Queuosine Biosynthesis Is Required for Sinorhizobium meliloti-Induced Cytoskeletal Modifications on HeLa Cells and Symbiosis with Medicago truncatula

Rhizobia are symbiotic soil bacteria able to intracellularly colonize legume nodule cells and form nitrogen-fixing symbiosomes therein. How the plant cell cytoskeleton reorganizes in response to rhizobium colonization has remained poorly understood especially because of the lack of an in vitro infection assay. Here, we report on the use of the heterologous HeLa cell model to experimentally tackle this question. We observed that the model rhizobium Sinorhizobium meliloti, and other rhizobia as well, were able to trigger a major reorganization of actin cytoskeleton of cultured HeLa cells in vitro. Cell deformation was associated with an inhibition of the three major small RhoGTPases Cdc42, RhoA and Rac1. Bacterial entry, cytoskeleton rearrangements and modulation of RhoGTPase activity required an intact S. meliloti biosynthetic pathway for queuosine, a hypermodifed nucleoside regulating protein translation through tRNA, and possibly mRNA, modification. We showed that an intact bacterial queuosine biosynthetic pathway was also required for effective nitrogen-fixing symbiosis of S. meliloti with its host plant Medicago truncatula, thus indicating that one or several key symbiotic functions of S. meliloti are under queuosine control. We discuss whether the symbiotic defect of que mutants may originate, at least in part, from an altered capacity to modify plant cell actin cytoskeleton.     

Protein-Protein Interactions in the β-Oxidation Part of the Phenylacetate Utilization Pathway: CRYSTAL STRUCTURE OF THE PaaF-PaaG HYDRATASE-ISOMERASE COMPLEX*

Microbial anaerobic and so-called hybrid pathways for degradation of aromatic compounds contain β-oxidation-like steps. These reactions convert the product of the opening of the aromatic ring to common metabolites. The hybrid phenylacetate degradation pathway is encoded in Escherichia coli by the paa operon containing genes for 10 enzymes. Previously, we have analyzed protein-protein interactions among the enzymes catalyzing the initial oxidation steps in the paa pathway (Grishin, A. M., Ajamian, E., Tao, L., Zhang, L., Menard, R., and Cygler, M. (2011) J. Biol. Chem. 286, 10735–10743). Here we report characterization of interactions between the remaining enzymes of this pathway and show another stable complex, PaaFG, an enoyl-CoA hydratase and enoyl-Coa isomerase, both belonging to the crotonase superfamily. These steps are biochemically similar to the well studied fatty acid β-oxidation, which can be catalyzed by individual monofunctional enzymes, multifunctional enzymes comprising several domains, or enzymatic complexes such as the bacterial fatty acid β-oxidation complex. We have determined the structure of the PaaFG complex and determined that although individually PaaF and PaaG are similar to enzymes from the fatty acid β-oxidation pathway, the structure of the complex is dissimilar from bacterial fatty acid β-oxidation complexes. The PaaFG complex has a four-layered structure composed of homotrimeric discs of PaaF and PaaG. The active sites of PaaF and PaaG are adapted to accept the intermediary components of the Paa pathway, different from those of the fatty acid β-oxidation. The association of PaaF and PaaG into a stable complex might serve to speed up the steps of the pathway following the conversion of phenylacetyl-CoA to a toxic and unstable epoxide-CoA by PaaABCE monooxygenase.     

Studies on the Assembly Characteristics of Large Subunit Ribosomal Proteins in S. cerevisae

During the assembly process of ribosomal subunits, their structural components, the ribosomal RNAs (rRNAs) and the ribosomal proteins (r-proteins) have to join together in a highly dynamic and defined manner to enable the efficient formation of functional ribosomes. In this work, the assembly of large ribosomal subunit (LSU) r-proteins from the eukaryote S. cerevisiae was systematically investigated. Groups of LSU r-proteins with specific assembly characteristics were detected by comparing the protein composition of affinity purified early, middle, late or mature LSU (precursor) particles by semi-quantitative mass spectrometry. The impact of yeast LSU r-proteins rpL25, rpL2, rpL43, and rpL21 on the composition of intermediate to late nuclear LSU precursors was analyzed in more detail. Effects of these proteins on the assembly states of other r-proteins and on the transient LSU precursor association of several ribosome biogenesis factors, including Nog2, Rsa4 and Nop53, are discussed.     

New Insights into Handling Missing Values in Environmental Epidemiological Studies

Missing data are unavoidable in environmental epidemiologic surveys. The aim of this study was to compare methods for handling large amounts of missing values: omission of missing values, single and multiple imputations (through linear regression or partial least squares regression), and a fully Bayesian approach. These methods were applied to the PARIS birth cohort, where indoor domestic pollutant measurements were performed in a random sample of babies'' dwellings. A simulation study was conducted to assess performances of different approaches with a high proportion of missing values (from 50% to 95%). Different simulation scenarios were carried out, controlling the true value of the association (odds ratio of 1.0, 1.2, and 1.4), and varying the health outcome prevalence. When a large amount of data is missing, omitting these missing data reduced statistical power and inflated standard errors, which affected the significance of the association. Single imputation underestimated the variability, and considerably increased risk of type I error. All approaches were conservative, except the Bayesian joint model. In the case of a common health outcome, the fully Bayesian approach is the most efficient approach (low root mean square error, reasonable type I error, and high statistical power). Nevertheless for a less prevalent event, the type I error is increased and the statistical power is reduced. The estimated posterior distribution of the OR is useful to refine the conclusion. Among the methods handling missing values, no approach is absolutely the best but when usual approaches (e.g. single imputation) are not sufficient, joint modelling approach of missing process and health association is more efficient when large amounts of data are missing.     

Effect of Peroxisome Proliferator-Activated Receptor-γ Coactivator-1 Alpha Variants on Spontaneous Clearance and Fibrosis Progression during Hepatitis C Virus Infection in Moroccan Patients

Virologica Sinica - Hepatitis C virus (HCV) is still one of the main causes of liver disease worldwide. Metabolic disorders, including non-alcoholic fatty liver disease (NAFLD), induced by HCV have...     

A single Na+-Pi cotransporter in Toxoplasma plays key roles in phosphate import and control of parasite osmoregulation

Inorganic ions such as phosphate, are essential nutrients required for a broad spectrum of cellular functions and regulation. During infection, pathogens must obtain inorganic phosphate (P_i) from the host. Despite the essentiality of phosphate for all forms of life, how the intracellular parasite Toxoplasma gondii acquires P_i from the host cell is still unknown. In this study, we demonstrated that Toxoplasma actively internalizes exogenous P_i by exploiting a gradient of Na⁺ ions to drive P_i uptake across the plasma membrane. The Na⁺-dependent phosphate transport mechanism is electrogenic and functionally coupled to a cipargarmin sensitive Na⁺-H⁺-ATPase. Toxoplasma expresses one transmembrane P_i transporter harboring PHO4 binding domains that typify the PiT Family. This transporter named TgPiT, localizes to the plasma membrane, the inward buds of the endosomal organelles termed VAC, and many cytoplasmic vesicles. Upon P_i limitation in the medium, TgPiT is more abundant at the plasma membrane. We genetically ablated the PiT gene, and ΔTgPiT parasites are impaired in importing P_i and synthesizing polyphosphates. Interestingly, ΔTgPiT parasites accumulate 4-times more acidocalcisomes, storage organelles for phosphate molecules, as compared to parental parasites. In addition, these mutants have a reduced cell volume, enlarged VAC organelles, defects in calcium storage and a slightly alkaline pH. Overall, these mutants exhibit severe growth defects and have reduced acute virulence in mice. In survival mode, ΔTgPiT parasites upregulate several genes, including those encoding enzymes that cleave or transfer phosphate groups from phosphometabolites, transporters and ions exchangers localized to VAC or acidocalcisomes. Taken together, these findings point to a critical role of TgPiT for P_i supply for Toxoplasma and also for protection against osmotic stresses.     

The emerging roles of lactate as a redox substrate and signaling molecule in adipose tissues

Journal of Physiology and Biochemistry - Thermogenic (brown and beige) adipose tissues improve glucose and lipid homeostasis and therefore represent putative targets to cure obesity and related...     

A Plasmodium homolog of ER tubule-forming proteins is required for parasite virulence

Reticulon and REEP family of proteins stabilize the high curvature of endoplasmic reticulum (ER) tubules. Plasmodium berghei Yop1 (PbYop1) is a REEP5 homolog in Plasmodium. Here, we characterize its function using a gene-knockout (Pbyop1∆). Pbyop1∆ asexual stage parasites display abnormal ER architecture and an enlarged digestive vacuole. The erythrocytic cycle of Pbyop1∆ parasites is severely attenuated and the incidence of experimental cerebral malaria is significantly decreased in Pbyop1∆-infected mice. Pbyop1∆ sporozoites have reduced speed, are slower to invade host cells but give rise to equal numbers of infected HepG2 cells, as WT sporozoites. We propose that PbYOP1’s disruption may lead to defects in trafficking and secretion of a subset of proteins required for parasite development and invasion of erythrocytes. Furthermore, the maintenance of ER morphology in different parasite stages is likely to depend on different proteins.