ISP1

ISP1 (PF3D7_1011000) is a dual-acylated inner membrane complex (IMC) protein that organises apical polarity and cytoskeletal architecture in invasive and sexual stages of Plasmodium (PMID:24244852, PMID:32395856). In P. berghei it localizes to the anterior apical end where IMC assembly is initiated, is expressed across developmental stages with a bias toward invasive stages, and is N-myristoylated, phosphorylated, and membrane-bound; gene disruption indicates it is likely essential for parasite development (PMID:24244852). Its apical positioning is coordinated with ISP3, whose loss mislocalizes ISP1 in ookinetes and sporozoites (PMID:24244852). Beyond N-myristoylation, ISP1 is palmitoylated on N-terminal cysteines by the IMC-resident palmitoyl-S-acyltransferase DHHC2, and this acylation controls its recruitment to the IMC and is required for zygote-to-ookinete differentiation (PMID:32395856); in P. falciparum an IMC-restricted acyltransferase, DHHC1, co-distributes with ISP1 during schizogony, consistent with a role in its palmitoylation and membrane targeting (PMID:25425642). Once IMC-anchored, ISP1 acts as a structural and organizing hub: it physically interacts with β-tubulin to tether and maintain subpellicular microtubules during zygote elongation (PMID:32395856), and in P. yoelii ookinetes it anchors the GCβ/CDC50A complex at the ookinete extrados site to initiate gliding motility (PMID:30146157). No structural model or higher-resolution mechanism of ISP1's tethering interactions has been characterized in the available corpus.

1. 2012 Low

   Establishing that ISP1 is a distinct member of a small family of IMC proteins with stage-specific expression framed it as a candidate structural component of the parasite pellicle rather than a diffuse cytoplasmic factor.

   Evidence Colocalization and comparative phylogenetic analysis across life-cycle stages in P. falciparum

   PMID:22389454

   Open questions at the time

   • This broad survey provides minimal ISP1-specific mechanistic detail
   • No function or binding partner assigned to ISP1 individually
   • Life-cycle expression not tied to any phenotype
2. 2013 Medium

   Localization and gene-disruption work answered where ISP1 sits and whether it matters, showing apical anterior positioning at the site of IMC initiation and likely essentiality, and revealing coordination with ISP3 in setting apical polarity.

   Evidence Live cell imaging, reverse genetics, and biochemical acylation/phosphorylation assays in P. berghei, plus imaging of ISP3-disruption parasites

   PMID:24244852

   Open questions at the time

   • Essentiality inferred from disruption difficulty rather than conditional knockout
   • Molecular basis of ISP1–ISP3 coordination not defined
   • No direct binding partners identified
3. 2014 Medium

   Identifying ISP1 as dual-acylated and co-distributing with the IMC-restricted acyltransferase DHHC1 addressed how it is targeted to the IMC membrane, linking its localization to palmitoylation machinery.

   Evidence Phylogenetic analysis, fluorescence colocalization, and biochemical acylation assays in P. falciparum

   PMID:25425642

   Open questions at the time

   • DHHC1-mediated palmitoylation of ISP1 shown by co-distribution, not direct enzymatic assay
   • Functional consequence of acylation not yet tested
   • Relationship between myristoylation and palmitoylation in targeting unresolved
4. 2018 Medium

   An interaction screen defined a downstream function of IMC-anchored ISP1: it anchors the GCβ/CDC50A complex at the ookinete extrados site to initiate gliding motility, connecting ISP1 to signalling that drives motility.

   Evidence Real-time imaging, protein–protein interaction screening of IMC proteins, and genetic/cell-biological experiments in P. yoelii

   PMID:30146157

   Open questions at the time

   • Direct binding interface between ISP1 and GCβ/CDC50A not mapped
   • How ISP1 achieves polarized (extrados-only) anchoring unknown
   • Single lab, single species
5. 2020 High

   Reciprocal biochemistry and genetics unified acylation, localization, and cytoskeletal function, showing DHHC2 palmitoylates N-terminal cysteines of ISP1 to drive IMC localization and zygote-to-ookinete differentiation, and that IMC-anchored ISP1 binds β-tubulin to tether subpellicular microtubules.

   Evidence In vitro palmitoylation assays, PAT-substrate experiments, reciprocal co-immunoprecipitation with β-tubulin, and genetic disruption with morphological readout in P. falciparum and P. yoelii

   PMID:32395856

   Open questions at the time

   • Structural basis of the ISP1–β-tubulin tethering interaction not resolved
   • Reconciliation of DHHC2 (this study) versus DHHC1 (2014 co-distribution) as the relevant acyltransferase not addressed
   • Whether β-tubulin tethering and GCβ/CDC50A anchoring are mechanistically linked is unknown

• It remains unresolved how ISP1's two documented downstream functions—microtubule tethering via β-tubulin and polarized anchoring of the GCβ/CDC50A gliding-motility complex—are structurally and mechanistically integrated at the apical IMC.
• No structural model of ISP1 or its binding interfaces
• Mechanism of polarized apical/extrados restriction unexplained
• Interplay between myristoylation, palmitoylation, and phosphorylation in regulating ISP1 not defined