NTPase isoforms and their molecular properties

The type I strains of T. gondii, which are acutely virulent in mice (Sibley and Boothroyd, 1992), contain two isoforms of NTPase (Bermudes et al.,

FIGURE 8.7 Behavior of NTPase in the tachyzoite-infected cell. NTPase is secreted from the dense granule of tachyzoite into the parasitophorous vacuolar space and associates to the intravacuolar network. Then, NTPase activity may be regulated by oxido-reduction change in its molecule affected by dithiol compound or unknown dithiol-disulfide oxidoreductase within the parasitophorous vacuole. Abbreviations used are as follows: DG, dense granule; PVM, parasitophorous vacuole membrane; IVN, intravacuolar network; N, nucleus.

FIGURE 8.7 Behavior of NTPase in the tachyzoite-infected cell. NTPase is secreted from the dense granule of tachyzoite into the parasitophorous vacuolar space and associates to the intravacuolar network. Then, NTPase activity may be regulated by oxido-reduction change in its molecule affected by dithiol compound or unknown dithiol-disulfide oxidoreductase within the parasitophorous vacuole. Abbreviations used are as follows: DG, dense granule; PVM, parasitophorous vacuole membrane; IVN, intravacuolar network; N, nucleus.

1994; Asai et al., 1995). One of the isoforms (NTPase-I in Asai et al., 1995; NTP3 in Bermudes et al., 1994) preferentially hydrolyzes triphosphate nucleosides, while another isoform (NTPase-II in Asai et al., 1995; NTP1 in Bermudes et al., 1994) hydrolyzes tri- and diphosphate nucleosides at approximately equal rates. The NTPase-I isoform appears to be present only in the type I virulent strains, while NTPase-II is universally present in all T. gondii strains (Asai et al., 1995). Although the presence of NTPase-I seems to be one of the primary factors of virulence in mice, no direct evidence for a role in virulence has been proven. Neospora caninum, which is avirulent in mice, has only NTPase-I type enzyme, and no NTPase-II type enzyme has been detected (Asai et al., 1998).

The complete cDNAs for NTPase-I and NTPase-II encode predicted open reading frames of identical size that differ in 16 of 628 amino acids between the two isoforms (Asai et al., 1995). The molecular mass of native NTPases are approximately 260 kDa, composed of four identical subunits with predicted molecular mass of 67 kDa (Asai et al., 1995). Both isoforms of the NTPase contain an N-terminal hydrophobic signal peptide (25 amino acids) that is absent in native NTPase purified from the tachy-zoites (Asai et al., 1995). It is supposed that this signal peptide serves as a signal for transport of NTPase to the dense granule. Other dense-granule proteins have similar N-terminal hydrophobic signal peptides (Cesbron-Delauw et al., 1996).

As mentioned above, the primary difference between these NTPase isoforms lies in their ability to hydrolyze nucleoside triphosphate versus diphosphate substrates. While NTPase-II hydrolyzes ATP to ADP and ADP to AMP at almost the same rate, both native and recombinant NTPase-Is hydrolyze ADP to AMP at a much slower rate - less than 1 percent of the rate for ATP (Asai et al., 1995, 1998). This suggests that T. gondii NTPase should be classified as NTP diphosphohydrolase (apyrase, EC 3.6.1.5), a new gene family of E-type ATPases. T. gondii NTPase has homology to apyrase, and an antibody against T. gondii NTPase recognizes apyrases of potato and Schistosoma mansoni (Vasconcelos et al., 1996), and Trypanosoma cruzi (Fietto et al., 2004). The abundant DTT-activated NTPase in N. caninum has no apyrase activity, and its substrate specificity is restricted to nucleoside triphosphate (Asai et al., 1998).

0 0

Post a comment