PPP1R9B

PPP1R9B (spinophilin/neurabin-2) is a scaffolding protein that targets the catalytic phosphatase PP1a (PPP1CA) to specific substrates, thereby coupling phosphatase activity to cell cycle control, translational regulation, and synaptic and cytoskeletal organization (PMID:26387546, PMID:33537097). As a PP1 regulatory subunit it directs dephosphorylation of the retinoblastoma pocket proteins pRb, p107, and p130 during G0/G1 and G1; loss of PPP1R9B or a tumor-derived point mutation (SPN-A566V) that disrupts the SPN-PP1 interaction sustains pocket-protein phosphorylation and promotes cancer stem-like properties, an effect that is amplified together with mutant p53 (PMID:26387546, PMID:33537097). Substrate selection by the PP1-spinophilin holoenzyme depends on substrate engagement of the PDZ domain adjoining its RVxF-ΦΦ-R-W motif rather than remodeling of the PP1 catalytic groove, which extends its substrate repertoire to the translational regulators 4E-BP1 and p70 S6K and links it to mTORC1-dependent translational control. Beyond its phosphatase-targeting role, spinophilin maintains cortical F-actin organization required for NK-cell immunological synapse formation and target conjugation (PMID:19130477), and in striatal neurons it mediates agonist-induced D2 dopamine receptor internalization, shaping locomotor responses to D2R agonists.

1. 2009 Medium

   Established that spinophilin acts as a cytoskeletal regulator in immune cells, not solely in neurons, by showing it organizes cortical F-actin at the NK immunological synapse.

   Evidence Confocal live imaging, shRNA knockdown, and F-actin staining with CD18/CD28 stimulation in the YTS NK-like cell line

   PMID:19130477

   Open questions at the time

   • Molecular link between spinophilin and the actin machinery not defined
   • Whether PP1 targeting is involved in this role untested
   • Single cell line, no in vivo validation
2. 2015 Medium

   Connected spinophilin to cell-cycle and cancer biology by showing it directs PP1a-mediated dephosphorylation of pRb, with loss driving cancer stemness.

   Evidence Reciprocal shRNA knockdown/overexpression, PPP1CA knockdown, flow cytometry and qRT-PCR in breast cancer lines and tumor samples

   PMID:26387546

   Open questions at the time

   • No structural basis for substrate targeting
   • Single lab, no independent replication
   • Direct PP1-spinophilin-pRb complex not biochemically reconstituted
3. 2021 Medium

   Resolved that the PP1-spinophilin holoenzyme regulates the cell cycle through phosphatase activity on pocket proteins, using a tumor mutation that separates complex assembly from catalysis.

   Evidence SPN-A566V mutational analysis, co-immunoprecipitation, phosphatase assays, and cell cycle/stemness assays in MCF10A, T47D, MDA-MB-468

   PMID:33537097

   Open questions at the time

   • Cooperativity with mutant p53 mechanistically undefined
   • In vivo tumorigenicity not tested
   • Single lab
4. 2024 Medium

   Defined the biochemical logic of substrate selection, showing PDZ-domain engagement rather than catalytic-groove remodeling determines targeting, and added 4E-BP1/p70 S6K to the substrate repertoire.

   Evidence PP1-PIP fusion proteins, in vitro phosphatase assays, and structural experiments (preprint)

   Open questions at the time

   • Preprint, not peer-reviewed
   • Cellular consequence for mTORC1 signaling not demonstrated in vivo
   • Fusion-protein constructs may not reflect native holoenzyme
5. 2025 Low

   Provided in vivo evidence that spinophilin mediates D2 dopamine receptor internalization and shapes locomotor responses, and catalogued a large spinophilin interactome enriched in mRNA processing/translation factors.

   Evidence Spinophilin knockout mice with locomotion assays and quinpirole interaction proteomics; Ultra-ID proximity labeling and nanobody pulldown in HEK293 (preprints)

   Open questions at the time

   • Preprints, single lab, awaits peer review
   • Direct D2R-spinophilin binding interface not mapped
   • Interactome hits largely uncharacterized at the mechanistic level

• How spinophilin integrates its distinct roles—PP1 substrate targeting, actin organization, receptor trafficking, and local translation—within a single cell type remains unresolved.
• No unifying model linking scaffolding domains to context-specific functions
• Tissue-specific substrate repertoire undefined
• Structural model of the native holoenzyme on physiological substrates lacking