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The Escherichia coli effector EspJ blocks Src kinase activity via amidation and ADP ribosylation
(2014)
The hallmark of enteropathogenic Escherichia coli (EPEC) infection is the formation of actin-rich pedestal-like structures, which are generated following phosphorylation of the bacterial effector Tir by cellular Src and Abl family tyrosine kinases. This leads to recruitment of the Nck-WIP-N-WASP complex that triggers Arp2/3-dependent actin polymerization in the host cell. The same phosphorylation-mediated signalling network is also assembled downstream of the Vaccinia virus protein A36 and the phagocytic Fc-gamma receptor FcγRIIa. Here we report that the EPEC type-III secretion system effector EspJ inhibits autophosphorylation of Src and phosphorylation of the Src substrates Tir and FcγRIIa. Consistent with this, EspJ inhibits actin polymerization downstream of EPEC, Vaccinia virus and opsonized red blood cells. We identify EspJ as a unique adenosine diphosphate (ADP) ribosyltransferase that directly inhibits Src kinase by simultaneous amidation and ADP ribosylation of the conserved kinase-domain residue, Src E310, resulting in glutamine-ADP ribose.
The Cytotoxic Necrotizing Factor 1 (CNF1) is a protein toxin which is a major virulence factor of pathogenic Escherichia coli strains. Here, we identified the Lutheran (Lu) adhesion glycoprotein/basal cell adhesion molecule (BCAM) as cellular receptor for CNF1 by co-precipitation of cell surface molecules with tagged toxin. The CNF1-Lu/BCAM interaction was verified by direct protein-protein interaction analysis and competition studies. These studies revealed amino acids 720 to 1014 of CNF1 as the binding site for Lu/BCAM. We suggest two cell interaction sites in CNF1: first the N-terminus, which binds to p37LRP as postulated before. Binding of CNF1 to p37LRP seems to be crucial for the toxin's action. However, it is not sufficient for the binding of CNF1 to the cell surface. A region directly adjacent to the catalytic domain is a high affinity interaction site for Lu/BCAM. We found Lu/BCAM to be essential for the binding of CNF1 to cells. Cells deficient in Lu/BCAM but expressing p37LRP could not bind labeled CNF1. Therefore, we conclude that LRP and Lu/BCAM are both required for toxin action but with different functions. Author Summary We study a crucial virulence factor produced by pathogenic Escherichia coli strains, the Cytotoxic Necrotizing Factor 1 (CNF1). More than 80% of urinary tract infections (UTIs), which are counted among the most common bacterial infections of humans, are caused by Uropathogenic Escherichia coli (UPEC) strains. We and others elucidated the molecular mechanism of the E. coli toxin CNF1. It constitutively activates Rho GTPases by a direct covalent modification. The toxin enters mammalian cells by receptor-mediated endocytosis. Here, we identified the protein receptor for CNF1 by co-precipitation of cell surface molecules with the tagged toxin and subsequent Maldi-TOF analysis. We identified the Lutheran (Lu) adhesion glycoprotein/basal cell adhesion molecule (BCAM) as receptor for CNF1 and located its interaction site to the C-terminal part of the toxin. We performed direct protein-protein interaction analysis and competition studies. Moreover, cells deficient in Lu/BCAM could not bind labeled CNF1. The identification of a toxin's cellular receptor and receptor binding region is an important task for understanding the pathogenic function of the toxin and, moreover, to make the toxin accessible for its use as a cellbiological and pharmacological tool, for example for the generation of immunotoxins.
The polarization of cells is essential for the proper functioning of most organs. Planar Cell Polarity (PCP), the polarization within the plane of an epithelium, is perpendicular to apical-basal polarity and established by the non-canonical Wnt/Fz-PCP signaling pathway. Within each tissue, downstream PCP effectors link the signal to tissue specific readouts such as stereocilia orientation in the inner ear and hair follicle orientation in vertebrates or the polarization of ommatidia and wing hairs in Drosophila melanogaster. Specific PCP effectors in the wing such as Multiple wing hairs (Mwh) and Rho Kinase (Rok) are required to position the hair at the correct position and to prevent ectopic actin hairs. In a genome-wide screen in vitro, we identified Combover (Cmb)/CG10732 as a novel Rho kinase substrate. Overexpression of Cmb causes the formation of a multiple hair cell phenotype (MHC), similar to loss of rok and mwh. This MHC phenotype is dominantly enhanced by removal of rok or of other members of the PCP effector gene family. Furthermore, we show that Cmb physically interacts with Mwh, and cmb null mutants suppress the MHC phenotype of mwh alleles. Our data indicate that Cmb is a novel PCP effector that promotes to wing hair formation, a function that is antagonized by Mwh.
While gene expression is a fundamental and tightly controlled cellular process that is regulated at multiple steps, the exact contribution of each step remains unknown in any organism. The absence of transcription initiation regulation for RNA polymerase II in the protozoan parasite Trypanosoma brucei greatly simplifies the task of elucidating the contribution of translation to global gene expression. Therefore, we have sequenced ribosome-protected mRNA fragments in T. brucei, permitting the genome-wide analysis of RNA translation and translational efficiency. We find that the latter varies greatly between life cycle stages of the parasite and ∼100-fold between genes, thus contributing to gene expression to a similar extent as RNA stability. The ability to map ribosome positions at sub-codon resolution revealed extensive translation from upstream open reading frames located within 5' UTRs and enabled the identification of hundreds of previously un-annotated putative coding sequences (CDSs). Evaluation of existing proteomics and genome-wide RNAi data confirmed the translation of previously un-annotated CDSs and suggested an important role for >200 of those CDSs in parasite survival, especially in the form that is infective to mammals. Overall our data show that translational control plays a prevalent and important role in different parasite life cycle stages of T. brucei.