ANKRD28

ANKRD28 is an ankyrin-repeat regulatory/scaffold subunit that couples protein Ser/Thr phosphatases to specific substrates, thereby shaping signaling outputs in the NF-κB pathway, nuclear transcriptional control, and cell migration (PMID:18186651, PMID:16564677, PMID:19118547). As part of the PP6 holoenzyme, it forms a heterotrimer with the PP6 catalytic subunit and SAPS-domain scaffolds (PP6R1 or PP6R3), with PP6R1 providing separable binding surfaces for PP6c and ANKRD28; loss of ANKRD28 or PP6R1 accelerates IκBε degradation upon TNFα, defining ANKRD28 as a positive PP6 co-regulator restraining NF-κB activation (PMID:18186651). Independently, ANKRD28 (PITK) acts as a PP1-targeting subunit that directs PP1 to nuclear foci to dephosphorylate hnRNP K at S284 and reprogram transcription, with its PP1 binding switched off by sequential CaMKIIδ-mediated phosphorylation of S1017 followed by GSK3 phosphorylation of S1013 flanking its PP1C-binding motif, which also governs its speckled-nuclear versus diffuse localization (PMID:16564677, PMID:17023142). Through its ankyrin repeats it binds the SH3 domain of DOCK180 in competition with ELMO, controlling focal adhesion organization, p130Cas phosphorylation, and Rac1-dependent migration (PMID:19118547), a function further tuned by Rab40c/CRL5-mediated ubiquitylation and lysosomal turnover of ANKRD28 that modulates PP6 activity in migrating cells (PMID:35512830). BRCA1 binds ANKRD28 in the cytoplasm and stabilizes IκBε through this interaction, linking BRCA1 to suppression of NF-κB signaling via the ANKRD28–PP6 axis (PMID:27026398). An ANKRD28–NUP98 fusion arising from a cryptic translocation in MDS/AML mislocalizes the protein to the nucleolus and drives oncogenic focus formation in a manner requiring the ANKRD28 C-terminus (PMID:17988990).

1. 2006 High

   Established that ANKRD28 (PITK) is a PP1-targeting subunit whose substrate-directing activity toward hnRNP K is controlled by phosphorylation of sites flanking its PP1C-binding motif, answering how a phosphatase is aimed at a specific nuclear substrate.

   Evidence In vivo phosphosite mapping, mutagenesis of S1013/S1017, PP1 binding and hnRNP K dephosphorylation assays, microarray transcription readout in cells

   PMID:16564677

   Open questions at the time

   • Direct structural basis of PP1 docking not resolved
   • Functional consequence of MEK5 induction for cell physiology not established
2. 2006 High

   Identified the upstream kinases governing the ANKRD28 phospho-switch, showing CaMKIIδ primes S1017 for GSK3 phosphorylation of S1013 and that this state dictates nuclear-speckle versus diffuse localization.

   Evidence In vitro kinase assays with CaMKIIδ and GSK3, mutagenesis, fluorescence localization imaging

   PMID:17023142

   Open questions at the time

   • Physiological signal triggering CaMKIIδ/GSK3 cascade not defined
   • Whether the same phospho-switch regulates PP6-associated functions untested
3. 2007 Medium

   Linked ANKRD28 to leukemogenesis by showing an ANKRD28-NUP98 fusion mislocalizes the protein and confers transforming activity, raising the question of how disrupted ANKRD28 function contributes to malignancy.

   Evidence FISH breakpoint mapping, RT-PCR, NIH/3T3 focus formation with C-terminal deletion, localization imaging

   PMID:17988990

   Open questions at the time

   • Single clinical case
   • Mechanism by which fusion transforms cells not defined
   • Relationship to ANKRD28's phosphatase-regulatory roles untested
4. 2008 High

   Defined ANKRD28 as a regulatory subunit of the PP6 holoenzyme assembled via SAPS-domain scaffolds and showed it restrains NF-κB by stabilizing IκBε, establishing its PP6-pathway role.

   Evidence FLAG Co-IP, mass spectrometry, gel filtration, siRNA knockdown with IκBε degradation assay

   PMID:18186651

   Open questions at the time

   • Whether ANKRD28 confers PP6 substrate specificity directly untested
   • Stoichiometry and architecture of the heterotrimer not resolved
5. 2008 High

   Revealed a PP6-independent role in migration: ANKRD28 binds the DOCK180 SH3 domain in competition with ELMO to shape focal adhesion dynamics and Rac1-dependent motility.

   Evidence Nano-LC/MS/MS Co-IP, RNAi knockdown, live-cell migration assay, focal adhesion imaging, p130Cas phosphorylation assay

   PMID:19118547

   Open questions at the time

   • Whether PP6 phosphatase activity participates in this migration role untested
   • Mechanism linking ANKRD28-DOCK180 binding to p130Cas hyperphosphorylation unclear
6. 2016 High

   Connected BRCA1 to the ANKRD28-PP6 axis, showing cytoplasmic BRCA1 binding stabilizes IκBε and thereby modulates NF-κB signaling through ANKRD28.

   Evidence Yeast two-hybrid, reciprocal endogenous Co-IP, proximity ligation, IκBε stabilization assay with BRCA1 truncation mutants

   PMID:27026398

   Open questions at the time

   • Whether BRCA1 alters PP6 catalytic activity or only complex stability not distinguished
   • Physiological context where this regulation operates not defined
7. 2022 High

   Identified post-translational control of ANKRD28 levels by Rab40c/CRL5-mediated ubiquitylation and lysosomal degradation, tying ANKRD28 turnover to PP6 activity and focal adhesion morphology in migrating cells.

   Evidence Co-IP, ubiquitylation assay, Rab40c knockout, phospho-FAK/MOB1 immunoblotting, focal adhesion imaging in MDA-MB-231

   PMID:35512830

   Open questions at the time

   • Ubiquitylation site(s) on ANKRD28 not mapped
   • Whether degradation is signal-regulated not established
8. 2024 Medium

   Extended the PP6 holoenzyme to colorectal cancer stem cell control, though the specific contribution of ANKRD28 within the complex was not directly tested.

   Evidence siRNA knockdown of PP6c, colony formation, xenograft proliferation, transcriptome analysis

   PMID:39014521

   Open questions at the time

   • ANKRD28's individual role in CSC regulation not assessed
   • Substrates mediating stemness phenotype unidentified

• How ANKRD28 partitions between its PP6-regulatory, PP1-targeting, and DOCK180-migration functions, and whether a unified structural/regulatory logic governs these roles, remains unresolved.
• No structure of any ANKRD28-containing complex
• Substrate specificity contributions of ANKRD28 not directly demonstrated
• Crosstalk between phospho-switch and degradation control unexplored