MLC7

MLC7 is a calmodulin-like myosin light chain of Toxoplasma gondii that operates as a component of the apical class XIV myosin H (MyoH) motor complex at the conoid, where this motor drives parasite motility, host-cell invasion, and egress (PMID:26760042). MLC7 associates with MyoH, whose tail carries three alpha-tubulin suppressor domains that anchor the motor complex to the conoid (PMID:26760042), and proximity-biotinylation places MLC7 together with MyoH and the conoid calmodulin-like proteins CaM1–CaM3, whose stability and distribution depend on an intact MyoH–MLC7 complex (PMID:28475612). MLC7 is one of several CaM-domain light chains in the parasite's myosin repertoire (PMID:21143563). Beyond its assignment to the MyoH motor complex and its conoid localization, no direct biochemical or structural characterization of MLC7 itself has been reported in the available corpus.

1. 2011 Low

   Before any functional assignment, it was unknown which calmodulin-like proteins served as myosin light chains in Toxoplasma; a repertoire survey established MLC7 as one of six CaM-domain-containing candidate light chains with a defined subcellular localization.

   Evidence Bioinformatic identification of CaM-like proteins with fluorescence-microscopy localization in T. gondii

   PMID:21143563

   Open questions at the time

   • No direct functional characterization of MLC7 itself
   • Cognate myosin partner not defined at this stage
   • No demonstration of light-chain activity
2. 2016 Medium

   It was unknown which myosin MLC7 partners with and what cellular process that motor serves; conditional disruption of MyoH showed MLC7 is a MyoH-associated light chain of a conoid motor complex required for motility, invasion, and egress.

   Evidence Conditional disruption of MyoH with subcellular localization and phenotypic analysis of motility/invasion/egress in T. gondii

   PMID:26760042

   Open questions at the time

   • MLC7's own contribution not tested by direct MLC7 loss-of-function
   • Direct MLC7–MyoH binding not biochemically reconstituted
   • Role of MLC7 in motor mechanochemistry not defined
3. 2017 Medium

   Whether MLC7 sits within a broader conoid apical complex and how it relates to other calmodulin-like proteins was unresolved; proximity biotinylation placed MLC7 with MyoH and CaM1–CaM3 and revealed that CaM stability/distribution depends on the MyoH–MLC7 complex.

   Evidence BirA proximity biotinylation with CaM1/2/3, CRISPR/Cas9 editing, auxin-induced degron depletion of MyoH, and super-resolution microscopy in T. gondii

   PMID:28475612

   Open questions at the time

   • Interaction inferred from biotinylation rather than reciprocal co-IP or reconstitution
   • MLC7 not directly depleted to test its role in CaM interdependence
   • Stoichiometry and architecture of the MyoH–MLC7–CaM assembly unknown

• The intrinsic function of MLC7 as a light chain — whether it regulates MyoH motor activity, Ca2+ responsiveness, or complex assembly — remains uncharacterized, as all evidence to date derives from perturbation of MyoH or the CaMs rather than of MLC7 itself.
• No direct MLC7 knockout/knockdown phenotype
• No structural model of the MyoH–MLC7 interface
• No test of whether MLC7 confers Ca2+ or calmodulin-dependent regulation