IMC32

IMC32 is an essential Toxoplasma gondii inner membrane complex (IMC) protein that lays the structural foundation of the daughter bud scaffold during endodyogeny (PMID:33593973). It localizes to the body portion of the IMC and is recruited to developing daughter buds early in replication; conditional depletion causes complete collapse of the IMC and is lethal (PMID:33593973). IMC32 anchors to the nascent bud membrane through an N-terminal palmitoylation site, whose mutation disrupts membrane association (PMID:33593973), while its C-terminal coiled-coil domains — containing two conserved functionally critical regions — are required both for correct localization and for its role in replication (PMID:33593973). Through these coiled-coil domains IMC32 physically binds IMC43 and BCC0, forming a tripartite essential daughter bud assembly complex (PMID:37782662, PMID:39024411). IMC43 acts upstream, controlling IMC32 localization at specific stages of endodyogeny (PMID:37782662), and BCC0's own recruitment depends on both IMC32 and IMC43, with its essential domain (residues 701–877) directly binding the IMC32 coiled-coil (PMID:39024411). Assembly proceeds by an IMC32–BCC0 subcomplex recruiting to nascent buds via IMC32 palmitoylation, which is subsequently locked into the scaffold by IMC32–IMC43 binding as bud elongation begins; BCC0 and IMC43 do not directly interact (PMID:39024411).

1. 2021 High

   Establishing whether IMC32 is a functionally important IMC component addressed whether it contributes structurally to daughter bud formation, and showed it is essential for the integrity of the IMC scaffold.

   Evidence Conditional degron depletion with fluorescence localization in T. gondii, revealing early daughter bud recruitment and lethal IMC collapse upon loss

   PMID:33593973

   Open questions at the time

   • Molecular partners not yet identified
   • Mechanism by which IMC collapse leads to death not defined
2. 2021 High

   Dissecting which sequence features drive IMC32 membrane targeting versus function distinguished a lipid-anchoring role from a protein-interaction role, defining a bipartite domain architecture.

   Evidence Site-directed mutagenesis of the N-terminal palmitoylation site and deletion mapping of C-terminal coiled-coil regions with functional complementation in T. gondii

   PMID:33593973

   Open questions at the time

   • Identity of binding partners engaged by the coiled-coil domains unknown at this stage
   • Palmitoylating enzyme not identified
3. 2023 High

   Determining what governs IMC32 stage-specific localization and what it binds revealed IMC43 as an upstream regulator and direct partner, defining a two-component essential complex.

   Evidence Auxin-inducible degron depletion of IMC43 showing IMC32 mislocalization, plus TurboID and pairwise yeast two-hybrid domain mapping of a direct IMC43–IMC32 interaction in T. gondii

   PMID:37782662

   Open questions at the time

   • Interaction not reconstituted with purified proteins
   • Whether additional components complete the complex unknown
4. 2024 High

   Identifying a third complex member and mapping the order of assembly addressed how the daughter bud scaffold nucleates, establishing a tripartite complex and a stepwise recruitment logic.

   Evidence Proximity labeling and yeast two-hybrid identifying BCC0 binding the IMC32 coiled-coil (residues 701–877), plus sequential depletion/localization epistasis and a negative BCC0–IMC43 yeast two-hybrid in T. gondii

   PMID:39024411

   Open questions at the time

   • Assembly order inferred from localization epistasis, not fully reconstituted in vitro
   • Downstream scaffold components recruited after this complex not defined

• How the IMC32-anchored complex is connected to downstream IMC alveolar/cytoskeletal architecture and what enzymes control IMC32 palmitoylation dynamics remain unresolved.
• Palmitoyltransferase acting on IMC32 not identified
• Structural basis of coiled-coil-mediated binding not resolved
• Link to mature IMC scaffold assembly uncharacterized