stabilization of F-actin by Salmonella effector SipA resembles the structural effects of inorganic phosphate and phalloidin

Entry of Salmonella into host enterocytes relies on its pathogenicity island 1 effector SipA. We found that SipA binds to F-actin in a 1:2 stoichiometry with sub-nanomolar affinity. A cryo-EM reconstruction revealed that SipA's globular core binds at the groove between actin strands, whereas the extended C-terminal arm penetrates deeply into the inter-strand space, stabilizing F-actin from within. The unusually strong binding of SipA is achieved by a combination of fast association via the core and very slow dissociation dictated by the arm. Similar to Pi, BeF3, and phalloidin, SipA potently inhibited actin depolymerization by actin depolymerizing factor (ADF)/cofilin, which correlated with increased filament stiffness, supporting the hypothesis that F-actin's mechanical properties contribute to the recognition of its nucleotide state by protein partners. The remarkably strong binding to F-actin maximizes the toxin's effects at the injection site while minimizing global influence on the cytoskeleton and preventing pathogen detection by the host cell.

Stabilization of F-actin by Salmonella effector SipA resembles the structural effects of inorganic phosphate and phalloidin

Entry of Salmonella into host enterocytes strictly relies on its pathogenicity island 1 effector SipA. We found that SipA binds to F-actin in a unique mode in a 1:2 stoichiometry with picomolar affinity. A cryo-EM reconstruction revealed that SipA's globular core binds at the grove between actin strands, whereas the extended C-terminal arm penetrates deeply into the inter-strand space, stabilizing F-actin from within. The unusually strong binding of SipA is achieved via a combination of fast association via the core and very slow dissociation dictated by the arm. Similarly to Pi, BeF3, and phalloidin, SipA potently inhibited actin depolymerization by ADF/cofilin, which correlated with the increased filament stiffness, supporting the hypothesis that F-actin's mechanical properties contribute to the recognition of its nucleotide state by protein partners. The remarkably strong binding to F-actin maximizes the toxin's effects at the injection site while minimizing global influence on the cytoskeleton and preventing pathogen detection by the host cell.

Structural basis of membrane skeleton organization in red blood cells

The spectrin-based membrane skeleton is a ubiquitous membrane-associated two-dimensional cytoskeleton underneath the lipid membrane of metazoan cells. Mutations of skeleton proteins impair the mechanical strength and functions of the membrane, leading to several different types of human diseases. Here, we report the cryo-EM structures of the native spectrin-actin junctional complex (from porcine erythrocytes), which is a specialized short F-actin acting as the central organizational unit of the membrane skeleton. While an α-/β-adducin hetero-tetramer binds to the barbed end of F-actin as a flexible cap, tropomodulin and SH3BGRL2 together create an absolute cap at the pointed end. The junctional complex is strengthened by ring-like structures of dematin in the middle actin layers and by patterned periodic interactions with tropomyosin over its entire length. This work serves as a structural framework for understanding the assembly and dynamics of membrane skeleton and offers insights into mechanisms of various ubiquitous F-actin-binding factors in other F-actin systems.

Vibrio variations on a type three theme

Mounting evidence suggests that Type 3 Secretion Systems (T3SS) are widespread among Vibrio species, and are present in strains isolated from diverse sources such as human clinical infections, environmental reservoirs, and diseased marine life. Experiments evaluating Vibrio parahaemolyticus and Vibrio cholerae T3SS mediated virulence suggest that Vibrio T3SS pathogenicity islands have a tripartite composition. A conserved 'core' region encodes functions essential for colonization and disease in vivo, including modulation of innate immune signaling pathways and actin dynamics, whereas regions flanking core sequences are variable among strains and encode effector proteins performing a diverse array of activities. Characterizing novel functions associated with Vibrio-specific effectors is, therefore, essential for understanding how vibrios employ T3SS mechanisms to cause disease in a broad range of hosts and how T3SS island composition potentially defines species-specific disease.

The extreme C terminus of the Pseudomonas aeruginosa effector ExoY is crucial for binding to its eukaryotic activator, F-actin

Bacterial nucleotidyl cyclase toxins are potent virulence factors that upon entry into eukaryotic cells are stimulated by endogenous cofactors to catalyze the production of large amounts of 3'5'-cyclic nucleoside monophosphates. The activity of the effector ExoY from Pseudomonas aeruginosa is stimulated by the filamentous form of actin (F-actin). Utilizing yeast phenotype analysis, site-directed mutagenesis, functional biochemical assays, and confocal microscopy, we demonstrate that the last nine amino acids of the C terminus of ExoY are crucial for the interaction with F-actin and, consequently, for ExoY's enzymatic activity in vitro and toxicity in a yeast model. We observed that isolated C-terminal sequences of P. aeruginosa ExoY that had been fused to a carrier protein bind to F-actin and that synthetic peptides corresponding to the extreme ExoY C terminus inhibit ExoY enzymatic activity in vitro and compete with the full-length enzyme for F-actin binding. Interestingly, we noted that various P. aeruginosa isolates of the PA14 family, including highly virulent strains, harbor ExoY variants with a mutation altering the C terminus of this effector. We found that these naturally occurring ExoY variants display drastically reduced enzymatic activity and toxicity. Our findings shed light on the molecular basis of the ExoY-F-actin interaction, revealing that the extreme C terminus of ExoY is critical for binding to F-actin in target cells and that some P. aeruginosa isolates carry C-terminally mutated, low-activity ExoY variants.

Vibrio parahaemolyticus Senses Intracellular K+ To Translocate Type III Secretion System 2 Effectors Effectively

Many Gram-negative bacterial symbionts and pathogens employ a type III secretion system (T3SS) to live in contact with eukaryotic cells. Because T3SSs inject bacterial proteins (effectors) directly into host cells, the switching of secretory substrates between translocators and effectors in response to host cell attachment is a crucial step for the effective delivery of effectors. Here, we show that the protein secretion switch of Vibrio parahaemolyticus T3SS2, which is a main contributor to the enteropathogenicity of a food poisoning bacterium, is regulated by two gatekeeper proteins, VgpA and VgpB. In the absence of these gatekeepers, effector secretion was activated, but translocator secretion was abolished, causing the loss of virulence. We found that the K⁺ concentration, which is high inside the host cell but low outside, is a key factor for VgpA- and VgpB-mediated secretion switching. Exposure of wild-type bacteria to K⁺ ions provoked both gatekeeper and effector secretions but reduced the level of secretion of translocators. The secretion protein profile of wild-type bacteria cultured with 0.1 M KCl was similar to that of gatekeeper mutants. Furthermore, depletion of K⁺ ions in host cells diminished the efficiency of T3SS2 effector translocation. Thus, T3SS2 senses the high intracellular concentration of K⁺ of the host cell so that T3SS2 effectors can be effectively injected.

The pathogenesis of many Gram-negative bacterial pathogens arises from a type III secretion system (T3SS), whereby bacterial proteins (effectors) are directly injected into host cells. The injected effectors then modify host cell functions. For effective delivery of effector proteins, bacteria need to both recognize host cell attachment and switch the type of secreted proteins. Here, we identified gatekeeper proteins that play important roles in a T3SS2 secretion switch of Vibrio parahaemolyticus, a causative agent of food-borne gastroenteritis. We also found that K⁺, which is present in high concentrations inside the host cell but in low concentrations outside, is a key factor for the secretion switch. Thus, V. parahaemolyticus senses the high intracellular K⁺ concentration, triggering the effective injection of effectors.

Natural Transformation in Vibrio parahaemolyticus: a Rapid Method To Create Genetic Deletions

The Gram-negative bacterium Vibrio parahaemolyticus is an opportunistic human pathogen and the leading cause of seafood-borne acute gastroenteritis worldwide. Recently, this bacterium was implicated as the etiologic agent of a severe shrimp disease with consequent devastating outcomes to shrimp farming. In both cases, acquisition of genetic material via horizontal transfer provided V. parahaemolyticus with new virulence tools to cause disease. Dissecting the molecular mechanisms of V. parahaemolyticus pathogenesis often requires manipulating its genome. Classically, genetic deletions in V. parahaemolyticus are performed using a laborious, lengthy, multistep process. Here, we describe a fast and efficient method to edit this bacterium's genome based on V. parahaemolyticus natural competence. Although this method is similar to one previously described, V. parahaemolyticus requires counterselection for curing of acquired plasmids due to its recalcitrant nature of retaining extrachromosomal DNA. We believe this approach will be of use to the Vibrio community.IMPORTANCE Spreading of vibrios throughout the world correlates with increased global temperatures. As they spread, they find new niches in which to survive, proliferate, and invade. Therefore, genetic manipulation of vibrios is of the utmost importance for studying these species. Here, we have delineated and validated a rapid method to create genetic deletions in Vibrio parahaemolyticus This study provides insightful methodology for studies with other Vibrio species.

Actin cytoskeleton-modulating T3SS2 effectors and their contribution to the Vibrio parahaemolyticus-induced diarrhea

To understand how bacterial pathogens cause diseases is the most important step in order to prevent the infection and develop an effective treatment. However, the past proceeding studies make us aware of quite-complicated interactions between the host and pathogenic bacteria. Vibrio parahaemolyticus, a food-born pathogen that is a subject of our study, causes inflammatory diarrhea in human upon ingestion of contaminated raw or undercooked seafood. Many virulence factors has been proposed since its discovery in Osaka around 70 years ago, while our research group has revealed that one of these virulence factors, type 3 secretion system 2 (T3SS2), is necessary for diarrhea induced by this bacterium. In addition, we recently found two novel T3SS2 effectors (VopO and VopV) that manipulate the actin cytoskeleton in infected host cells. In this article, I would like to show our findings with regard to biological activities of the effectors and their contributions to the T3SS2-induced enterotoxicity.

How Bacteria Subvert Animal Cell Structure and Function

Bacterial pathogens encode a wide variety of effectors and toxins that hijack host cell structure and function. Of particular importance are virulence factors that target actin cytoskeleton dynamics critical for cell shape, stability, motility, phagocytosis, and division. In addition, many bacteria target organelles of the general secretory pathway (e.g., the endoplasmic reticulum and the Golgi complex) and recycling pathways (e.g., the endolysosomal system) to establish and maintain an intracellular replicative niche. Recent research on the biochemistry and structural biology of bacterial effector proteins and toxins has begun to shed light on the molecular underpinnings of these host-pathogen interactions. This exciting work is revealing how pathogens gain control of the complex and dynamic host cellular environments, which impacts our understanding of microbial infectious disease, immunology, and human cell biology.