AAP4

AAP4 (Apical Annuli Protein 4) is a structural component of the apical annuli of Toxoplasma gondii, pore-like complexes embedded in the inner membrane complex (IMC) sutures at the apical end of the parasite (PMID:31470470). Within the annuli, AAP4 localizes to the largest (~400 nm diameter) ring-like structure, defining its position as a peripheral scaffolding element of this complex (PMID:31470470). Genetic knockout produces only a modest reduction of in vitro fitness and does not impair acute virulence in mice, indicating AAP4 is dispensable for acute infection (PMID:31470470). Beyond its structural assignment within the apical annuli, no biochemical activity or molecular partners for AAP4 have been characterized in the available corpus.

1. 2019 High

   Before this work the molecular composition of the apical annuli was undefined; identifying AAP4 as a component localizing to the largest ring established it as a defined structural constituent of this IMC-embedded complex.

   Evidence Reciprocal proximity biotinylation (BioID) proteomics with orthogonal super-resolution microscopy in Toxoplasma gondii

   PMID:31470470

   Open questions at the time

   • No biochemical activity or direct binding partners assigned to AAP4
   • The function of the apical annuli themselves not resolved
   • Mechanism by which AAP4 is recruited to the IMC sutures unknown
2. 2019 Medium

   To test whether AAP4 is required for the parasite, knockout was assessed and showed only a modest in vitro fitness cost with no loss of acute virulence, establishing AAP4 as non-essential for acute infection.

   Evidence Genetic knockout with in vitro fitness assay and in vivo mouse virulence assay in Toxoplasma gondii

   PMID:31470470

   Open questions at the time

   • Whether AAP4 contributes to chronic infection or other life-cycle stages not tested
   • Whether other apical annuli proteins functionally compensate not addressed
   • The cellular process disrupted by loss of AAP4 not identified
3. 2019 Low

   To place AAP4 in an evolutionary context, phylogenetic analysis found it restricted to coccidian apicomplexans that replicate by internal budding, raising a possible functional link to that replication mode.

   Evidence Phylogenetic analysis combined with BioID localization data

   PMID:31470470

   Open questions at the time

   • Inference is computational with no direct functional experiment linking AAP4 to internal budding
   • No mechanistic role in budding demonstrated

• The biochemical function of AAP4, its direct interaction partners within the apical annuli, and the physiological role of the annuli themselves remain unresolved.
• No molecular activity assigned to AAP4
• Function of the apical annuli complex not established
• Contribution to internal budding untested experimentally