Affinage · Toxoplasma gondii

ISP3

IMC sub-compartment protein ISP3 · UniProt A0A125YUT6

Annotated
2026-09-11
11 papers in source corpus 3 papers cited in narrative 3 extracted findings

Mechanistic narrative

Synthesis pass · prose summary of the discoveries below

ISP3 is a Toxoplasma gondii inner membrane complex (IMC) sub-compartment protein that localizes to the central and basal region of the IMC (PMID:20844581). Its membrane targeting depends on N-terminal residues predicted for coordinated myristoylation and palmitoylation, and this sub-compartment addressing is hierarchically controlled by ISP1: loss of ISP1 relocalizes ISP3 to the apical cap, establishing ISP1 as a gatekeeper of ISP3 targeting (PMID:20844581). Depletion of the palmitoyl acyltransferase TgDHHC2 causes ISP3 (along with ISP1) to lose membrane association, identifying TgDHHC2 as the likely enzyme responsible for palmitoylating ISP3, while TgDHHC14 depletion does not affect ISP3 localization (PMID:32329212). Structurally, ISP3 adopts a pleckstrin homology (PH) fold that lacks detectable phospholipid-binding activity; conserved surface residue mapping indicates the canonical lipid-binding site has been repurposed to mediate protein–protein interactions, consistent with a role in recruiting partner proteins to the IMC (PMID:24675080). The specific partner proteins recruited by ISP3 have not been identified in the available corpus.

Mechanistic history

Synthesis pass · year-by-year structured walk · 2 steps
  1. 2010 High

    Established where ISP3 resides in the parasite and how its sub-compartment identity is set, revealing a hierarchical targeting logic among IMC proteins.

    Evidence Fluorescence microscopy of tagged ISP3 in wild-type and ISP1-knockout parasites plus N-terminal acylation site mutagenesis in T. gondii

    PMID:20844581

    Open questions at the time
    • Direct enzymatic demonstration of myristoylation/palmitoylation not shown
    • Mechanism by which ISP1 excludes ISP3 from the apical cap not defined
    • Functional consequence of correct central/basal targeting not established
  2. 2014 High

    Resolved the ISP3 fold and its molecular activity, showing a PH fold that has lost lipid binding and is instead configured for protein–protein interaction.

    Evidence X-ray crystallography at 2.32 Å, phospholipid-binding assays, and conserved surface/crystal-packing analysis of TgISP3

    PMID:24675080

    Open questions at the time
    • No specific interacting partner identified or co-crystallized
    • Repurposed interaction surface inferred from surface mapping, not from a bound complex

Mechanism profile

Synthesis pass · controlled-vocabulary classification
Partners
ISP1 (HIERARCHICAL GATEKEEPER OF ISP3 SUB-COMPARTMENT TARGETING) [#0]TGDHHC2 (PALMITOYL ACYLTRANSFERASE REQUIRED FOR ISP3 MEMBRANE ANCHORING) [#2]

Evidence

Reading pass · 3 per-paper findings extracted from the source corpus
Year Finding Method Journal Conf PMIDs
2010 ISP3 localizes to the central plus basal region of the Toxoplasma gondii IMC, and this targeting is dependent upon N-terminal residues predicted for coordinated myristoylation and palmitoylation. Disruption of ISP1 causes dramatic relocalization of ISP3 (and ISP2) to the apical cap, demonstrating that ISP1 acts as a gatekeeper that hierarchically controls ISP3 sub-compartment targeting. Fluorescence microscopy of tagged ISP3 in wild-type and ISP1-knockout parasites; N-terminal myristoylation/palmitoylation site mutagenesis PLoS pathogens High 20844581
2014 Crystal structures of TgISP3 (and TgISP1) at 2.32 Å (and 2.10 Å) resolution reveal that both proteins adopt a pleckstrin homology (PH) fold. Biochemical assays showed no detectable phospholipid-binding activity despite the PH fold; instead, mapping of conserved surface residues and crystal packing analysis suggests the phospholipid-binding site has been repurposed to coordinate protein–protein interactions, likely recruiting partner proteins to the IMC. X-ray crystallography; phospholipid-binding biochemical assays; conserved surface residue mapping; crystal packing analysis The Journal of biological chemistry High 24675080
2020 Depletion of the palmitoyl acyltransferase TgDHHC2 causes ISP3 (along with ISP1) to lose membrane association, identifying TgDHHC2 as the likely palmitoylating enzyme (writer) responsible for ISP3 membrane anchoring. TgDHHC14 depletion does not affect ISP1/3 localization, indicating sequential and distinct PAT requirements for different IMC sub-compartments. Conditional knockdown of TgDHHC2 and TgDHHC14 followed by fluorescence microscopy to assess ISP3 membrane association Cellular microbiology Medium 32329212

Source papers

Stage 0 corpus · 11 papers · ranked by NIH iCite citations
Year Title Journal Citations PMID
2010 A novel family of Toxoplasma IMC proteins displays a hierarchical organization and functions in coordinating parasite division. PLoS pathogens 192 20844581
2016 Virus-Like Nanoparticle Vaccine Confers Protection against Toxoplasma gondii. PloS one 33 27548677
2011 Subcellular antigen location influences T-cell activation during acute infection with Toxoplasma gondii. PloS one 33 21829561
2014 Identification and characterization of Toxoplasma SIP, a conserved apicomplexan cytoskeleton protein involved in maintaining the shape, motility and virulence of the parasite. Cellular microbiology 28 25088010
2016 Cross-protection induced by Toxoplasma gondii virus-like particle vaccine upon intraperitoneal route challenge. Acta tropica 15 27586039
2021 A Novel Toxoplasma Inner Membrane Complex Suture-Associated Protein Regulates Suture Protein Targeting and Colocalizes with Membrane Trafficking Machinery. mBio 12 34634933
2014 The inner membrane complex sub-compartment proteins critical for replication of the apicomplexan parasite Toxoplasma gondii adopt a pleckstrin homology fold. The Journal of biological chemistry 12 24675080
2025 Co-dependent formation of the Toxoplasma gondii subpellicular microtubules and inner membrane skeleton. mBio 11 40801525
2009 Toxoplasma gondii: further studies on the subpellicular network. Memorias do Instituto Oswaldo Cruz 11 19820829
2024 Apical annuli are specialised sites of post-invasion secretion of dense granules in Toxoplasma. eLife 10 38270431
2020 Two palmitoyl acyltransferases involved sequentially in the biogenesis of the inner membrane complex of Toxoplasma gondii. Cellular microbiology 7 32329212