ERK7

Toxoplasma gondii ERK7 is an autoactivating MAP kinase that localizes to the apical cap of the inner membrane complex, just basal to the conoid, where it governs biogenesis and stability of the parasite's apical complex (PMID:32073987, PMID:34607461). ERK7 is tethered at this site through an essential trimeric scaffold: it binds AC9 directly via a conserved C-terminal motif, and AC9, AC10, and ERK7 assemble through multiple independent pairwise interactions into the AC9:AC10:ERK7 complex that concentrates the kinase in the apical cap cytoskeleton (PMID:32409604, PMID:35130732). AC9 doubles as an inhibitory scaffold, engaging the ERK7 active site to displace nucleotide and hold the kinase in an 'off' state at its site of action (PMID:32409604). Functionally, ERK7 activity protects the apical complex from premature degradation during replication by antagonizing the E3 ubiquitin ligase CSAR1: normally CSAR1 clears maternal cytoskeleton from the residual body, but upon ERK7 loss it mislocalizes to the apical complex and drives aberrant proteolysis of conoid, apical polar ring, and subpellicular microtubule components, with CSAR1 disruption fully suppressing the ERK7 knockdown phenotype (PMID:37027006, PMID:34607461). Loss of ERK7 therefore abolishes conoid assembly, microneme secretion, gliding motility, host cell invasion, and downstream intracellular proliferation, and severely attenuates virulence in mice (PMID:32073987, PMID:27150970, PMID:32914221). ERK7 abundance and localization are independent of SPY-mediated O-fucosylation (PMID:40888172).

1. 2016 Medium

   Establishing whether ERK7 matters for the parasite lifecycle, a clean knockout showed the kinase is required for tachyzoite proliferation and virulence, framing ERK7 as a functionally important target.

   Evidence Double homologous recombination knockout with in vitro proliferation and mouse virulence assays in strain GT1

   PMID:27150970

   Open questions at the time

   • Mechanism of the proliferation defect not resolved
   • No subcellular localization or molecular partners defined
2. 2020 High

   Localizing ERK7 to the apical cap and demonstrating conoid loss on depletion pinpointed the kinase as essential for apical complex biogenesis, and separately identified host cell invasion as the specific lifecycle step it enables.

   Evidence AID conditional knockdown with conoid/motility/invasion readouts and fluorescence localization; complementation-controlled invasion versus attachment timing assays in GT1

   PMID:32073987 PMID:32914221

   Open questions at the time

   • Kinase substrates in apical complex assembly not identified
   • How ERK7 activity is coupled to conoid maturation unresolved
3. 2020 High

   Asking how ERK7 is delivered and controlled at the apical cap, structural and biochemical work showed AC9 is a direct binding partner that both tethers the kinase and inhibits it by displacing active-site nucleotide, defining an unusual scaffold-as-inhibitor mechanism.

   Evidence TurboID interactome, ERK7–AC9 crystal structure, and C-terminal motif deletion/mutagenesis

   PMID:32409604

   Open questions at the time

   • How and when ERK7 is released into an active state in vivo not defined
   • Physiological substrates of the active kinase unknown
4. 2021 High

   Defining the full structural consequence of ERK7 loss, high-resolution imaging and proteomics showed depletion collapses the apical polar ring and subpellicular microtubules and impairs secretion, phenocopying AC9/AC10 loss and cementing ERK7 as a keystone of apical cytoskeleton integrity.

   Evidence AID knockdown with U-ExM, electron microscopy, and comparative proteomics

   PMID:34607461

   Open questions at the time

   • Direct effectors linking ERK7 activity to microtubule and APR stability not identified
5. 2022 High

   Resolving the architecture that anchors the kinase, interaction mapping showed AC9, AC10, and ERK7 form an essential trimeric complex organized by multivalent pairwise contacts, with AC10 a foundational scaffold component.

   Evidence Yeast two-hybrid with deletion mapping, proximity biotinylation, and conditional knockdown of individual components

   PMID:35130732

   Open questions at the time

   • Stoichiometry and higher-order assembly of the complex not defined
   • How the scaffold gates kinase activity temporally unresolved
6. 2023 High

   Explaining why apical structures are lost, interactome and epistasis experiments revealed ERK7 protects the apical complex by preventing mislocalization of the E3 ligase CSAR1, whose disruption fully suppresses the ERK7 depletion phenotype.

   Evidence Proximity biotinylation/mass spectrometry, CSAR1 genetic disruption with suppressor epistasis, and localization microscopy

   PMID:37027006

   Open questions at the time

   • Whether CSAR1 is a direct ERK7 phosphorylation substrate not established
   • Molecular basis of CSAR1 spatial restriction to the residual body unknown
7. 2025 Medium

   Testing a candidate post-translational regulator, deletion of the SPY O-fucosyltransferase left ERK7 abundance and localization unchanged, excluding O-fucosylation as a determinant of ERK7 levels or targeting.

   Evidence Proteomics and immunofluorescence in SPYΔ versus wild-type parasites

   PMID:40888172

   Open questions at the time

   • Other post-translational modifications regulating ERK7 not tested

• The direct in vivo substrates of ERK7 kinase activity, and the trigger that relieves AC9-mediated inhibition to activate the kinase, remain unidentified.
• No phosphorylation substrates mapped
• Activation trigger and timing of ERK7 catalysis in situ unknown