title: Analysis of targeting sequences demonstrates that trafficking to the Toxoplasma gondii plastid branches off the secretory system.
abstract: Apicomplexan parasites possess a plastid-like organelle called the apicoplast. Most proteins in the Toxoplasma gondii apicoplast are encoded in the nucleus and imported post-translationally. T. gondii apicoplast proteins often have a long N-terminal extension that directs the protein to the apicoplast. It can be modeled as a bipartite targeting sequence that contains a signal sequence and a plastid transit peptide. We identified two nuclearly encoded predicted plastid proteins and made fusions with green fluorescent protein to study protein domains required for apicoplast targeting. The N-terminal 42 amino acids of the apicoplast ribosomal protein S9 directs secretion of green fluorescent protein, indicating that targeting to the apicoplast proceeds through the secretory system. Large sections of the S9 predicted transit sequence can be deleted with no apparent impact on the ability to direct green fluorescent protein to the apicoplast. The predicted transit peptide domain of the S9 targeting sequence directs protein to the mitochondrion in vivo. The transit peptide can also direct import of green fluorescent protein into chloroplasts in vitro. These data substantiate the model that protein targeting to the apicoplast involves two distinct mechanisms: the first involving the secretory system and the second sharing features with typical chloroplast protein import.
authors.affiliation: Department of Pathobiology, School of Public Health and Community Medicine, University of Washington, Seattle, WA 98195, USA.
authors.forename: A
authors.initials: A
authors.lastname: DeRocher
authors.forename: C B
authors.initials: CB
authors.lastname: Hagen
authors.forename: J E
authors.initials: JE
authors.lastname: Froehlich
authors.forename: J E
authors.initials: JE
authors.lastname: Feagin
authors.forename: M
authors.initials: M
authors.lastname: Parsons
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D030801:Animals, Genetically Modified; D002467:Cell Nucleus / Q000235:genetics; D002736:Chloroplasts / Q000235:genetics / Q000378:metabolism; D005786:Gene Expression Regulation; D049452:Green Fluorescent Proteins; D008164:Luminescent Proteins / Q000235:genetics; D008928:Mitochondria / Q000378:metabolism; D008969:Molecular Sequence Data; D015388:Organelles / Q000235:genetics / Q000648:ultrastructure*; D018087:Plastids; D014176:Protein Biosynthesis; D015800:Protozoan Proteins / Q000235:genetics*; D000091243:Ribosomal Protein S9; D012269:Ribosomal Proteins / Q000235:genetics*; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid; D014122:Toxoplasma / Q000235:genetics* / Q000648:ultrastructure*; D014158:Transcription, Genetic; D014162:Transfection
chemical_list: D008164:Luminescent Proteins; D015800:Protozoan Proteins; D000091243:Ribosomal Protein S9; D012269:Ribosomal Proteins; D049452:Green Fluorescent Proteins