title: In Vivo Biotinylation of the Toxoplasma Parasitophorous Vacuole Reveals Novel Dense Granule Proteins Important for Parasite Growth and Pathogenesis.
abstract: Toxoplasma gondii is an obligate intracellular parasite that invades host cells and replicates within a unique parasitophorous vacuole. To maintain this intracellular niche, the parasite secretes an array of dense granule proteins (GRAs) into the nascent parasitophorous vacuole. These GRAs are believed to play key roles in vacuolar remodeling, nutrient uptake, and immune evasion while the parasite is replicating within the host cell. Despite the central role of GRAs in the Toxoplasma life cycle, only a subset of these proteins have been identified, and many of their roles have not been fully elucidated. In this report, we utilize the promiscuous biotin ligase BirA* to biotinylate GRA proteins secreted into the vacuole and then identify those proteins by affinity purification and mass spectrometry. Using GRA-BirA* fusion proteins as bait, we have identified a large number of known and candidate GRAs and verified localization of 13 novel GRA proteins by endogenous gene tagging. We proceeded to functionally characterize three related GRAs from this group (GRA38, GRA39, and GRA40) by gene knockout. While Δgra38 and Δgra40 parasites showed no altered phenotype, disruption of GRA39 results in slow-growing parasites that contain striking lipid deposits in the parasitophorous vacuole, suggesting a role in lipid regulation that is important for parasite growth. In addition, parasites lacking GRA39 showed dramatically reduced virulence and a lower tissue cyst burden in vivo Together, the findings from this work reveal a partial vacuolar proteome of T. gondii and identify a novel GRA that plays a key role in parasite replication and pathogenesis.
  
  
  
  Most intracellular pathogens reside inside a membrane-bound vacuole within their host cell that is extensively modified by the pathogen to optimize intracellular growth and avoid host defenses. In Toxoplasma, this vacuole is modified by a host of secretory GRA proteins, many of which remain unidentified. Here we demonstrate that in vivo biotinylation of proximal and interacting proteins using the promiscuous biotin ligase BirA* is a powerful approach to rapidly identify vacuolar GRA proteins. We further demonstrate that one factor identified by this approach, GRA39, plays an important role in the ability of the parasite to replicate within its host cell and cause disease.
authors.affiliation: Department of Microbiology, Immunology and Molecular Genetics, University of California, Los Angeles, Los Angeles, California, USA Department of Pediatrics, David Geffen School of Medicine at the University of California, Los Angeles, Los Angeles, California, USA.
authors.forename: Santhosh M
authors.initials: SM
authors.lastname: Nadipuram
authors.affiliation: Department of Microbiology, Immunology and Molecular Genetics, University of California, Los Angeles, Los Angeles, California, USA.
authors.forename: Elliot W
authors.initials: EW
authors.lastname: Kim
authors.affiliation: Department of Biological Chemistry and Institute of Genomics and Proteomics, University of California, Los Angeles, Los Angeles, California, USA.
authors.forename: Ajay A
authors.initials: AA
authors.lastname: Vashisht
authors.affiliation: Department of Microbiology, Immunology and Molecular Genetics, University of California, Los Angeles, Los Angeles, California, USA.
authors.forename: Andrew H
authors.initials: AH
authors.lastname: Lin
authors.affiliation: Department of Microbiology, Immunology and Molecular Genetics, University of California, Los Angeles, Los Angeles, California, USA.
authors.forename: Hannah N
authors.initials: HN
authors.lastname: Bell
authors.affiliation: Department of Molecular Microbiology and Immunology, The Johns Hopkins University Bloomberg School of Public Health, Baltimore, Maryland, USA.
authors.forename: Isabelle
authors.initials: I
authors.lastname: Coppens
authors.affiliation: Department of Biological Chemistry and Institute of Genomics and Proteomics, University of California, Los Angeles, Los Angeles, California, USA Molecular Biology Institute, University of California Los Angeles, Los Angeles, California, USA.
authors.forename: James A
authors.initials: JA
authors.lastname: Wohlschlegel
authors.affiliation: Department of Microbiology, Immunology and Molecular Genetics, University of California, Los Angeles, Los Angeles, California, USA Molecular Biology Institute, University of California Los Angeles, Los Angeles, California, USA pbradley@ucla.edu.
authors.forename: Peter J
authors.initials: PJ
authors.lastname: Bradley
mesh_terms: D019921:Biotinylation; D002478:Cells, Cultured; D002846:Chromatography, Affinity; D005347:Fibroblasts / Q000469:parasitology; D006801:Humans; D013058:Mass Spectrometry; D015800:Protozoan Proteins / Q000032:analysis*; D013194:Staining and Labeling; D014122:Toxoplasma / Q000254:growth & development* / Q000472:pathogenicity*; D014617:Vacuoles / Q000737:chemistry* / Q000469:parasitology*; D037521:Virulence Factors / Q000032:analysis*
chemical_list: D015800:Protozoan Proteins; D037521:Virulence Factors