title: The Toxoplasma homolog of Plasmodium apical membrane antigen-1 (AMA-1) is a microneme protein secreted in response to elevated intracellular calcium levels.
abstract: A monoclonal antibody (MAb) has been generated against a novel 63 kDa surface/apical antigen of Toxoplasma gondii tachyzoites which is identified here as TgAMA-1, the Toxoplasma homolog of Plasmodium apical membrane antigen-1 (AMA-1). Sequence analysis, phase partitioning in Triton X-114, and labeling of TgAMA-1 with iodonaphthalene azide all suggest that TgAMA-1 is a type I transmembrane protein. There is a high degree of sequence similarity between TgAMA-1 and Plasmodium AMA-1, most notably in the position of conserved cysteine residues within the protein's predicted extracellular domain. In contrast to full length Plasmodium AMA-1, which has previously been localized to the rhoptries, it is shown here by immunofluorescence and immunoelectron microscopy that intracellular TgAMA-1 is found in the micronemes. A 53 kDa N-terminal proteolytic fragment of TgAMA-1 is constitutively secreted from the parasite at 37 degrees C. As is the case with other microneme proteins, the proteolytic processing and secretion of TgAMA-1 is dramatically enhanced in response to treatments which increase intracellular calcium levels.
authors.affiliation: Department of Microbiology and Molecular Genetics, University of Vermont, 214 Stafford Hall, Burlington, VT 05405, USA.
authors.forename: C G
authors.initials: CG
authors.lastname: Donahue
authors.forename: V B
authors.initials: VB
authors.lastname: Carruthers
authors.forename: S D
authors.initials: SD
authors.lastname: Gilk
authors.forename: G E
authors.initials: GE
authors.lastname: Ward
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D000911:Antibodies, Monoclonal; D000953:Antigens, Protozoan; D015153:Blotting, Western; D002118:Calcium / Q000378:metabolism*; D004591:Electrophoresis, Polyacrylamide Gel; D008565:Membrane Proteins / Q000737:chemistry / Q000276:immunology / Q000302:isolation & purification / Q000378:metabolism*; D008856:Microscopy, Fluorescence; D016253:Microscopy, Immunoelectron; D008969:Molecular Sequence Data; D008970:Molecular Weight; D010961:Plasmodium / Q000737:chemistry* / Q000276:immunology; D011499:Protein Processing, Post-Translational; D015800:Protozoan Proteins / Q000737:chemistry / Q000276:immunology / Q000302:isolation & purification / Q000378:metabolism*; D016415:Sequence Alignment; D014122:Toxoplasma / Q000737:chemistry / Q000276:immunology / Q000378:metabolism* / Q000648:ultrastructure
chemical_list: D000911:Antibodies, Monoclonal; D000953:Antigens, Protozoan; D008565:Membrane Proteins; D015800:Protozoan Proteins; C090077:apical membrane antigen I, Plasmodium; D002118:Calcium