APH

APH (acylated pleckstrin homology domain-containing protein) is a lipid-sensing effector required for regulated microneme exocytosis in apicomplexan parasites, a process essential for motility, invasion, and egress (PMID:26962945). Resident on the microneme surface, APH senses phosphatidic acid (PA) generated by DAG-kinase-1 from diacylglycerol, placing APH downstream of this lipid second-messenger conversion so that the DAG/PA balance controls the timing of secretion (PMID:26962945). Structural analysis defined the molecular basis of PA recognition: the PH domain inserts into PA-enriched bilayers and clusters multiple phosphate head-groups at its membrane-binding surface (PMID:29910186), while acylation contributes to APH membrane association and its recruitment to PA-enriched membranes (PMID:29910186).

1. 2016 High

   Established that APH is a microneme-surface lipid sensor whose PA-sensing function is necessary for regulated secretion, defining a molecular link between membrane lipid composition and exocytosis.

   Evidence Genetic depletion with localization and lipid-sensing assays in Toxoplasma gondii

   PMID:26962945

   Open questions at the time

   • Does not resolve how PA sensing is mechanically coupled to the exocytic machinery
   • Does not identify downstream effectors of APH
2. 2016 Medium

   Placed APH downstream of DAG-kinase-1 in the DAG/PA signaling balance, showing that PA generated by DAG-kinase-1 is the ligand APH senses to trigger exocytosis.

   Evidence Genetic epistasis combining DAG-kinase-1 depletion with APH functional analysis in Toxoplasma gondii

   PMID:26962945

   Open questions at the time

   • Pathway position inferred from combinatorial depletion rather than direct biochemical reconstitution
   • Upstream PLC activation and DAG dynamics not directly linked to APH in this evidence
3. 2018 High

   Resolved the atomic basis of PA recognition, showing the PH domain inserts into PA-enriched membranes and clusters phosphate head-groups, defining the structural determinants of lipid sensing.

   Evidence Atomic-resolution structure with phospholipid-membrane interaction assays and mutagenesis of PA-binding residues in Toxoplasma gondii and Plasmodium falciparum

   PMID:29910186

   Open questions at the time

   • Does not establish how PA binding is transmitted to trigger vesicle fusion
   • In vivo consequences of specific PA-binding residue mutations on secretion not fully mapped
4. 2018 Medium

   Showed that acylation contributes to APH membrane association, adding a second membrane-targeting determinant alongside PA head-group binding.

   Evidence Biochemical fractionation, localization, and lipid-binding assays in Toxoplasma gondii

   PMID:29910186

   Open questions at the time

   • Acylation sites not fully mutagenized to establish causality
   • Relative contribution of acylation versus PA binding to membrane targeting not quantified

• How PA sensing by APH is mechanically coupled to the microneme fusion machinery to drive exocytosis remains unresolved.
• No identified downstream effector connecting APH to vesicle fusion
• No reconstitution of PA-triggered secretion
• Regulation of APH acylation state during signaling unknown