Mechanism of actin filament nucleation by the bacterial effector VopL

Disulfidptosis: a novel cell death modality induced by actin cytoskeleton collapse and a promising target for cancer therapeutics

Disulfidptosis is a novel discovered form of programmed cell death (PCD) that diverges from apoptosis, necroptosis, ferroptosis, and cuproptosis, stemming from disulfide stress-induced cytoskeletal collapse. In cancer cells exhibiting heightened expression of the solute carrier family 7 member 11 (SLC7A11), excessive cystine importation and reduction will deplete nicotinamide adenine dinucleotide phosphate (NADPH) under glucose deprivation, followed by an increase in intracellular disulfide stress and aberrant disulfide bond formation within actin networks, ultimately culminating in cytoskeletal collapse and disulfidptosis. Disulfidptosis involves crucial physiological processes in eukaryotic cells, such as cystine and glucose uptake, NADPH metabolism, and actin dynamics. The Rac1-WRC pathway-mediated actin polymerization is also implicated in this cell death due to its contribution to disulfide bond formation. However, the precise mechanisms underlying disulfidptosis and its role in tumors are not well understood. This is probably due to the multifaceted functionalities of SLC7A11 within cells and the complexities of the downstream pathways driving disulfidptosis. This review describes the critical roles of SLC7A11 in cells and summarizes recent research advancements in the potential pathways of disulfidptosis. Moreover, the less-studied aspects of this newly discovered cell death process are highlighted to stimulate further investigations in this field.

Hemolysin Coregulated Protein (HCP) from Vibrio Cholerae Interacts with the Host Cell Actin Cytoskeleton

Vibrio cholerae (V. cholerae), the etiological agent of cholera, employs various virulence factors to adapt and thrive within both aquatic and human host environments. Among these factors, the type VI secretion system (T6SS) stands out as one of the crucial determinants of its pathogenicity. Valine glycine repeat protein G1 (VgrG1) and hemolysin coregulated protein (HCP) are considered major effector molecules of T6SS. Previous studies have highlighted that VgrG1 interacts with HCP proteins. Additionally, it has been shown that VgrG1 possesses an actin cross-linking domain (ACD) with actin-binding activity. Interestingly, it was reported that purified HCP protein treatment increased the stress fibers within cells. Therefore, we hypothesize that HCP may interact with host cell actin, potentially playing a role in the cytoskeletal rearrangement during V. cholerae infection. To test this hypothesis, we characterized HCP from the V. cholerae O139 serotype and demonstrated its interaction with actin monomers. In silico analysis and experimental validation revealed the presence of an actin-binding site within HCP. Furthermore, overexpression of HCP resulted in its colocalization with actin stress fibers in host cells. Our findings establish HCP as an effector molecule for potent host cell actin cytoskeleton remodeling during V. cholerae infection, providing new insights into bacterial pathogenicity mechanisms. Understanding the interplay between bacterial effectors and host cell components is crucial for developing targeted therapeutic interventions against cholera and related infectious diseases.

Pointed-end processive elongation of actin filaments by Vibrio effectors VopF and VopL

According to the cellular actin dynamics paradigm, filaments grow at their barbed ends and depolymerize predominantly from their pointed ends to form polar structures and do productive work. We show that actin can elongate at the pointed end when assisted by Vibrio VopF/L toxins, which act as processive polymerases. In cells, processively moving VopF/L speckles are inhibited by factors blocking the pointed but not barbed ends. Multispectral single-molecule imaging confirmed that VopF molecules associate with the pointed end, actively promoting its elongation even in the presence of profilin. Consequently, VopF/L can break the actin cytoskeleton's polarity by compromising actin-based cellular processes. Therefore, actin filament design allows processive growth at both ends, which suggests unforeseen possibilities for cellular actin organization, particularly in specialized cells and compartments.

Functional Mimicry of Eukaryotic Actin Assembly by Pathogen Effector Proteins

The actin cytoskeleton lies at the heart of many essential cellular processes. There are hundreds of proteins that cells use to control the size and shape of actin cytoskeletal networks. As such, various pathogens utilize different strategies to hijack the infected eukaryotic host actin dynamics for their benefit. These include the control of upstream signaling pathways that lead to actin assembly, control of eukaryotic actin assembly factors, encoding toxins that distort regular actin dynamics, or by encoding effectors that directly interact with and assemble actin filaments. The latter class of effectors is unique in that, quite often, they assemble actin in a straightforward manner using novel sequences, folds, and molecular mechanisms. The study of these mechanisms promises to provide major insights into the fundamental determinants of actin assembly, as well as a deeper understanding of host-pathogen interactions in general, and contribute to therapeutic development efforts targeting their respective pathogens. This review discusses mechanisms and highlights shared and unique features of actin assembly by pathogen effectors that directly bind and assemble actin, focusing on eukaryotic actin nucleator functional mimics Rickettsia Sca2 (formin mimic), Burkholderia BimA (Ena/VASP mimic), and Vibrio VopL (tandem WH2-motif mimic).

Adaptations of Vibrio parahaemolyticus to Stress During Environmental Survival, Host Colonization, and Infection

Vibrio parahaemolyticus (Vp) is an aquatic Gram-negative bacterium that may infect humans and cause gastroenteritis and wound infections. The first pandemic of Vp associated infection was caused by the serovar O3:K6 and epidemics caused by the other serovars are increasingly reported. The two major virulence factors, thermostable direct hemolysin (TDH) and/or TDH-related hemolysin (TRH), are associated with hemolysis and cytotoxicity. Vp strains lacking tdh and/or trh are avirulent and able to colonize in the human gut and cause infection using other unknown factors. This pathogen is well adapted to survive in the environment and human host using several genetic mechanisms. The presence of prophages in Vp contributes to the emergence of pathogenic strains from the marine environment. Vp has two putative type-III and type-VI secretion systems (T3SS and T6SS, respectively) located on both the chromosomes. T3SS play a crucial role during the infection process by causing cytotoxicity and enterotoxicity. T6SS contribute to adhesion, virulence associated with interbacterial competition in the gut milieu. Due to differential expression, type III secretion system 2 (encoded on chromosome-2, T3SS2) and other genes are activated and transcribed by interaction with bile salts within the host. Chromosome-1 encoded T6SS1 has been predominantly identified in clinical isolates. Acquisition of genomic islands by horizontal gene transfer provides enhanced tolerance of Vp toward several antibiotics and heavy metals. Vp consists of evolutionarily conserved targets of GTPases and kinases. Expression of these genes is responsible for the survival of Vp in the host and biochemical changes during its survival. Advanced genomic analysis has revealed that various genes are encoded in Vp pathogenicity island that control and expression of virulence in the host. In the environment, the biofilm gene expression has been positively correlated to tolerance toward aerobic, anaerobic, and micro-aerobic conditions. The genetic similarity analysis of toxin/antitoxin systems of Escherichia coli with VP genome has shown a function that could induce a viable non-culturable state by preventing cell division. A better interpretation of the Vp virulence and other mechanisms that support its environmental fitness are important for diagnosis, treatment, prevention and spread of infections. This review identifies some of the common regulatory pathways of Vp in response to different stresses that influence its survival, gut colonization and virulence.

Spire stimulates nucleation by Cappuccino and binds both ends of actin filaments

The actin nucleators Spire and Cappuccino synergize to promote actin assembly, but the mechanism of their synergy is controversial. Together these proteins promote the formation of actin meshes, which are conserved structures that regulate the establishment of oocyte polarity. Direct interaction between Spire and Cappuccino is required for oogenesis and for in vitro synergistic actin assembly. This synergy is proposed to be driven by elongation and the formation of a ternary complex at filament barbed ends, or by nucleation and interaction at filament pointed ends. To mimic the geometry of Spire and Cappuccino in vivo, we immobilized Spire on beads and added Cappuccino and actin. Barbed ends, protected by Cappuccino, grow away from the beads while pointed ends are retained, as expected for nucleation-driven synergy. We found that Spire is sufficient to bind barbed ends and retain pointed ends of actin filaments near beads and we identified Spire’s barbed-end binding domain. Loss of barbed-end binding increases nucleation by Spire and synergy with Cappuccino in bulk pyrene assays and on beads. Importantly, genetic rescue by the loss-of-function mutant indicates that barbed-end binding is not necessary for oogenesis. Thus, increased nucleation is a critical element of synergy both in vitro and in vivo.

A Novel Mouse Model of Enteric Vibrio parahaemolyticus Infection Reveals that the Type III Secretion System 2 Effector VopC Plays a Key Role in Tissue Invasion and Gastroenteritis

The Gram-negative marine bacterium Vibrio parahaemolyticus is a common cause of infectious gastroenteritis due to the ingestion of contaminated seafood. Most virulent V. parahaemolyticus strains encode two type III secretion systems (T3SS1 and T3SS2); however, the roles they and their translocated effectors play in causing intestinal disease remain unclear. While studies have identified T3SS1 effectors as responsible for killing epithelial cells in culture, the T3SS2 effectors caused massive epithelial cell disruption in a rabbit ileal loop model. Additional models are thus needed to clarify the pathogen-host interactions that drive V. parahaemolyticus-associated gastroenteritis. Germfree mice were infected with a pathogenic clinical isolate of V. parahaemolyticus, RIMD2210633 (RIMD). The pathogen was found to adhere to as well as invade the cecal mucosa, accompanied by severe inflammation and dramatic mucosal damage, including widespread sloughing of infected epithelial cells. Mice infected with a V. parahaemolyticus strain lacking the T3SS1 (POR2) also developed severe pathology, similar to that seen with RIMD. In contrast, the ΔT3SS2 strain (POR3) appeared unable to invade the intestinal mucosa or cause any mucosal pathology. Confirming a role for TS332 effectors, a strain expressing the T3SS2 but lacking VopC (POR2ΔvopC), a T3SS2 effector implicated in epithelial cell invasion in culture, was strongly attenuated in invading the intestinal mucosa and in causing gastroenteritis, although infection with this mutant resulted in more pathology than the ΔT3SS2 strain. We thus present an experimental system that enables further characterization of T3SS effectors as well as the corresponding host inflammatory response involved in the gastroenteritis caused by invasive V. parahaemolyticus IMPORTANCE Vibrio parahaemolyticus causes severe gastroenteritis following consumption of contaminated seafood. Global warming has allowed this pathogen to spread worldwide, contributing to recent outbreaks. Clinical isolates are known to harbor an array of virulence factors, including T3SS1 and T3SS2; however, the precise role these systems play in intestinal disease remains unclear. There is an urgent need to improve our understanding of how V. parahaemolyticus infects hosts and causes disease. We present a novel mouse model for this facultative intracellular pathogen and observe that the T3SS2 is essential to pathogenicity. Moreover, we show that the T3SS2 effector VopC, previously shown to be a Rac and Cdc42 deamidase that facilitates bacterial uptake by nonphagocytic cells, also plays a key role in the ability of V. parahaemolyticus to invade the intestinal mucosa and cause gastroenteritis. This experimental model thus provides a valuable tool for future elucidation of virulence mechanisms used by this facultative intracellular pathogen during in vivo infection.

Presence of actin binding motif in VgrG-1 toxin of Vibrio cholerae reveals the molecular mechanism of actin cross-linking

Type VI secretion systems (T6SS) plays a crucial role in Vibrio cholerae mediated pathogenicity. Tip of T6SS is homologous to gp27/gp5 complex or tail spike of T4 bacteriophage. VgrG-1 of V. cholerae T6SS is unusual among other VgrG because its effector domain is trans-located into the cytosol of eukaryotic cells with an additional actin cross-linking domain (ACD) at its C terminal end. ACD of VgrG-1 (VgrG-1-ACD) causes T6SS dependent host cell cytotoxicity through actin cytoskeleton disruption to prevent bacterial engulfment by macrophages. ACD mediated actin cross-linking promotes survival of the bacteria in the small intestine of humans, along with other virulence factors; establishes successful infection with the onset of diarrhoea in humans. Our studies demonstrated VgrG-1-ACD can bind to actin besides actin cross-linking activity. Computational analysis of ACD revealed the presence of actin binding motif (ABM). Mutations in ABM lead to loss of actin binding in vitro. VgrG-1-ACD having the mutated ABM cannot cross-link actin efficiently in vitro and manifests less actin cytoskeleton disruption when transfected in HeLa cells.

Rickettsia Sca2 Recruits Two Actin Subunits for Nucleation but Lacks WH2 Domains

The Rickettsia ∼1800-amino-acid autotransporter protein surface cell antigen 2 (Sca2) promotes actin polymerization on the surface of the bacterium to drive its movement using an actin comet-tail mechanism. Sca2 mimics eukaryotic formins in that it promotes both actin filament nucleation and elongation and competes with capping protein to generate filaments that are long and unbranched. However, despite these functional similarities, Sca2 is structurally unrelated to eukaryotic formins and achieves these functions through an entirely different mechanism. Thus, while formins are dimeric, Sca2 functions as a monomer. However, Sca2 displays intramolecular interactions and functional cooperativity between its N- and C-terminal domains that are crucial for actin nucleation and elongation. Here, we map the interaction of N- and C- terminal fragments of Sca2 and their contribution to actin binding and nucleation. We find that both the N- and C-terminal regions of Sca2 interact with actin monomers but only weakly, whereas the full-length protein binds two actin monomers with high affinity. Moreover, deletions at both ends of the N- and C-terminal regions disrupt their ability to interact with each other, suggesting that they form a contiguous ring-like structure that wraps around two actin subunits, analogous to the formin homology-2 domain. The discovery of Sca2 as an actin nucleator followed the identification of what appeared to be a repeat of three Wiskott-Aldrich syndrome homology 2 (WH2) domains in the middle of the molecule, consistent with the presence of WH2 domains in most actin nucleators. However, we show here that contrary to previous assumptions, Sca2 does not contain WH2 domains. Instead, our analysis indicates that the region containing the putative WH2 domains is folded as a globular domain that cooperates with other parts of the Sca2 molecule for actin binding and nucleation.

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.

T3SS effector VopL inhibits the host ROS response, promoting the intracellular survival of Vibrio parahaemolyticus

The production of antimicrobial reactive oxygen species by the nicotinamide dinucleotide phosphate (NADPH) oxidase complex is an important mechanism for control of invading pathogens. Herein, we show that the gastrointestinal pathogen Vibrio parahaemolyticus counteracts reactive oxygen species (ROS) production using the Type III Secretion System 2 (T3SS2) effector VopL. In the absence of VopL, intracellular V. parahaemolyticus undergoes ROS-dependent filamentation, with concurrent limited growth. During infection, VopL assembles actin into non-functional filaments resulting in a dysfunctional actin cytoskeleton that can no longer mediate the assembly of the NADPH oxidase at the cell membrane, thereby limiting ROS production. This is the first example of how a T3SS2 effector contributes to the intracellular survival of V. parahaemolyticus, supporting the establishment of a protective intracellular replicative niche.

Vibrio Parahaemolyticus: A Review on Distribution, Pathogenesis, Virulence Determinants and Epidemiology

Vibrio parahaemolyticus is a Gram-negative, halophilic bacterium isolated from marine environments globally. After the consumption of contaminated seafood, V. parahaemolyticus causes acute gastroenteritis. To initiate infection, a wide range of virulence factors are required. A complex group of genes is known to participate in the pathogenicity of V. parahaemolyticus; however, to understand the full mechanism of infection, extensive research is yet required. V. parahaemolyticus has become the leading cause of seafood-related gastroenteritis in Japan, the United States and several other parts of the world. In addition, outbreaks caused by the pandemic clone of this organism are escalating and spreading universally. To minimize the risk of V. parahaemolyticus infection and warrant the safety of seafood, collaboration between governments and scientists is required. We herein provide an updated review of the pathogenicity determinants and distribution of V. parahaemolyticus to deliver a better understanding of the significance of V. parahaemolyticus and its host-pathogen interactions.

Actin filament assembly by bacterial factors VopL/F: Which end is up?

Competing models have been proposed for actin filament nucleation by the bacterial proteins VopL/F. In this issue, Burke et al. (2017. J. Cell Biol. https://doi.org/10.1083/jcb.201608104) use direct observation to demonstrate that VopL/F bind the barbed and pointed ends of actin filaments but only nucleate new filaments from the pointed end.

The bacterial virulence factors VopL and VopF nucleate actin from the pointed end

How the bacterial virulence factors VopL/F from Vibrio catalyze actin nucleation is unclear. Using multicolor TIRF microscopy imaging, Burke et al. find that VopL and VopF stimulate actin assembly via identical pointed-end nucleation mechanisms.

VopL and VopF (VopL/F) are tandem WH2-domain actin assembly factors used by infectious Vibrio species to induce actin assembly in host cells. There is disagreement about the filament assembly mechanism of VopL/F, including whether they associate with the filament barbed or pointed end. Here, we used multicolor total internal reflection fluorescence microscopy to directly observe actin assembly with fluorescently labeled VopL/F. In actin monomer assembly reactions, VopL/F exclusively nucleate actin filament assemblies, remaining only briefly associated with the pointed end. VopL/F do not associate with the ends of preassembled filaments. In assembly reactions with saturating profilin, ∼85% of VopL/F molecules also promote nucleation from the pointed end, whereas a smaller fraction (<15%) associate for ∼25 s with the barbed end of preassembled filaments, inhibiting their elongation. We conclude that VopL/F function primarily as actin nucleation factors that remain briefly (∼100 s) associated with the pointed end.

Actin-based motility of bacterial pathogens: mechanistic diversity and its impact on virulence

A diverse spectrum of intracellular bacterial pathogens that inhabit the cytosol have evolved the ability to polymerize actin on their surface to power intracellular actin-based motility (ABM). These include species of Listeria, Burkholderia and Rickettsia, as well as Shigella and Mycobacteria Here, we provide an overview of the roles of bacterial ABM in survival and virulence. Moreover, we survey the molecular mechanisms of actin polymerization in host cells and describe how bacterial pathogens mimic or harness the full diversity of these mechanisms for ABM. Finally, we present ABM through a new lens by comparing motility mechanisms between related species of Listeria, Burkholderia, and Rickettsia Through these comparisons, we hope to illuminate how exploitation of different actin polymerization mechanisms influences ABM as well as pathogenicity and virulence in humans and other animals.

Identification and Characterization of a Candidate Wolbachia pipientis Type IV Effector That Interacts with the Actin Cytoskeleton

Many bacteria live as intracellular symbionts, causing persistent infections within insects. One extraordinarily common infection is that of Wolbachia pipientis, which infects 40% of insect species and induces reproductive effects. The bacteria are passed from generation to generation both vertically (through the oocyte) and horizontally (by environmental transmission). Maintenance of the infection within Drosophila melanogaster is sensitive to the regulation of actin, as Wolbachia inefficiently colonizes strains hemizygous for the profilin or villin genes. Therefore, we hypothesized that Wolbachia must depend on the host actin cytoskeleton. In this study, we identify and characterize a Wolbachia protein (WD0830) that is predicted to be secreted by the bacterial parasite. Expression of WD0830 in a model eukaryote (the yeast Saccharomyces cerevisiae) induces a growth defect associated with the appearance of aberrant, filamentous structures which colocalize with rhodamine-phalloidin-stained actin. Purified WD0830 bundles actin in vitro and cosediments with actin filaments, suggesting a direct interaction of the two proteins. We characterized the expression of WD0830 throughout Drosophila development and found it to be upregulated in third-instar larvae, peaking in early pupation, during the critical formation of adult tissues, including the reproductive system. In transgenic flies, heterologously expressed WD0830 localizes to the developing oocyte. Additionally, overexpression of WD0830 results in increased Wolbachia titers in whole flies, in stage 9 and 10 oocytes, and in embryos, compared to controls, suggesting that the protein may facilitate Wolbachia's replication or transmission. Therefore, this candidate secreted effector may play a role in Wolbachia's infection of and persistence within host niches.

IMPORTANCE

The obligate intracellular Wolbachia pipientis is a ubiquitous alphaproteobacterial symbiont of arthropods and nematodes and is related to the rickettsial pathogens Ehrlichia spp. and Anaplasma spp. Studies of Wolbachia cell biology suggest that this bacterium relies on host actin for efficient proliferation and transmission between generations. Here, we identified and characterized a Wolbachia protein that localizes to and manipulates the eukaryotic actin cytoskeleton, is expressed by Wolbachia during host development, and alters Wolbachia titers and localization in transgenic fruit flies. We hypothesize that WD0830 may be utilized by the bacterium to facilitate replication in or invasion of different niches during host development.

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.

The WH2 Domain and Actin Nucleation: Necessary but Insufficient

Two types of sequences, proline-rich domains (PRDs) and the WASP-homology 2 (WH2) domain, are found in most actin filament nucleation and elongation factors discovered thus far. PRDs serve as a platform for protein-protein interactions, often mediating the binding of profilin-actin. The WH2 domain is an abundant actin monomer-binding motif comprising ∼17 amino acids. It frequently occurs in tandem repeats, and functions in nucleation by recruiting actin subunits to form the polymerization nucleus. It is found in Spire, Cordon Bleu (Cobl), Leiomodin (Lmod), Arp2/3 complex activators (WASP, WHAMM, WAVE, etc.), the bacterial nucleators VopL/VopF and Sca2, and some formins. Yet, it is argued here that the WH2 domain plays only an auxiliary role in nucleation, always synergizing with other domains or proteins for this activity.

Biochemical Activities of the Wiskott-Aldrich Syndrome Homology Region 2 Domains of Sarcomere Length Short (SALS) Protein

Drosophila melanogaster sarcomere length short (SALS) is a recently identified Wiskott-Aldrich syndrome protein homology 2 (WH2) domain protein involved in skeletal muscle thin filament regulation. SALS was shown to be important for the establishment of the proper length and organization of sarcomeric actin filaments. Here, we present the first detailed characterization of the biochemical activities of the tandem WH2 domains of SALS (SALS-WH2). Our results revealed that SALS-WH2 binds both monomeric and filamentous actin and shifts the monomer-filament equilibrium toward the monomeric actin. In addition, SALS-WH2 can bind to but fails to depolymerize phalloidin- or jasplakinolide-bound actin filaments. These interactions endow SALS-WH2 with the following two major activities in the regulation of actin dynamics: SALS-WH2 sequesters actin monomers into non-polymerizable complexes and enhances actin filament disassembly by severing, which is modulated by tropomyosin. We also show that profilin does not influence the activities of the WH2 domains of SALS in actin dynamics. In conclusion, the tandem WH2 domains of SALS are multifunctional regulators of actin dynamics. Our findings suggest that the activities of the WH2 domains do not reconstitute the presumed biological function of the full-length protein. Consequently, the interactions of the WH2 domains of SALS with actin must be tuned in the cellular context by other modules of the protein and/or sarcomeric components for its proper functioning.

Actin: Structure, Function, Dynamics, and Interactions with Bacterial Toxins

Actin is one of the most abundant proteins in any eukaryotic cell and an indispensable component of the cytoskeleton. In mammalian organisms, six highly conserved actin isoforms can be distinguished, which differ by only a few amino acids. In non-muscle cells, actin polymerizes into actin filaments that form actin structures essential for cell shape stabilization, and participates in a number of motile activities like intracellular vesicle transport, cytokinesis, and also cell locomotion. Here, we describe the structure of monomeric and polymeric actin, the polymerization kinetics, and its regulation by actin-binding proteins. Probably due to its conserved nature and abundance, actin and its regulating factors have emerged as prefered targets of bacterial toxins and effectors, which subvert the host actin cytoskeleton to serve bacterial needs.

Bacterial nucleators: actin' on actin

The actin cytoskeleton is a key target of numerous microbial pathogens, including protozoa, fungi, bacteria and viruses. In particular, bacterial pathogens produce and deliver virulence effector proteins that hijack actin dynamics to enable bacterial invasion of host cells, allow movement within the host cytosol, facilitate intercellular spread or block phagocytosis. Many of these effector proteins directly or indirectly target the major eukaryotic actin nucleator, the Arp2/3 complex, by either mimicking nucleation promoting factors or activating upstream small GTPases. In contrast, this review is focused on a recently identified class of effector proteins from Gram-negative bacteria that function as direct actin nucleators. These effector proteins mimic functional activities of formins, WH2-nucleators and Ena/VASP assembly promoting factors demonstrating that bacteria have coopted the complete set of eukaryotic actin assembly pathways. Structural and functional analyses of these nucleators have revealed several motifs and/or mechanistic activities that are shared with eukaryotic actin nucleators. However, functional effects of these proteins during infection extend beyond plain actin polymerization leading to interference with other host cell functions such as vesicle trafficking, cell cycle progression and cell death. Therefore, their use as model systems could not only help in the understanding of the mechanistic details of actin polymerization but also provide novel insights into the connection between actin dynamics and other cellular pathways.

Virulent Burkholderia species mimic host actin polymerases to drive actin-based motility

Burkholderia pseudomallei and B. mallei are bacterial pathogens that cause melioidosis and glanders, whereas their close relative B. thailandensis is non-pathogenic. All use the trimeric autotransporter BimA to facilitate actin-based motility, host cell fusion, and dissemination. Here, we show that BimA orthologs mimic different host actin-polymerizing proteins. B. thailandensis BimA activates the host Arp2/3 complex. In contrast, B. pseudomallei and B. mallei BimA mimic host Ena/VASP actin polymerases in their ability to nucleate, elongate, and bundle filaments by associating with barbed ends, as well as in their use of WH2 motifs and oligomerization for activity. Mechanistic differences among BimA orthologs resulted in distinct actin filament organization and motility parameters, which affected the efficiency of cell fusion during infection. Our results identify bacterial Ena/VASP mimics and reveal that pathogens imitate the full spectrum of host actin-polymerizing pathways, suggesting that mimicry of different polymerization mechanisms influences key parameters of infection.

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

A novel bacterial type III secretion effector, VopV, from the enteric pathogen Vibrio parahaemolyticus has been identified as a key factor in pathogenicity due to its interaction with cytoskeletal actin. One of the repeat units in the long repetitive region of VopV, named VopV_rep1, functions as an actin-binding module. Despite its importance in pathogenesis, the manner in which the effector binds to actin and the subsequent effects on actin dynamics remain unclear. Here, we report the molecular basis of the VopV_rep1/actin interaction. VopV_rep1 exists as an unstructured protein in solution but potently and specifically binds filamentous actin (F-actin) and not globular actin (G-actin). The F-actin/VopV_rep1 complex was directly visualized at 9.6-Å resolution using electron cryomicroscopy (cryoEM) and helical image reconstitution. The density map revealed the binding site of VopV_rep1 at the interface between two actin strands, which is close to the binding site of the bicyclic heptapeptide toxin phalloidin. Consistent with this observation, VopV_rep1 alone prevented the depolymerization of F-actin. Overall, VopV_rep1 demonstrated unique characteristics in comparison to known actin-binding proteins, but was relatively similar to phalloidin. The phalloidin-like behavior, targeting the interstrand region of actin filaments to stabilize the filament structure, likely contributes to the pathogenicity of V. parahaemolyticus.

Dimeric WH2 repeats of VopF sequester actin monomers into non-nucleating linear string conformations: An X-ray scattering study

VopF and VopL are highly similar virulence-factors of Vibrio cholerae and Vibrio parahaemolyticus respectively that disrupt the host's actin cytoskeleton, using a unique organization in dimerized WH2 repeats. Association of dimerized WH2 domains with the barbed face of actin confers multifunctional activities to VopF in vitro, including G-actin sequestration and filament nucleation, barbed end tracking and uncapping. Here, small angle X-ray scattering (SAXS) measurements of complexes of VopF with actin and structural modeling reveal that VopF stabilizes linear actin-strings that differ from canonical actin filament architectures but represent non-polymerizable sequestered forms of actin. The results exclude that VopL binds the pointed end of actin filaments in the template filament nucleation mechanism derived from crystallographic studies.

The pathogenesis, detection, and prevention of Vibrio parahaemolyticus

Vibrio parahaemolyticus, a Gram-negative motile bacterium that inhabits marine and estuarine environments throughout the world, is a major food-borne pathogen that causes life-threatening diseases in humans after the consumption of raw or undercooked seafood. The global occurrence of V. parahaemolyticus accentuates the importance of investigating its virulence factors and their effects on the human host. This review describes the virulence factors of V. parahaemolyticus reported to date, including hemolysin, urease, two type III secretion systems and two type VI secretion systems, which both cause both cytotoxicity in cultured cells and enterotoxicity in animal models. We describe various types of detection methods, based on virulence factors, that are used for quantitative detection of V. parahaemolyticus in seafood. We also discuss some useful preventive measures and therapeutic strategies for the diseases mediated by V. parahaemolyticus, which can reduce, to some extent, the damage to humans and aquatic animals attributable to V. parahaemolyticus. This review extends our understanding of the pathogenic mechanisms of V. parahaemolyticus mediated by virulence factors and the diseases it causes in its human host. It should provide new insights for the diagnosis, treatment, and prevention of V. parahaemolyticus infection.

Subversion of the cytoskeleton by intracellular bacteria: lessons from Listeria, Salmonella and Vibrio

Entry into host cells and intracellular persistence by invasive bacteria are tightly coupled to the ability of the bacterium to disrupt the eukaryotic cytoskeletal machinery. Herein we review the main strategies used by three intracellular pathogens to harness key modulators of the cytoskeleton. Two of these bacteria, namely Listeria monocytogenes and Salmonella enterica serovar Typhimurium, exhibit quite distinct intracellular lifestyles and therefore provide a comprehensive panel for the understanding of the intricate bacteria-cytoskeleton interplay during infections. The emerging intracellular pathogen Vibrio parahaemolyticus is depicted as a developing model for the uncovering of novel mechanisms used to hijack the cytoskeleton.

Regulation of Vibrio parahaemolyticus T3SS2 gene expression and function of T3SS2 effectors that modulate actin cytoskeleton

Vibrio parahaemolyticus is a leading causative agent of seafood-borne gastroenteritis worldwide. Most clinical isolates from patients with diarrhoea possess two sets of genes for the type III secretion system (T3SS) on each chromosome (T3SS1 and T3SS2). T3SS is a protein secretion system that delivers effector proteins directly into eukaryotic cells. The injected effectors modify the normal cell functions by altering or disrupting the normal cell signalling pathways. Of the two sets of T3SS genes present in V. parahaemolyticus, T3SS2 is essential for enterotoxicity in several animal models. Recent studies have elucidated the biological activities of several T3SS2 effectors and their roles in virulence. This review focuses on the regulation of T3SS2 gene expression and T3SS2 effectors that specifically target the actin cytoskeleton.

Filament assembly by Spire: key residues and concerted actin binding

The most recently identified class of actin nucleators, WASp homology domain 2 (WH2) nucleators, use tandem repeats of monomeric actin-binding WH2 domains to facilitate actin nucleation. WH2 domains are involved in a wide variety of actin regulatory activities. Structurally, they are expected to clash with interprotomer contacts within the actin filament. Thus, the discovery of their role in nucleation was surprising. Here we use Drosophila Spire (Spir) as a model system to investigate both how tandem WH2 domains can nucleate actin and what differentiates nucleating WH2-containing proteins from their non-nucleating counterparts. We found that the third WH2 domain in Spir (Spir-C or SC) plays a unique role. In the context of a short nucleation construct (containing only two WH2 domains), placement of SC in the N-terminal position was required for the most potent nucleation. We found that the native organization of the WH2 domains with respect to each other is necessary for binding to actin with positive cooperativity. We identified two residues within SC that are critical for its activity. Using this information, we were able to convert a weak synthetic nucleator into one with activity equal to a native Spir construct. Lastly, we found evidence that SC binds actin filaments, in addition to monomers.

Mechanism of actin filament pointed-end capping by tropomodulin

Proteins that cap the ends of the actin filament are essential regulators of cytoskeleton dynamics. Whereas several proteins cap the rapidly growing barbed end, tropomodulin (Tmod) is the only protein known to cap the slowly growing pointed end. The lack of structural information severely limits our understanding of Tmod's capping mechanism. We describe crystal structures of actin complexes with the unstructured amino-terminal and the leucine-rich repeat carboxy-terminal domains of Tmod. The structures and biochemical analysis of structure-inspired mutants showed that one Tmod molecule interacts with three actin subunits at the pointed end, while also contacting two tropomyosin molecules on each side of the filament. We found that Tmod achieves high-affinity binding through several discrete low-affinity interactions, which suggests a mechanism for controlled subunit exchange at the pointed end.

Mutagenetic and electron microscopy analysis of actin filament severing by Cordon-Bleu, a WH2 domain protein

Cordon-Bleu (Cobl) is a regulator of actin dynamics in neural development and ciliogenesis. Its function is associated with three adjacent actin binding WASP Homology 2 (WH2) domains. We showed that these WH2 repeats confer multifunctional regulation of actin dynamics, which makes Cobl a « dynamizer » of actin assembly, inducing fast turnover of actin filaments and oscillatory polymerization regime via nucleation, severing, and rapid depolymerization activities. Cobl is the most efficient severer of actin filaments characterized so far. To understand which primary sequence elements determine the filament severing activity of the WH2 repeats, here we combine a mutagenetic/domain swapping approach of the minimal fully active Cobl-KAB construct, which comprises the lysine rich region K preceding the two first WH2 domains A and B. The mutated Cobl constructs display variable loss of the original filament nucleating activities of native Cobl-KAB, without any strict correlation with a loss in actin binding, which emphasizes the functional importance of the electrostatic environment of WH2 domains. Filament severing displayed the greatest stringency and was abolished in all mutated forms of Cobl-KAB. Filament severing and re-annealing by Cobl-KAB, which is key in its rapid remodeling of a population of actin filaments, and most likely responsible for its function in ciliogenesis, was analyzed by electron microscopy in comparison with Spire and ADF.

Manipulation of intestinal epithelial cell function by the cell contact-dependent type III secretion systems of Vibrio parahaemolyticus

Vibrio parahaemolyticus elicits gastroenteritis by deploying Type III Secretion Systems (TTSS) to deliver effector proteins into epithelial cells of the human intestinal tract. The bacteria must adhere to the human cells to allow colonization and operation of the TTSS translocation apparatus bridging the bacterium and the host cell. This article first reviews recent advances in identifying the molecules responsible for intercellular adherence. V. parahaemolyticus possesses two TTSS, each of which delivers an exclusive set of effectors and mediates unique effects on the host cell. TTSS effectors primarily target and alter the activation status of host cell signaling proteins, thereby bringing about changes in the regulation of cellular behavior. TTSS1 is responsible for the cytotoxicity of V. parahaemolyticus, while TTSS2 is necessary for the enterotoxicity of the pathogen. Recent publications have elucidated the function of several TTSS effectors and their importance in the virulence of the bacterium. This review will explore the ability of the TTSS to manipulate activities of human intestinal cells and how this modification of cell function favors bacterial colonization and persistence of V. parahaemolyticus in the host.

The bacterial effector VopL organizes actin into filament-like structures

VopL is an effector protein from Vibrio parahaemolyticus that nucleates actin filaments. VopL consists of a VopL C-terminal domain (VCD) and an array of three WASP homology 2 (WH2) motifs. Here, we report the crystal structure of the VCD dimer bound to actin. The VCD organizes three actin monomers in a spatial arrangement close to that found in the canonical actin filament. In this arrangement, WH2 motifs can be modeled into the binding site of each actin without steric clashes. The data suggest a mechanism of nucleation wherein VopL creates filament-like structures, organized by the VCD with monomers delivered by the WH2 array, that can template addition of new subunits. Similarities with Arp2/3 complex and formin proteins suggest that organization of monomers into filament-like structures is a general and central feature of actin nucleation.

Virulence determinants for Vibrio parahaemolyticus infection

Vibrio parahaemolyticus is a marine microorganism that causes acute gastroenteritis associated with the consumption of contaminated raw or under cooked seafood. During infection, the bacterium utilizes a wide variety of virulence factors, including adhesins, toxins and type III secretion systems, to cause both cytotoxicity in cultured cells and enterotoxicity in animal models. Herein, we describe recent discoveries on the regulation and characterization of the virulence factors from V. para. Determining how this bacterial pathogen uses virulence factors to mediate pathogenicity improves our understanding of V. para. infections and more generally, host-pathogen interactions.

Measurement and analysis of in vitro actin polymerization

The polymerization of actin underlies force generation in numerous cellular processes. While actin polymerization can occur spontaneously, cells maintain control over this important process by preventing actin filament nucleation and then allowing stimulated polymerization and elongation by several regulated factors. Actin polymerization, regulated nucleation, and controlled elongation activities can be reconstituted in vitro, and used to probe the signaling cascades cells use to control when and where actin polymerization occurs. Introducing a pyrene fluorophore allows detection of filament formation by an increase in pyrene fluorescence. This method has been used for many years and continues to be broadly used, owing to its simplicity and flexibility. Here we describe how to perform and analyze these in vitro actin polymerization assays, with an emphasis on extracting useful descriptive parameters from kinetic data.

The role of type III secretion system 2 in Vibrio parahaemolyticus pathogenicity

Vibrio parahaemolyticus, a Gram-negative marine bacterial pathogen, is emerging as a major cause of food-borne illnesses worldwide due to the consumption of raw seafood leading to diseases including gastroenteritis, wound infection, and septicemia. The bacteria utilize toxins and type III secretion system (T3SS) to trigger virulence. T3SS is a multi-subunit needle-like apparatus used to deliver bacterial proteins, termed effectors, into the host cytoplasm which then target various eukaryotic signaling pathways. V. parahaemolyticus carries two T3SSs in each of its two chromosomes, named T3SS1 and T3SS2, both of which play crucial yet distinct roles during infection: T3SS1 causes cytotoxicity whereas T3SS2 is mainly associated with enterotoxicity. Each T3SS secretes a unique set of effectors that contribute to virulence by acting on different host targets and serving different functions. Emerging studies on T3SS2 of V. parahaemolyticus, reveal its regulation, translocation, discovery, characterization of its effectors, and development of animal models to understand the enterotoxicity. This review on recent findings for T3SS2 of V. parahaemolyticus highlights a novel mechanism of invasion that appears to be conserved by other marine bacteria.

Subversion of cell signaling by pathogens

Pathogens exploit several eukaryotic signaling pathways during an infection. They have evolved specific effectors and toxins to hijack host cell machinery for their own benefit. Signaling molecules are preferentially targeted by pathogens because they globally regulate many cellular processes. Both viruses and bacteria manipulate and control pathways that regulate host cell survival and shape, including MAPK signaling, G-protein signaling, signals controlling cytoskeletal dynamics, and innate immune responses.

Interactions of isolated C-terminal fragments of neural Wiskott-Aldrich syndrome protein (N-WASP) with actin and Arp2/3 complex

Wiskott-Aldrich syndrome proteins (WASP) are a family of proteins that all catalyze actin filament branching with the Arp2/3 complex in a variety of actin-based motile processes. The constitutively active C-terminal domain, called VCA, harbors one or more WASP homology 2 (WH2) domains that bind G-actin, whereas the CA extension binds the Arp2/3 complex. The VCA·actin·Arp2/3 entity associates with a mother filament to form a branched junction from which a daughter filament is initiated. The number and function of WH2-bound actin(s) in the branching process are not known, and the stoichiometry of the VCA·actin·Arp2/3 complex is debated. We have expressed the tandem WH2 repeats of N-WASP, either alone (V) or associated with the C (VC) and CA (VCA) extensions. We analyzed the structure of actin in complex with V, VC, and VCA using protein crystallography and hydrodynamic and spectrofluorimetric methods. The partial crystal structure of the VC·actin 1:1 complex shows two actins in the asymmetric unit with extensive actin-actin contacts. In solution, each of the two WH2 domains in V, VC, and VCA binds G-actin in 1:2 complexes that participate in barbed end assembly. V, VC, and VCA enhance barbed end depolymerization like profilin but neither nucleate nor sever filaments, in contrast with other WH2 repeats. VCA binds the Arp2/3 complex in a 1:1 complex even in the presence of a large excess of VCA. VCA·Arp2/3 binds one actin in a latrunculin A-sensitive fashion, in a 1:1:1 complex, indicating that binding of the second actin to VCA is weakened in the ternary complex.

Multiple forms of Spire-actin complexes and their functional consequences

Spire is a WH2 domain-containing actin nucleator essential for establishing an actin mesh during oogenesis. In vitro, in addition to nucleating filaments, Spire can sever them and sequester actin monomers. Understanding how Spire is capable of these disparate functions and which are physiologically relevant is an important goal. To study severing, we examined the effect of Drosophila Spire on preformed filaments in bulk and single filament assays. We observed rapid depolymerization of actin filaments by Spire, which we conclude is largely due to its sequestration activity and enhanced by its weak severing activity. We also studied the solution and crystal structures of Spire-actin complexes. We find structural and functional differences between constructs containing four WH2 domains (Spir-ABCD) and two WH2 domains (Spir-CD) that may provide insight into the mechanisms of nucleation and sequestration. Intriguingly, we observed lateral interactions between actin monomers associated with Spir-ABCD, suggesting that the structures built by these four tandem WH2 domains are more complex than originally imagined. Finally, we propose that Spire-actin mixtures contain both nuclei and sequestration structures.

How a single residue in individual β-thymosin/WH2 domains controls their functions in actin assembly

β-Thymosin (βT) and WH2 domains are widespread, intrinsically disordered actin-binding peptides that display significant sequence variability and different regulations of actin self-assembly in motile and morphogenetic processes. Here, we reveal the structural mechanisms by which, in their 1:1 stoichiometric complexes with actin, they either inhibit assembly by sequestering actin monomers like Thymosin-β4, or enhance motility by directing polarized filament assembly like Ciboulot βT. We combined mutational, functional or structural analysis by X-ray crystallography, SAXS (small angle X-ray scattering) and NMR on Thymosin-β4, Ciboulot, TetraThymosinβ and the long WH2 domain of WASP-interacting protein. The latter sequesters G-actin with the same molecular mechanisms as Thymosin-β4. Functionally different βT/WH2 domains differ by distinct dynamics of their C-terminal half interactions with G-actin pointed face. These C-terminal interaction dynamics are controlled by the strength of electrostatic interactions with G-actin. At physiological ionic strength, a single salt bridge with actin located next to their central LKKT/V motif induces G-actin sequestration in both isolated long βT and WH2 domains. The results open perspectives for elucidating the functions of βT/WH2 domains in other modular proteins.

The C terminus of formin FMNL3 accelerates actin polymerization and contains a WH2 domain-like sequence that binds both monomers and filament barbed ends

Formin proteins are actin assembly factors that accelerate filament nucleation then remain on the elongating barbed end and modulate filament elongation. The formin homology 2 (FH2) domain is central to these activities, but recent work has suggested that additional sequences enhance FH2 domain function. Here we show that the C-terminal 76 amino acids of the formin FMNL3 have a dramatic effect on the ability of the FH2 domain to accelerate actin assembly. This C-terminal region contains a WASp homology 2 (WH2)-like sequence that binds actin monomers in a manner that is competitive with other WH2 domains and with profilin. In addition, the C terminus binds filament barbed ends. As a monomer, the FMNL3 C terminus inhibits actin polymerization and slows barbed end elongation with moderate affinity. As a dimer, the C terminus accelerates actin polymerization from monomers and displays high affinity inhibition of barbed end elongation. These properties are not common to all formin C termini, as those of mDia1 and INF2 do not behave similarly. Interestingly, mutation of two aliphatic residues, which blocks high affinity actin binding by the WH2-like sequence, has no effect on the ability of the C terminus to enhance FH2-mediated polymerization. However, mutation of three successive basic residues at the C terminus of the WH2-like sequence compromises polymerization enhancement. These results illustrate that the C termini of formins are highly diverse in their interactions with actin.