APH

APH (PF3D7_0414600) is a lipid-sensing protein anchored to the surface of micronemes that provides spatial control for microneme exocytosis during apicomplexan host-cell invasion (PMID:29910186, PMID:31492901). Its pleckstrin-homology domain directly senses phosphatidic acid (PA): structural and functional dissection showed that APH inserts into lipid bilayers and clusters multiple PA head-groups at its binding surface, defining a conserved PA-sensing hub (PMID:29910186). Surface anchoring is achieved through N-terminal lipid modification, with APH confirmed as an N-myristoyltransferase substrate that is myristoylated at its N-terminal glycine in P. falciparum (PMID:34695132). Functionally, inducible deletion in P. yoelii established that APH is required for microneme exocytosis, erythrocyte invasion, and asexual growth: APH loss impaired secretion of the microneme protein MTRAP and reduced post-internalization echinocytosis, with APH colocalizing largely with MTRAP and less with AMA1, indicating a direct role in MTRAP secretion and only an indirect role in AMA1 secretion (PMID:34628068). APH marks micronemes independently of the HOPS/CORVET tethering machinery, as its apical localization persists when VPS16/VPS18 are depleted despite mislocalization of secreted microneme cargo (PMID:40198740). Beyond PA-sensing and its role positioning microneme fusion at the apical membrane, the corpus does not characterize additional binding partners or downstream fusion machinery for APH.

1. 2018 High

   Establishing how APH engages membranes answered whether the protein is a genuine lipid sensor, revealing a PA-recognition mechanism rather than a generic membrane anchor.

   Evidence Crystal structure of the APH PH domain with phospholipid-binding assays (largely in T. gondii, addressing conserved PfAPH hub)

   PMID:29910186

   Open questions at the time

   • Structural work performed primarily on TgAPH, not the P. falciparum protein directly
   • Does not establish the in vivo functional consequence of PA sensing
   • Downstream fusion partners engaged after PA binding not identified
2. 2019 Medium

   Placing APH in the secretion sequence framed its PA-binding activity as the trigger initiating microneme membrane fusion at the apical plasma membrane, upstream of rhoptry discharge.

   Evidence APH localization and function cited as a mechanistic comparator framework within the RASP2 study

   PMID:31492901

   Open questions at the time

   • This paper cites prior APH work rather than generating new APH data
   • Temporal ordering relative to rhoptry discharge inferred, not directly measured for APH
   • PA generation source at the apical membrane not defined here
3. 2021 High

   Inducible deletion answered whether APH is functionally required for invasion, demonstrating it is needed for microneme exocytosis and defining a direct role in MTRAP versus an indirect role in AMA1 secretion.

   Evidence DiCre inducible knockout in P. yoelii with secretion, invasion, echinocytosis, colocalization, and electron microscopy readouts

   PMID:34628068

   Open questions at the time

   • Molecular basis for differential MTRAP vs AMA1 dependence not resolved
   • Direct physical interaction between APH and MTRAP not shown
   • Performed in P. yoelii; not confirmed by equivalent P. falciparum knockout
4. 2021 Medium

   Identifying the N-terminal modification answered how APH is targeted to the microneme surface, confirming N-myristoylation as the lipid anchor and APH as an NMT substrate.

   Evidence Chemical proteomics with YnMyr metabolic labeling, enrichment, and MS/MS of the modified N-terminal peptide, with NMT inhibitor control in P. falciparum

   PMID:34695132

   Open questions at the time

   • Single method for this specific protein
   • Whether myristoylation alone or combined with palmitoylation drives stable membrane anchoring not tested
   • Functional consequence of blocking APH myristoylation not directly measured
5. 2025 Medium

   Testing APH localization under HOPS/CORVET depletion distinguished microneme biogenesis/surface anchoring from cargo delivery, showing APH surface anchoring is independent of the tethering complex even when secreted cargo is mislocalized.

   Evidence Conditional knockdown of VPS16/VPS18 with live imaging of APH-mCherry as a microneme surface marker in P. falciparum

   PMID:40198740

   Open questions at the time

   • APH used as a marker rather than the primary subject of perturbation
   • Machinery that delivers APH itself to the microneme surface not identified
   • Does not resolve how cargo mislocalization arises independently of APH anchoring

• How APH couples PA sensing to the downstream membrane-fusion machinery and to selective MTRAP secretion remains unresolved.
• No direct APH binding partners or fusion effectors identified in the corpus
• Mechanistic basis for direct MTRAP versus indirect AMA1 dependence unknown
• P. falciparum-specific functional knockout phenotype not established