CAM1

CAM1 (CaM1) is a calmodulin-like protein of Toxoplasma gondii that localizes to the conoid at the parasite's apical tip and functions in the motility-dependent steps of the lytic cycle (PMID:28475612). CaM1 and its paralog CaM2 are individually dispensable but their combined loss is synthetic lethal, placing the two in an essential redundant pathway (PMID:28475612). Depletion of CaM1 impairs parasite motility, host-cell invasion, and egress, and these defects occur downstream of microneme and rhoptry secretion (PMID:28475612). At the conoid, CaM1 spatially overlaps and physically associates with the conoid-resident motor myosin H (MyoH) and its light chain MLC7 (PMID:28475612), and MyoH is in turn required for correct conoid localization of CaM1, since its disruption redistributes CaM1 away from the apical tip (PMID:28475612). Beyond this conoid/MyoH-associated role, no further mechanistic detail (biochemical calcium-binding activity, direct substrate or reconstituted interaction) has been characterized in the available corpus.

1. 2017 High

   Establishing whether CaM1 is essential resolved why single-gene knockouts had been uninformative: CaM1 and CaM2 act redundantly, so only their combined loss is lethal, defining an essential shared pathway.

   Evidence CRISPR/Cas9 single and double knockouts in T. gondii

   PMID:28475612

   Open questions at the time

   • Does not define which molecular function CaM1 and CaM2 share
   • Degree of functional interchangeability between the two paralogs not quantified
2. 2017 High

   Super-resolution imaging placed CaM1 at the conoid overlapping MyoH, pinpointing the subcellular site where CaM1 acts within the apical motility machinery.

   Evidence Super-resolution microscopy of CaM1 and MyoH in T. gondii

   PMID:28475612

   Open questions at the time

   • Spatial overlap does not prove direct molecular contact
   • Dynamics of CaM1 recruitment during the lytic cycle not resolved
3. 2017 Medium

   Phenotypic depletion assays assigned CaM1 to motility, invasion, and egress acting downstream of secretion, distinguishing its role from the upstream microneme/rhoptry discharge machinery.

   Evidence Auxin-induced degron depletion with motility, invasion, and egress assays in T. gondii

   PMID:28475612

   Open questions at the time

   • Individual CaM1 dispensability means the specific contribution is inferred from the combined loss-of-function context
   • Molecular step within motility that CaM1 controls not defined
4. 2017 Medium

   Proximity biotinylation identified MyoH and its light chain MLC7 as CaM1 neighbors, offering a candidate molecular partnership that could explain how CaM1 supports the apical motor.

   Evidence BioID proximity biotinylation (BirA-CaM1 fusion) in T. gondii

   PMID:28475612

   Open questions at the time

   • Proximity labeling is indirect and was not confirmed by reciprocal Co-IP or reconstitution
   • Whether CaM1 binds MyoH directly as a light chain remains untested
5. 2017 Medium

   Genetic disruption of MyoH showed it is required for CaM1/CaM2 conoid localization, establishing a directional dependency in which the motor scaffolds the calmodulin-like proteins at the apical tip.

   Evidence MyoH genetic disruption followed by immunofluorescence localization of CaM1/CaM2 in T. gondii

   PMID:28475612

   Open questions at the time

   • Whether the localization dependence reflects direct binding or an indirect structural requirement is unknown
   • Reciprocal test (effect of CaM1/CaM2 loss on MyoH localization or activity) not reported

• The biochemical activity of CaM1 (calcium binding, target selectivity, whether it functions as a MyoH light chain) and the basis of its redundancy with CaM2 remain unresolved.
• No direct biochemical demonstration of calcium binding or effector engagement
• No reconstitution or reciprocal Co-IP confirming a CaM1-MyoH complex
• Molecular basis for CaM1/CaM2 functional redundancy not defined