CAM3

CaM3 is an essential calmodulin-like protein of Toxoplasma gondii that operates at the conoid to drive the apical motility machinery underlying parasite invasion and egress (PMID:28475612). It localizes to the conoid at the parasite's apical tip, overlapping with the conoid-associated motor myosin H (MyoH) (PMID:28475612). Functionally, CaM3 acts downstream of microneme and rhoptry secretion to support gliding motility, host-cell invasion, and egress, since conditional depletion blocks these processes without impairing secretion (PMID:28475612). Proximity biotinylation places CaM3 in the apical MyoH complex, labeling MyoH and its light chain MLC7, and disruption of MyoH triggers CaM3 degradation, indicating that CaM3 stability and function are coupled to the MyoH motor (PMID:28475612). Beyond this conoid-localized, MyoH-associated role, no further biochemical or structural mechanism has been characterized in the available corpus.

1. 2017 High

   Establishing whether CaM3 is dispensable answered whether this calmodulin-like protein has a non-redundant role in the parasite lytic cycle.

   Evidence CRISPR/Cas9 deletion (refractory) and auxin-induced degron depletion with growth assays in T. gondii

   PMID:28475612

   Open questions at the time

   • Does not identify which downstream molecular activity of CaM3 is essential
   • No structural or biochemical characterization of the protein
2. 2017 Medium

   Defining CaM3's subcellular position answered where in the parasite it acts, pinpointing it to the apical motility apparatus.

   Evidence Super-resolution microscopy showing conoid localization overlapping MyoH in T. gondii

   PMID:28475612

   Open questions at the time

   • Co-localization only, no functional rescue at the conoid
   • Does not establish direct physical association from imaging alone
3. 2017 High

   Dissecting the step at which CaM3 acts answered whether it functions in secretion or in the downstream motor-driven events of the lytic cycle.

   Evidence AID conditional depletion with motility, invasion, egress, and secretion assays in T. gondii

   PMID:28475612

   Open questions at the time

   • Does not resolve the molecular target CaM3 regulates during motility
   • Does not distinguish direct motor regulation from indirect effects
4. 2017 Medium

   Identifying CaM3's molecular neighbors answered how it is integrated into the motility machinery, linking it to the MyoH motor complex.

   Evidence BirA proximity biotinylation identifying MyoH and MLC7, plus MyoH disruption assessing CaM3 stability in T. gondii

   PMID:28475612

   Open questions at the time

   • Proximity labeling, not reciprocal Co-IP, so direct binding is unproven
   • Mechanism by which MyoH loss destabilizes CaM3 is undefined
   • No demonstration that CaM3 modulates MyoH motor activity

• How CaM3 mechanistically regulates the MyoH motor and what Ca2+-dependent signals it transduces at the conoid remain unresolved.
• No direct binding assay or structure of a CaM3-MyoH/MLC7 complex
• No test of whether CaM3 confers Ca2+ sensitivity to motor function
• No reciprocal Co-IP validating the interaction