AAP1

AAP1 (apical annuli protein 1) is a component of the apical annuli of Toxoplasma gondii, structures embedded within the sutures of the inner membrane complex (IMC) where alveolar vesicles connect (PMID:31470470). Reciprocal proximity biotinylation and super-resolution microscopy place AAP1 at these annuli, which are organized as concentric rings of 200–400 nm diameter (PMID:31470470). Beyond this localization, the molecular function of AAP1 has not been characterized in the available corpus; its narrow conservation among coccidian apicomplexans that replicate by internal budding points to a role tied to that replication mode, and its co-detection with Centrin2 and shared domain signatures with centrosomal proteins hint at a structural link to the centrosome, but neither has been functionally tested (PMID:31470470).

1. 2019 Medium

   Establishing where AAP1 acts: it was unknown what protein composition defined the apical annuli, and localizing AAP1 to IMC sutures defined it as a bona fide annuli component.

   Evidence Reciprocal BioID proximity biotinylation and super-resolution microscopy in Toxoplasma gondii

   PMID:31470470

   Open questions at the time

   • No functional perturbation (knockdown/knockout) of AAP1 to test its role at the annuli
   • Molecular activity of AAP1 not defined
   • Mechanism of recruitment to IMC sutures unknown
2. 2019 Medium

   Resolving the architecture of the annuli: the physical organization was unknown, and imaging showed AAP1-containing annuli form concentric rings of defined dimensions within IMC sutures.

   Evidence Super-resolution microscopy with BioID proteome mapping in Toxoplasma gondii

   PMID:31470470

   Open questions at the time

   • Does not establish AAP1's specific position or stoichiometry within the ring structure
   • No structural model of AAP1 itself
3. 2019 Low

   Probing a possible centrosomal connection: it was unknown whether the annuli relate to the centrosome, and AAP1's co-localization with Centrin2 plus centrosomal domain signatures raised a candidate structural/functional link.

   Evidence BioID proteome mapping, domain analysis, and co-detection with Centrin2 in Toxoplasma gondii

   PMID:31470470

   Open questions at the time

   • Centrosomal relationship inferred from co-detection and domain analysis, not functionally validated for AAP1
   • No direct interaction assay between AAP1 and Centrin2 beyond proximity/co-detection
4. 2019 Low

   Assessing conservation to infer function: it was unclear which parasites possess AAP1, and phylogenetic analysis showed restriction to coccidians that replicate by internal budding, suggesting a role in that process.

   Evidence Phylogenetic/comparative sequence analysis

   PMID:31470470

   Open questions at the time

   • Purely computational inference with no functional experiment linking AAP1 to internal budding
   • Role in replication not directly demonstrated

• The molecular function of AAP1 and its mechanistic role in internal budding or in any centrosome-annuli relationship remain undefined.
• No loss-of-function phenotype reported
• No biochemical activity or defined interaction network beyond proximity data
• No test of whether AAP1 is required for internal budding replication