Isolation and Characterization of TgVP1, a Type I Vacuolar H⁺-translocating Pyrophosphatase fromToxoplasma gondii

Abstract

Here we report the isolation and characterization of a type I vacuolar-type H⁺-pyrophosphatase (V-PPase), TgVP1, from an apicomplexan,Toxoplasma gondii, a parasitic protist that is particularly amenable to molecular and genetic manipulation. The 816-amino acid TgVP1 polypeptide is 50% sequence-identical (65% similar) to the prototypical type I V-PPase from Arabidopsis thaliana, AVP1, and contains all the sequence motifs characteristic of this pump category. Unlike AVP1 and other known type I enzymes, however, TgVP1 contains a 74-residue N-terminal extension encompassing a 42-residue N-terminal signal peptide sequence, sufficient for targeting proteins to the secretory pathway of T. gondii. Providing that the coding sequence for the entire N-terminal extension is omitted from the plasmid, transformation of Saccharomyces cerevisiae with plasmid-borne TgVP1 yields a stable and functional translation product that is competent in aminomethylenediphosphonate (AMDP)-inhibitable K⁺-activated pyrophosphate (PP_i) hydrolysis and PP_i-energized H⁺ translocation. Immunofluorescence microscopy of both free and intracellular T. gondii tachyzoites using purified universal V-PPase polyclonal antibodies reveals a punctate apical distribution for the enzyme. Equivalent studies of the tachyzoites during host cell invasion, by contrast, disclose a transverse radial distribution in which the V-PPase is associated with a collar-like structure that migrates along the length of the parasite in synchrony with and in close apposition to the penetration furrow. Although treatment of T. gondii with AMDP concentrations as high as 100 μm had no discernible effect on the efficiency of host cell invasion and integration, concentrations commensurate with the I₅₀ for the inhibition of TgVP1 activity in vitro (0.9 μm) do inhibit cell division and elicit nuclear enlargement concomitant with the inflation and eventual disintegration of acidocalcisome-like vesicular structures. A dynamic association of TgVP1 with the host cell invasion apparatus is invoked, one in which the effects of inhibitory V-PPase substrate analogs are exerted after rather than during host cell invasion.