HOOK

In Toxoplasma gondii, HOOK is a component of a parasite-adapted FHF-like adaptor complex that couples dynein-based transport to the Ca2+-triggered secretory events driving host cell invasion (PMID:37933960, PMID:37093045). HOOK assembles with an FTS homolog and two parasite-specific partners (the coccidian-specific interactor HIP/TGGT1_306920 and TGGT1_316650) into a partially conserved HOOK-FTS-FHIP complex that localizes to the parasite apical tip in an apically enriched vesicular pattern (PMID:37933960, PMID:37093045). Loss of HOOK or FTS impairs the rapid apical trafficking of micronemes and blocks microneme protein secretion and host cell invasion, and depletion of FTS or HIP phenocopies and exacerbates these defects across egress, motility, and attachment, establishing the intact complex as required for apical microneme positioning and exocytosis (PMID:37933960, PMID:37093045). HOOK is itself phosphorylated in a CDPK1-dependent manner, placing it downstream of the Ca2+-dependent kinase cascade that governs microneme secretion (PMID:37933960, PMID:36712004).

1. 2023 High

   Whether Toxoplasma possesses a HOOK-based transport adaptor and which partners it uses was unknown; identifying an FHF-like complex established the molecular architecture linking HOOK to the parasite apical secretory machinery.

   Evidence Co-immunoprecipitation/complex characterization and fluorescence colocalization imaging in T. gondii

   PMID:37933960 PMID:37093045

   Open questions at the time

   • Role of dynein engagement in complex function not directly demonstrated in the timeline
   • Function of the two parasite-specific partners at the biochemical level not resolved
   • Stoichiometry and structural basis of complex assembly not defined
2. 2023 High

   How HOOK connects to the Ca2+ signaling that triggers secretion was unknown; showing it is a CDPK1-dependent phosphosubstrate placed HOOK downstream of the master Ca2+ kinase controlling microneme trafficking.

   Evidence Sub-minute phosphoproteomics and bio-orthogonal kinase substrate labeling in T. gondii

   PMID:37933960 PMID:36712004

   Open questions at the time

   • Functional consequence of specific phosphosites on HOOK activity not defined
   • Whether phosphorylation controls complex assembly or dynein coupling not established
3. 2023 High

   Whether HOOK and FTS are functionally required for invasion was untested; loss-of-function established them as essential for rapid apical microneme trafficking, secretion, and invasion.

   Evidence Genetic knockout/conditional depletion of HOOK and FTS with secretion and invasion assays in T. gondii

   PMID:37933960 PMID:37093045

   Open questions at the time

   • Molecular step at which trafficking fails not pinpointed
   • Direct link between HOOK phosphorylation and the trafficking phenotype not established
4. 2023 Medium

   Whether the full complex or HOOK alone drives the phenotype was unresolved; depletion of FTS or the coccidian-specific HIP phenocopied and worsened HOOK loss, establishing the intact HOOK-FTS-HIP complex as required for optimal microneme positioning and exocytosis.

   Evidence Conditional depletion with motility, egress, attachment, and invasion assays in T. gondii

   PMID:37093045

   Open questions at the time

   • Basis for the more severe HIP/FTS phenotype relative to HOOK not explained
   • Contribution of each subunit to dynein recruitment not dissected

• How CDPK1 phosphorylation of HOOK is mechanistically coupled to dynein-driven apical microneme transport and Ca2+-regulated exocytosis remains unresolved.
• No structural model of the parasite FHF complex
• Direct demonstration of dynein-motor coupling absent
• Functional role of individual HOOK phosphosites undefined