Mass spectrometric analysis of Ehrlichia chaffeensis tandem repeat proteins reveals evidence of phosphorylation and absence of glycosylation

Anaplasmataceae: Dichotomous Autophagic Interplay for Infection

Autophagy is a vital conserved degradative process that maintains cellular homeostasis by recycling or eliminating dysfunctional cellular organelles and proteins. More recently, autophagy has become a well-recognized host defense mechanism against intracellular pathogens through a process known as xenophagy. On the host-microbe battlefield many intracellular bacterial pathogens have developed the ability to subvert xenophagy to establish infection. Obligately intracellular bacterial pathogens of the Anaplasmataceae family, including Ehrlichia chaffeensis, Anaplasma phaogocytophilium and Orientia tsutsugamushi have developed a dichotomous strategy to exploit the host autophagic pathway to obtain nutrients while escaping lysosomal destruction for intracellular survival within the host cell. In this review, the recent findings regarding how these master manipulators engage and inhibit autophagy for infection are explored. Future investigation to understand mechanisms used by Anaplasmataceae to exploit autophagy may advance novel antimicrobial therapies and provide new insights into how intracellular microbes exploit autophagy to survive.

Ehrlichia chaffeensis Outer Membrane Protein 1-Specific Human Antibody-Mediated Immunity Is Defined by Intracellular TRIM21-Dependent Innate Immune Activation and Extracellular Neutralization

Antibodies are essential for immunity against Ehrlichia chaffeensis, and protective mechanisms involve blocking of ehrlichial attachment or complement and Fcγ-receptor-dependent destruction. In this study, we determined that major outer membrane protein 1 (OMP-19) hypervariable region 1 (HVR1)-specific human monoclonal antibodies (huMAbs) are protective through conventional extracellular neutralization and, more significantly, through a novel intracellular TRIM21-mediated mechanism.

Antibodies are essential for immunity against Ehrlichia chaffeensis, and protective mechanisms involve blocking of ehrlichial attachment or complement and Fcγ-receptor-dependent destruction. In this study, we determined that major outer membrane protein 1 (OMP-19) hypervariable region 1 (HVR1)-specific human monoclonal antibodies (huMAbs) are protective through conventional extracellular neutralization and, more significantly, through a novel intracellular TRIM21-mediated mechanism. Addition of OMP-1-specific huMAb EHRL-15 (IgG1) prevented infection by blocking attachment/entry, a mechanism previously reported; conversely, OMP-1-specific huMAb EHRL-4 (IgG3) engaged intracellular TRIM21 and initiated an immediate innate immune response and rapid intracellular degradation of ehrlichiae. EHRL-4-TRIM21-mediated inhibition was significantly impaired in TRIM21 knockout THP-1 cells. EHRL-4 interacted with cytosolic Fc receptor TRIM21, observed by confocal microscopy and confirmed by co-immunoprecipitation. E. chaffeensis-EHRL-4-TRIM21 complexes caused significant upregulation of proinflammatory cytokine/chemokine transcripts and resulted in rapid (<30 min) nuclear accumulation of NF-κB and TRIM21 and ehrlichial destruction. We investigated the role of TRIM21 in the autophagic clearance of ehrlichiae in the presence of EHRL-4. Colocalization between EHRL-4-opsonized ehrlichiae, polyubiquitinated TRIM21, autophagy regulators (ULK1 and beclin 1) and effectors (LC3 and p62), and lysosome-associated membrane protein 2 (LAMP2) was observed. Moreover, autophagic flux defined by conversion of LC3I to LC3II and accumulation and degradation of p62 was detected, and EHRL-4-mediated degradation of E. chaffeensis was abrogated by the autophagy inhibitor 3-methyladenine. Our results demonstrate that huMAbs are capable of inhibiting E. chaffeensis infection by distinct effector mechanisms: extracellularly by neutralization and intracellularly by engaging TRIM21, which mediates a rapid innate immune response that mobilizes the core autophagy components, triggering localized selective autophagic degradation of ehrlichiae.

Ehrlichia chaffeensis TRP120 nucleomodulin binds DNA with disordered tandem repeat domain

Ehrlichia chaffeensis, the causative agent of human monocytotropic ehrlichiosis, secretes several effector proteins that bind host DNA to modulate host gene expression. The tandem repeat protein 120 (TRP120), one of the largest effector proteins, has four nearly identical tandem repeat (TR) regions that each consists of 80 amino acids. In addition to playing a role in ehrlichial binding and internalization, TRP120 translocates to the host nucleus where it is thought to function as a transcription factor that modulates gene expression. However, sequence analysis of TRP120 does not identify the presence of DNA-binding or trans-activation domains typical of classical eukaryotic transcription factors. Thus, the mechanism by which TRP120 binds DNA and modulates gene expression remains elusive. Herein, we expressed the TR regions of the TRP120 protein, and characterized its solution structure and ability to bind DNA. TRP120, expressed as either a one or two TR repeat, is a monomer in solution, and is mostly disordered as determined by circular dichroism (CD) and nuclear magnetic resonance (NMR) spectroscopy. Using NMR spectroscopy, we further show that the 1 TR construct selectively binds GC-rich DNA. Although low pH was required for TRP120 TR-DNA interaction, acidic pH alone does not induce any significant structural changes in the TR region. This suggests that TRP120 folds into an ordered structure upon forming a protein-DNA complex, and thus folding of TRP120 TR is coupled with DNA binding.

Ehrlichia chaffeensis Tandem Repeat Effector Targets Differentially Influence Infection

Ehrlichia chaffeensis infects mononuclear phagocytes and survives intracellularly by exploiting host cell processes to evade host defenses. The mechanisms involved are not fully defined, but appear to rely largely on a subset of tandem repeat proteins (TRP) effectors. E. chaffeensis TRPs are type 1 secreted effectors that interact with a functionally diverse group of host cell targets associated with various biological processes. In this study, we investigated the influence of TRP host target proteins on ehrlichial infection by RNA interference. In total, 138 TRP-interacting host proteins identified by yeast two-hybrid were targeted by siRNA and the infection level determined by real-time qPCR. Knockdown of 124 (89%) TRP target proteins had significant influence on infection either by inhibiting (85%) or promoting (15%) ehrlichial infection. Notably, knockdown of 18 host proteins which interacted with TRP120 promoted the infection, suggesting that these targets may be degraded to promote infection. Host proteins that interact with TRPs are involved in cellular processes, including cell signaling, vesicle trafficking and intracellular transport, transcriptional regulation, metabolism, protein posttranslational modification, and apoptosis. Selected host targets were examined by immunofluorescent microscopy during infection and were found to localize with the morulae, or in the host cell cytoplasm adjacent to morulae. This study confirms that the majority of host proteins known to interact with TRP effectors influence infection and further extends the current knowledge that E. chaffeensis TRPs participate in a complex array of host protein interactions in order to reprogram the host cell and promote intracellular survival.

Anaplasma phagocytophilum APH0032 Is Exposed on the Cytosolic Face of the Pathogen-Occupied Vacuole and Co-opts Host Cell SUMOylation

Anaplasma phagocytophilum, a member of the family Anaplasmataceae and the obligate intracellular bacterium that causes granulocytic anaplasmosis, resides in a host cell-derived vacuole. Bacterial proteins that localize to the A. phagocytophilum-occupied vacuole membrane (AVM) are critical host-pathogen interfaces. Of the few bacterial AVM proteins that have been identified, the domains responsible for AVM localization and the host cell pathways that they co-opt are poorly defined. APH0032 is an effector that is expressed and localizes to the AVM late during the infection cycle. Herein, the APH0032 domain that is essential for associating with host cell membranes was mapped. Immunofluorescent labeling of infected cells that had been differentially permeabilized confirmed that APH0032 is exposed on the AVM's cytosolic face, signifying its potential to interface with host cell processes. SUMOylation is the covalent attachment of a member of the small ubiquitin-like modifier (SUMO) family of proteins to lysines in target substrates. Previous work from our laboratory determined that SUMOylation is important for A. phagocytophilum survival and that SUMOylated proteins decorate the AVM. Algorithmic prediction analyses identified APH0032 as a candidate for SUMOylation. Endogenous APH0032 was precipitated from infected cells using a SUMO affinity matrix, confirming that the effector co-opts SUMOylation during infection. APH0032 pronouncedly colocalized with SUMO1, but not SUMO2/3 moieties on the AVM. Ectopic expression of APH0032 in A. phagocytophilum infected host cells significantly boosted the bacterial load. This study delineates the first domain of any Anaplasmataceae protein that is essential for associating with the pathogen-occupied vacuole membrane, demonstrates the importance of APH0032 to infection, and identifies it as the second A. phagocytophilum effector that co-opts SUMOylation, thus underscoring the relevance of this post-translational modification to infection.

Hacker within! Ehrlichia chaffeensis Effector Driven Phagocyte Reprogramming Strategy

Ehrlichia chaffeensis is a small, gram negative, obligately intracellular bacterium that preferentially infects mononuclear phagocytes. It is the etiologic agent of human monocytotropic ehrlichiosis (HME), an emerging life-threatening tick-borne zoonosis. Mechanisms by which E. chaffeensis establishes intracellular infection, and avoids host defenses are not well understood, but involve functionally relevant host-pathogen interactions associated with tandem and ankyrin repeat effector proteins. In this review, we discuss the recent advances in our understanding of the molecular and cellular mechanisms that underlie Ehrlichia host cellular reprogramming strategies that enable intracellular survival.

The Anaplasma phagocytophilum effector AmpA hijacks host cell SUMOylation

SUMOylation, the covalent attachment of a member of the small ubiquitin-like modifier (SUMO) family of proteins to lysines in target substrates, is an essential post-translational modification in eukaryotes. Microbial manipulation of SUMOylation recently emerged as a key virulence strategy for viruses and facultative intracellular bacteria, the latter of which have only been shown to deploy effectors that negatively regulate SUMOylation. Here, we demonstrate that the obligate intracellular bacterium, Anaplasma phagocytophilum, utilizes an effector, AmpA (A. phagocytophilum post-translationally modified protein A) that becomes SUMOylated in host cells and this is important for the pathogen's survival. We previously discovered that AmpA (formerly APH1387) localizes to the A. phagocytophilum-occupied vacuolar membrane (AVM). Algorithmic prediction analyses denoted AmpA as a candidate for SUMOylation. We verified this phenomenon using a SUMO affinity matrix to precipitate both native AmpA and ectopically expressed green fluorescent protein (GFP)-tagged AmpA. SUMOylation of AmpA was lysine dependent, as SUMO affinity beads failed to precipitate a GFP-AmpA protein when its lysine residues were substituted with arginine. Ectopically expressed and endogenous AmpA were poly-SUMOylated, which was consistent with the observation that AmpA colocalizes with SUMO2/3 at the AVM. Only late during the infection cycle did AmpA colocalize with SUMO1, which terminally caps poly-SUMO2/3 chains. AmpA was also detected in the cytosol of infected host cells, further supporting its secretion and likely participation in interactions that aid pathogen survival. Indeed, whereas siRNA-mediated knockdown of Ubc9 - a necessary enzyme for SUMOylation - slightly bolstered A. phagocytophilum infection, pharmacologically inhibiting SUMOylation in infected cells significantly reduced the bacterial load. Ectopically expressed GFP-AmpA served as a competitive agonist against native AmpA in infected cells, while lysine-deficient GFP-AmpA was less effective, implying that modification of AmpA lysines is important for infection. Collectively, these data show that AmpA becomes directly SUMOylated during infection, representing a novel tactic for A. phagocytophilum survival.

Ehrlichia moonlighting effectors and interkingdom interactions with the mononuclear phagocyte

Ehrlichia chaffeensis is an obligately intracellular gram negative bacterium with a small genome that thrives in mammalian mononuclear phagocytes by exploiting eukaryotic processes. Herein, we discuss the latest findings on moonlighting tandem repeat protein effectors and their secretion mechanisms, and novel molecular interkingdom interactions that provide insight into the intracellular pathobiology of ehrlichiae.

Recombinant Ehrlichia P29 protein induces a protective immune response in a mouse model of ehrlichiosis

Ehrlichioses are emerging tick-borne bacterial diseases of humans and animals for which no vaccines are available. The diseases are caused by obligately intracellular bacteria belonging to the genus Ehrlichia. Several immunoreactive proteins of ehrlichiae have been identified based on their reactivity with immune sera from human patients and animals. These include the major outer membrane proteins, ankyrin repeat proteins and tandem repeat proteins (TRP). Polyclonal antibodies directed against the tandem repeats (TRs) of Ehrlichia chaffeensis TRP32, TRP47 and TRP120 have been shown to provide protection in mice. In the present study, we evaluated E. muris P29, which is the ortholog of E. chaffeensis TRP47 and E. canis TRP36, as a subunit vaccine in a mouse model of ehrlichiosis. Our study indicated that unlike E. chaffeensis TRP47 and E. canis TRP36, orthologs of E. muris (P29) and E. muris-like agent (EMLA) do not contain tandem repeats. Immunization of mice with recombinant E. muris P29 induced significant protection against a challenge infection. The protection induced by E. muris P29 was associated with induction of strong antibody responses. In contrast to development of P29-specific IgG antibodies following immunization, development of P29-specific IgG antibodies, but not IgM antibodies, was impaired during persistent E. muris infection. Furthermore, our study indicated that CD4+ T cells target P29 during E. muris infection and differentiate into IFN-γ-producing Th1 effector/memory cells. In conclusion, our study indicated that orthologs of E. muris P29 showed considerable variation in the central tandem repeat region among different species, induction of P29-specific IgG antibody response was impaired during persistent E. muris infection, and rP29 induced protective immune responses.

TolC-dependent secretion of an ankyrin repeat-containing protein of Rickettsia typhi

Rickettsia typhi, the causative agent of murine (endemic) typhus, is an obligate intracellular pathogen with a life cycle involving both vertebrate and invertebrate hosts. In this study, we characterized a gene (RT0218) encoding a C-terminal ankyrin repeat domain-containing protein, named Rickettsia ankyrin repeat protein 1 (RARP-1), and identified it as a secreted effector protein of R. typhi. RT0218 showed differential transcript abundance at various phases of R. typhi intracellular growth. RARP-1 was secreted by R. typhi into the host cytoplasm during in vitro infection of mammalian cells. Transcriptional analysis revealed that RT0218 was cotranscribed with adjacent genes RT0217 (hypothetical protein) and RT0216 (TolC) as a single polycistronic mRNA. Given one of its functions as a facilitator of extracellular protein secretion in some Gram-negative bacterial pathogens, we tested the possible role of TolC in the secretion of RARP-1. Using Escherichia coli C600 and an isogenic tolC insertion mutant as surrogate hosts, our data demonstrate that RARP-1 is secreted in a TolC-dependent manner. Deletion of either the N-terminal signal peptide or the C-terminal ankyrin repeats abolished RARP-1 secretion by wild-type E. coli. Importantly, expression of R. typhi tolC in the E. coli tolC mutant restored the secretion of RARP-1, suggesting that TolC has a role in RARP-1 translocation across the outer membrane. This work implies that the TolC component of the putative type 1 secretion system of R. typhi is involved in the secretion process of RARP-1.

Ehrlichia chaffeensis tandem repeat proteins and Ank200 are type 1 secretion system substrates related to the repeats-in-toxin exoprotein family

Ehrlichia chaffeensis has type 1 and 4 secretion systems (T1SS and T4SS), but the substrates have not been identified. Potential substrates include secreted tandem repeat protein (TRP) 47, TRP120, and TRP32, and the ankyrin repeat protein, Ank200, that are involved in molecular host–pathogen interactions including DNA binding and a network of protein–protein interactions with host targets associated with signaling, transcriptional regulation, vesicle trafficking, and apoptosis. In this study we report that E. chaffeensis TRP47, TRP32, TRP120, and Ank200 were not secreted in the Agrobacterium tumefaciens Cre recombinase reporter assay routinely used to identify T4SS substrates. In contrast, all TRPs and the Ank200 proteins were secreted by the Escherichia coli complemented with the hemolysin secretion system (T1SS), and secretion was reduced in a T1SS mutant (ΔTolC), demonstrating that these proteins are T1SS substrates. Moreover, T1SS secretion signals were identified in the C-terminal domains of the TRPs and Ank200, and a detailed bioinformatic analysis of E. chaffeensis TRPs and Ank200 revealed features consistent with those described in the repeats-in-toxins (RTX) family of exoproteins, including glycine- and aspartate-rich tandem repeats, homology with ATP-transporters, a non-cleavable C-terminal T1SS signal, acidic pIs, and functions consistent with other T1SS substrates. Using a heterologous E. coli T1SS, this investigation has identified the first Ehrlichia T1SS substrates supporting the conclusion that the T1SS and corresponding substrates are involved in molecular host–pathogen interactions that contribute to Ehrlichia pathobiology. Further investigation of the relationship between Ehrlichia TRPs, Ank200, and the RTX exoprotein family may lead to a greater understanding of the importance of T1SS substrates and specific functions of T1SS in the pathobiology of obligately intracellular bacteria.

Proteomic analysis of Anaplasma phagocytophilum during infection of human myeloid cells identifies a protein that is pronouncedly upregulated on the infectious dense-cored cell

Anaplasma phagocytophilum is an obligate intracellular bacterium that invades neutrophils to cause the emerging infectious disease human granulocytic anaplasmosis. A. phagocytophilum undergoes a biphasic developmental cycle, transitioning between an infectious dense-cored cell (DC) and a noninfectious reticulate cell (RC). To gain insights into the organism's biology and pathogenesis during human myeloid cell infection, we conducted proteomic analyses on A. phagocytophilum organisms purified from HL-60 cells. A total of 324 proteins were unambiguously identified, thereby verifying 23.7% of the predicted A. phagocytophilum proteome. Fifty-three identified proteins had been previously annotated as hypothetical or conserved hypothetical. The second most abundant gene product, after the well-studied major surface protein 2 (P44), was the hitherto hypothetical protein APH_1235. APH_1235 homologs are found in other Anaplasma and Ehrlichia species but not in other bacteria. The aph_1235 RNA level is increased 70-fold in the DC form relative to that in the RC form. Transcriptional upregulation of and our ability to detect APH_1235 correlate with RC to DC transition, DC exit from host cells, and subsequent DC binding and entry during the next round of infection. Immunoelectron microscopy pronouncedly detects APH_1235 on DC organisms, while detection on RC bacteria minimally, at best, exceeds background. This work represents an extensive study of the A. phagocytophilum proteome, discerns the complement of proteins that is generated during survival within human myeloid cells, and identifies APH_1235 as the first known protein that is pronouncedly upregulated on the infectious DC form.

Ehrlichia chaffeensis transcriptome in mammalian and arthropod hosts reveals differential gene expression and post transcriptional regulation

BACKGROUND

Human monocytotropic ehrlichiosis is an emerging life-threatening zoonosis caused by obligately intracellular bacterium, Ehrlichia chaffeensis. E. chaffeensis is transmitted by the lone star tick, Amblyomma americanum, and replicates in mononuclear phagocytes in mammalian hosts. Differences in the E. chaffeensis transcriptome in mammalian and arthropod hosts are unknown. Thus, we determined host-specific E. chaffeensis gene expression in human monocyte (THP-1) and in Amblyomma and Ixodes tick cell lines (AAE2 and ISE6) using a whole genome microarray.

METHODOLOGY/PRINCIPAL FINDINGS

The majority (∼80%) of E. chaffeensis genes were expressed during infection in human and tick cells. There were few differences observed in E. chaffeensis gene expression between the vector Amblyomma and non-vector Ixodes tick cells, but extensive host-specific and differential gene expression profiles were detected between human and tick cells, including higher transcriptional activity in tick cells and identification of gene subsets that were differentially expressed in the two hosts. Differentially and host-specifically expressed ehrlichial genes encoded major immunoreactive tandem repeat proteins (TRP), the outer membrane protein (OMP-1) family, and hypothetical proteins that were 30-80 amino acids in length. Consistent with previous observations, high expression of p28 and OMP-1B genes was detected in human and tick cells, respectively. Notably, E. chaffeensis genes encoding TRP32 and TRP47 were highly upregulated in the human monocytes and expressed as proteins; however, although TRP transcripts were expressed in tick cells, the proteins were not detected in whole cell lysates demonstrating that TRP expression was post transcriptionally regulated.

CONCLUSIONS/SIGNIFICANCE

Ehrlichia gene expression is highly active in tick cells, and differential gene expression among a wide variety of host-pathogen associated genes occurs. Furthermore, we demonstrate that genes associated with host-pathogen interactions are differentially expressed and regulated by post transcriptional mechanisms.

Tyrosine-phosphorylated Ehrlichia chaffeensis and Ehrlichia canis tandem repeat orthologs contain a major continuous cross-reactive antibody epitope in lysine-rich repeats

A small subset of major immunoreactive proteins have been identified in Ehrlichia chaffeensis and Ehrlichia canis, including three molecularly and immunologically characterized pairs of immunoreactive tandem repeat protein (TRP) orthologs with major continuous species-specific epitopes within acidic tandem repeats (TR) that stimulate strong antibody responses during infection. In this study, we identified a fourth major immunoreactive TR-containing ortholog pair and defined a major cross-reactive epitope in homologous nonidentical 24-amino-acid lysine-rich TRs. Antibodies from patients and dogs with ehrlichiosis reacted strongly with recombinant TR regions, and epitopes were mapped to the N-terminal TR region (18 amino acids) in E. chaffeensis and the complete TR (24 amino acids) in E. canis. Two less-dominant epitopes were mapped to adjacent glutamate/aspartate-rich and aspartate/tyrosine-rich regions in the acidic C terminus of E. canis TRP95 but not in E. chaffeensis TRP75. Major immunoreactive proteins in E. chaffeensis (75-kDa) and E. canis (95-kD) whole-cell lysates and supernatants were identified with TR-specific antibodies. Consistent with other ehrlichial TRPs, the TRPs identified in ehrlichial whole-cell lysates and the recombinant proteins migrated abnormally slow electrophoretically a characteristic that was demonstrated with the positively charged TR and negatively charged C-terminal domains. E. chaffeensis TRP75 and E. canis TRP95 were immunoprecipitated with anti-pTyr antibody, demonstrating that they are tyrosine phosphorylated during infection of the host cell.

Global proteomic analysis of two tick-borne emerging zoonotic agents: anaplasma phagocytophilum and ehrlichia chaffeensis

Anaplasma phagocytophilum and Ehrlichia chaffeensis are obligatory intracellular α-proteobacteria that infect human leukocytes and cause potentially fatal emerging zoonoses. In the present study, we determined global protein expression profiles of these bacteria cultured in the human promyelocytic leukemia cell line, HL-60. Mass spectrometric (MS) analyses identified a total of 1,212 A. phagocytophilum and 1,021 E. chaffeensis proteins, representing 89.3 and 92.3% of the predicted bacterial proteomes, respectively. Nearly all bacterial proteins (≥99%) with known functions were expressed, whereas only approximately 80% of “hypothetical” proteins were detected in infected human cells. Quantitative MS/MS analyses indicated that highly expressed proteins in both bacteria included chaperones, enzymes involved in biosynthesis and metabolism, and outer membrane proteins, such as A. phagocytophilum P44 and E. chaffeensis P28/OMP-1. Among 113 A. phagocytophilum p44 paralogous genes, 110 of them were expressed and 88 of them were encoded by pseudogenes. In addition, bacterial infection of HL-60 cells up-regulated the expression of human proteins involved mostly in cytoskeleton components, vesicular trafficking, cell signaling, and energy metabolism, but down-regulated some pattern recognition receptors involved in innate immunity. Our proteomics data represent a comprehensive analysis of A. phagocytophilum and E. chaffeensis proteomes, and provide a quantitative view of human host protein expression profiles regulated by bacterial infection. The availability of these proteomic data will provide new insights into biology and pathogenesis of these obligatory intracellular pathogens.

Molecular and cellular pathobiology of Ehrlichia infection: targets for new therapeutics and immunomodulation strategies

Ehrlichia are small obligately intracellular bacteria in the order Rickettsiales that are transmitted by ticks and associated with emerging life-threatening human zoonoses. Vaccines are not available for human ehrlichiosis, and therapeutic options are limited to a single antibiotic class. New technologies for exploring host-pathogen interactions have yielded recent advances in understanding the molecular interactions between Ehrlichia and the eukaryotic host cell and identified new targets for therapeutic and vaccine development, including those that target pathogen virulence mechanisms or disrupt the processes associated with ehrlichial effector proteins. Animal models have also provided insight into immunopathological mechanisms that contribute significantly to understanding severe disease manifestations, which should lead to the development of immunomodulatory approaches for treating patients nearing or experiencing severe disease states. In this review, we discuss the recent advances in our understanding of molecular and cellular pathobiology and the immunobiology of Ehrlichia infection. We identify new molecular host-pathogen interactions that can be targets of new therapeutics, and discuss prospects for treating the immunological dysregulation during acute infection that leads to life-threatening complications.

Anaplasma phagocytophilum APH_0032 is expressed late during infection and localizes to the pathogen-occupied vacuolar membrane

Anaplasma phagocytophilum infects neutrophils and myeloid, endothelial, and tick cell lines to reside within a host cell-derived vacuole that is indispensible for its survival. Here, we identify APH_0032 as an Anaplasma-derived protein that associates with the A. phagocytophilum-occupied vacuolar membrane (AVM). APH_0032 is a 66.1 kDa acidic protein that electrophoretically migrates with an apparent molecular weight of 130 kDa. It contains a predicted transmembrane domain and tandemly arranged direct repeats that comprise 46% of the protein. APH_0032 is undetectable on Anaplasma organisms bound to the surfaces of HL-60 cells, but is detected on the AVM and surfaces of intravacuolar bacteria beginning 24 h post-infection. APH_0032 localizes to the AVM in HL-60, THP-1, HMEC-1, and ISE6 cells. APH_0032, along with APH_1387, which encodes a confirmed AVM protein, is transcribed during A. phagocytophilum infection of tick salivary glands and murine neutrophils. APH_0032 localizes to the AVM in neutrophils recovered from infected mice. The Legionella pneumophila Dot/IcM type IV secretion system (T4SS) can heterologously secrete a CyaA-tagged version of the A. phagocytophilum VirB/D T4SS effector, AnkA, but fails to secrete CyaA-tagged APH_0032 or APH_1387. These data confirm APH_0032 as an Anaplasma-derived AVM protein and hint that neither it nor APH_1387 are T4SS effectors.