The Toxoplasma homolog of Plasmodium apical membrane antigen-1 (AMA-1) is a microneme protein secreted in response to elevated intracellular calcium levels

A monoclonal antibody (MAb) has been generated against a; novel 63 kDa surf ace/apical antigen of Toxoplasma gondii tachyzoites which is identified her e as TgAMA-1, the Toxoplasma homolog of Plasmodium apical membrane antigen- 1 (AMA-1). Sequence analysis, phase partitioning in Triton X-114, and label ing of TgAMA-1 with iodonaphthalene azide all suggest that TgAMA-1 is a typ e I transmembrane protein. There is a high degree of sequence similarity be tween TgAMA-1 and Plasmodium AMA-I, most notably in the position of conserv ed cysteine residues within the protein's predicted extracellular domain. I n contrast to full length Plasmodium AMA-1, which has previously been local ized to the rhoptries, it is shown here by immunofluorescence and immunoele ctron microscopy that intracellular TgAMA-1 is found in the micronemes. A 5 3 kDa N-terminal proteolytic fragment of TgAMA-1 is constitutively secreted from the parasite at 37 degreesC. As is the case with other microneme prot eins, the proteolytic processing and secretion of TgAMA-1 is dramatically e nhanced in response to treatments which increase intracellular calcium leve ls. (C) 2000 Elsevier Science B.V. All rights reserved.