TGME49_228160

Acid phosphatase activity in Toxoplasma gondii is a secreted enzyme activity associated with the parasite's regulated secretory organelles and its manipulation of the host cell. A 36–38 kDa antigen recognized by monoclonal antibody S16 carries acid phosphatase activity and localizes to dense granules and rhoptries (PMID:8542988), and this antigen is an excretory (secreted) protein of the tachyzoite (PMID:11511000). The dense granule protein GRA44, which contains a putative acid phosphatase domain, is processed by parasite secretory pathway proteases into two fragments that are secreted into the parasitophorous vacuole, where it associates with the MYR1-containing translocon and is required for translocation of effectors that upregulate host cell c-Myc, phenocopying disruption of the MYR complex (PMID:32075881). Distinct from the parasite's own enzyme, host lysosomal acid phosphatase is a cytochemical readout of the host-defense interface: living intracellular parasites actively prevent lysosomal fusion and exclude host acid phosphatase from their vacuole (PMID:4343243), and forcing lysosomal fusion (e.g. by hydroxyurea treatment) delivers acid phosphatase to the vacuole and destroys the parasite (PMID:16936937). How the parasite acid phosphatase enzymatic activity mechanistically contributes to effector translocation, and its full substrate repertoire, have not been characterized in the available corpus.

1. 1972 High

   Established that intracellular survival of Toxoplasma depends on excluding host lysosomal contents, using host acid phosphatase as the lysosomal marker to show living parasites block lysosome fusion.

   Evidence EM acid phosphatase cytochemistry with lysosomal tracers (thorotrast, ferritin) in macrophages, fibroblasts, and HeLa cells

   PMID:4343243

   Open questions at the time

   • Does not identify the parasite factor(s) that block lysosomal fusion
   • Concerns host, not parasite, acid phosphatase
2. 1977 Low

   First attempt to localize the parasite's own acid phosphatase, placing activity in anterior and peripheral granules of the parasite body.

   Evidence Three cytochemical methods (lead nitrate, Gomori, Naphthol AS-BI) on endodozoites

   PMID:929689

   Open questions at the time

   • Inconsistent activity patterns across detection methods
   • No molecular identity or functional validation
3. 1978 Low

   Reported no detectable parasite acid phosphatase in RH-strain organelles under the conditions tested, highlighting method/condition dependence of cytochemical detection.

   Evidence Transmission EM cytochemistry for acid phosphatase

   PMID:215069

   Open questions at the time

   • Negative result restricted to specific experimental conditions
   • Single method, no molecular confirmation
4. 1995 Medium

   Gave the parasite acid phosphatase a molecular and subcellular identity by assigning activity to a 36–38 kDa S16 antigen in dense granules and rhoptries.

   Evidence EM acid phosphatase cytochemistry, Western blot, immunoprecipitation, IEF, and immunogold labeling

   PMID:8542988

   Open questions at the time

   • Substrate and enzymatic function not defined
   • No gene identity linked to the antigen at this stage
5. 2001 Low

   Confirmed the S16/acid phosphatase antigen is secreted by mapping it as an excretory protein on the tachyzoite proteome.

   Evidence 2D gel electrophoresis of tachyzoite lysate plus Western blot

   PMID:11511000

   Open questions at the time

   • Proteome mapping only, no functional data
   • Does not link secretion to a downstream role
6. 2006 Low

   Showed that overriding the fusion block (via hydroxyurea) delivers host acid phosphatase to the vacuole and kills the parasite, reinforcing lysosomal exclusion as a survival mechanism.

   Evidence Ultrastructural acid phosphatase cytochemistry with acridine orange/LysoTracker fluorescence in Vero cells

   PMID:16936937

   Open questions at the time

   • Mechanistic inference is indirect and condition-specific
   • Does not define how hydroxyurea triggers fusion
7. 2020 Medium

   Connected an acid-phosphatase-domain dense granule protein (GRA44) to effector export, showing it is processed, secreted into the vacuole, binds the MYR1 translocon, and is required for host c-Myc upregulation.

   Evidence Reciprocal co-immunoprecipitation, conditional knockdown, Western blot, and c-Myc host-cell phenotype assay

   PMID:32075881

   Open questions at the time

   • Whether the acid phosphatase catalytic activity itself is required is unresolved
   • No substrate identified for the GRA44 phosphatase domain
   • Relationship between GRA44 and the historic S16 antigen not established in the corpus

• The enzymatic substrate(s) of the Toxoplasma acid phosphatase and how its catalytic activity mechanistically contributes to effector translocation and host manipulation remain undefined.
• No substrate identified
• Catalytic requirement for GRA44's role untested
• No structural model of the acid phosphatase domain