Tandemly repeated genes encode nucleoside triphosphate hydrolase isoforms secreted into the parasitophorous vacuole of Toxoplasma gondii

The obligate intracellular parasite Toxoplasma gondii produces a nucleoside triphosphate hydrolase (NTPase) (nucleoside-triphosphatase, EC 3.6.1.15) activable by dithiol-containing compounds. We have isolated the genomic DNA for the NTPase from the RH strain of Toxoplasma and determined the nucleotide sequence of three tandemly arranged open reading frames termed NTP1, NTP2, and NTP3. We have also isolated and sequenced cDNAs for NTP1 and NTP3; no cDNA for NTP2 was obtained. The two cDNA clones encode proteins that are more than 97% identical at the amino acid level but significantly differ within two small domains, indicating the presence of NTPase isoforms. Both possess N-terminal signal sequences and two regions with partial homology to certain known ATP binding motifs: the glycine-rich loop common to many ATP binding proteins and the beta-phosphate binding domain found in the hexokinase-actin-hsp70 family. Antiserum against a NTP1-fusion protein immunoprecipitated NTPase activity from extracellular parasites that was increased in activity by treatment with dithiothreitol, confirming the identity of the cloned genes. By immunofluorescence, the NTPase is located in vesicular structures within the parasite, and in infected cells it is secreted into the vacuolar space and becomes partially associated with the parasitophorous vacuolar membrane. Since the vacuolar membrane is freely permeable to small molecules of < 1300 Da, host cell ATP may serve as a substrate for the NTPase and supply the energy for parasite-directed processes in the vacuolar space.