ITPKA

ITPKA is a bifunctional protein that couples inositol phosphate signaling to actin cytoskeleton remodeling, with roles spanning neuronal morphogenesis, vertebrate neural development, and cancer cell motility (PMID:25620569, PMID:36688944). Its C-terminal catalytic domain phosphorylates Ins(1,4,5)P3, while an N-terminal actin-binding domain forms homodimers that engage F-actin; in the full-length enzyme these activities are reciprocally coupled, with embedding in an actin network doubling kinase activity and the product Ins(1,3,4,5)P4 feeding back to inhibit spontaneous actin polymerization (PMID:25620569). A cryo-EM structure of the ITPKA–F-actin complex defined a short linear F-actin binding motif and the residues required for filament engagement [PMID:bio_10.1101_2025.04.16.649135]. The actin-bundling activity, rather than the kinase activity, is the primary driver of migration and invasion in lung cancer cells, with kinase activity contributing only a modest ATP-stimulated boost (PMID:36688944), while in neurons both activities independently restrain NGF-induced neurite outgrowth (PMID:25892505). Beyond its canonical lipid substrate, ITPKA phosphorylates PYCR1 at Ser29 to block its Stub1-mediated ubiquitination and promote glioma growth (PMID:39170313), and it acts as a scaffold by binding the microtubule end-binding protein EB3 in a PKA-Ser119-phosphorylation-dependent manner during synaptic plasticity (PMID:30466786) and by binding MDM2 to stabilize p53 and drive senescence (PMID:33879633). ITPKA is required for anterior neural development in Xenopus (PMID:40703653), its transcription is activated by TFAP2A (PMID:32015686) and repressed by miR-203 (PMID:33879633).

1. 2015 High

   Established the structural and functional coupling between ITPKA's two activities, answering whether actin engagement and kinase catalysis influence one another.

   Evidence In vitro F-actin reconstitution, STED microscopy, InsP3 kinase assay, and domain-deletion analysis of full-length ITPKA

   PMID:25620569

   Open questions at the time

   • Did not resolve atomic-level contacts of the actin-binding motif
   • In vitro reconstitution may not capture cellular regulation
2. 2015 Medium

   Demonstrated that both the F-actin binding and kinase activities of ITPKA independently regulate a cellular morphogenetic process (neurite outgrowth).

   Evidence Over-expression of wild-type, kinase-dead, and isolated N-terminal domain constructs with quantitative neurite outgrowth assays in PC12 cells

   PMID:25892505

   Open questions at the time

   • Gain-of-function overexpression rather than endogenous loss-of-function
   • Single cell model and single lab
3. 2018 Medium

   Identified ITPKA as a scaffold for microtubule end-binding protein EB3 under phosphoregulatory control, extending its cytoskeletal role to microtubules during synaptic plasticity.

   Evidence Co-immunoprecipitation, PKA phosphorylation assay, and Ser119 phospho-mutant analysis in a chemically induced LTP paradigm

   PMID:30466786

   Open questions at the time

   • Single Co-IP without reciprocal structural validation
   • Functional consequence of EB3 binding for plasticity not directly demonstrated
4. 2020 Medium

   Linked ITPKA to EMT/metastasis through DBN1 interaction and identified TFAP2A as a transcriptional activator, clarifying upstream regulation and a downstream effector partner.

   Evidence Co-IP for ITPKA–DBN1, ChIP/reporter assay for TFAP2A, and migration/invasion assays with knockdown in lung adenocarcinoma cells

   PMID:32015686

   Open questions at the time

   • Direct binding interface with DBN1 not mapped
   • Single lab
5. 2021 Medium

   Revealed a non-cytoskeletal tumor-suppressive arm in which ITPKA binds MDM2 to stabilize p53 and drive senescence, and identified miR-203 as a negative regulator.

   Evidence Co-IP (ITPKA–MDM2), p53 stability assay, overexpression/knockdown senescence and growth assays, and miR-203 functional experiments in ovarian cancer cells

   PMID:33879633

   Open questions at the time

   • Mechanism of how ITPKA binding affects MDM2 activity not resolved
   • Context-dependent (apparently opposite to pro-invasive roles elsewhere)
6. 2024 Medium

   Expanded ITPKA's catalytic repertoire to a protein substrate, showing it phosphorylates PYCR1-Ser29 to block ubiquitination, coupling kinase activity to protein stability in glioma.

   Evidence Co-IP, in vitro kinase assay with Ser29 phospho-site mapping, Stub1 ubiquitination assay, and in vivo tumorigenicity assay

   PMID:39170313

   Open questions at the time

   • Whether PYCR1 is a direct versus indirect substrate in cells not fully resolved
   • Single lab, single study
7. 2025 Medium

   Provided the structural basis of F-actin engagement by defining a short linear F-actin binding motif and its essential residues.

   Evidence Cryo-EM of the ITPKA–F-actin complex, SLiMFold computational pipeline, and SFM mutagenesis with binding affinity measurements (preprint)

   PMID:bio_10.1101_2025.04.16.649135

   Open questions at the time

   • Not yet peer-reviewed
   • Functional consequences of the SFM mutations in cells not established
8. 2025 Medium

   Established a required in vivo developmental role for ITPKA in vertebrate anterior neural patterning.

   Evidence Morpholino knockdown of Itpka in Xenopus laevis with RNA rescue and marker gene analysis

   PMID:40703653

   Open questions at the time

   • Did not dissect which ITPKA activity (kinase vs actin) drives the phenotype
   • Morpholino off-target effects not fully excluded beyond rescue

• It remains unresolved how the kinase, actin-bundling, and scaffolding activities of ITPKA are differentially deployed across neurons, development, and distinct tumor contexts where they appear to have opposing outcomes.
• No unified model integrating tumor-suppressive (p53) and pro-invasive (actin/PYCR1) roles
• Endogenous regulation of activity switching unknown