MAST3

MAST3 is a serine/threonine kinase that operates as a node controlling phosphatase activity, inflammatory signaling, and growth regulation across neuronal and non-neuronal contexts (PMID:28167675, PMID:21994190, PMID:40312366). Its best-defined activity is in striatal neurons, where MAST3 directly phosphorylates ARPP-16 at Ser46 to convert it into a selective inhibitor of B55α- and B56δ-containing PP2A heterotrimers, establishing a basal brake on PP2A substrate dephosphorylation (PMID:28167675). This axis is wired into cAMP signaling: PKA phosphorylates and inhibits MAST3 while also phosphorylating ARPP-16 at a competing site, so that the two kinase inputs mutually antagonize each other to produce a switch-like control of PP2A disinhibition (PMID:28613156). Through its PDZ domain MAST3 binds the C-terminal tail of PTEN and promotes PTEN C-terminal phosphorylation (PMID:15951562), and it modulates TLR4-dependent NF-κB activity and downstream inflammatory gene expression (PMID:18650832, PMID:21994190). In breast cancer cells MAST3 interacts directly with YAP and drives its phosphorylation and ubiquitin-proteasome degradation independently of the canonical MST–LATS cascade, thereby suppressing proliferation and invasion (PMID:40312366). De novo missense variants in the MAST3 kinase domain elevate ARPP-16 phosphorylation in a gain-of-function manner and define a neurodevelopmental disorder, consistent with MAST3 expression being restricted to excitatory cortical neurons postnatally (PMID:34185323).

1. 2005 Medium

   Established the first molecular partner and a structural basis for MAST3 target engagement, showing the PDZ domain recruits a substrate whose C-terminus it then phosphorylates.

   Evidence Co-IP, PTEN chimera/mutation panel, and in vitro kinase assay defining residues 350-403 of PTEN as the PDZ-binding region

   PMID:15951562

   Open questions at the time

   • Functional consequence of PTEN C-terminal phosphorylation by MAST3 not resolved
   • Cellular context and physiological role of the MAST3–PTEN interaction unaddressed
2. 2008 Medium

   Placed MAST3 within innate immune signaling by showing it is required for TLR4-specific NF-κB output, the first link of MAST3 to inflammation.

   Evidence siRNA knockdown with NF-κB reporter assay in antigen-presenting cells and lymphocytes

   PMID:18650832

   Open questions at the time

   • Direct kinase substrate in the TLR4/NF-κB cascade not identified
   • Reporter readout only; endogenous target unknown
3. 2011 Medium

   Extended the inflammatory role to a defined transcriptional program, showing MAST3 dosage tunes NF-κB-related inflammatory gene expression.

   Evidence MAST3 overexpression/knockdown with genome-wide expression profiling in epithelial and macrophage lines

   PMID:21994190

   Open questions at the time

   • Whether transcriptional changes are direct or downstream of an undefined kinase event is unresolved
   • No enzyme-substrate link to the NF-κB machinery
4. 2017 High

   Defined the central catalytic function of MAST3: direct phosphorylation of ARPP-16 at Ser46 to selectively inhibit specific PP2A heterotrimers, with genetic knockout confirming basal PP2A inhibition in vivo.

   Evidence In vitro and in vivo kinase assays, conditional ARPP-16/19 knockout with phospho-substrate readouts, and Co-IP of ARPP-16 with PP2A

   PMID:28167675

   Open questions at the time

   • Structural basis of ARPP-16 Ser46 selectivity for B55α/B56δ PP2A not defined
   • Full set of physiological PP2A substrates downstream unenumerated
5. 2017 High

   Embedded the MAST3–ARPP-16 axis in cAMP signaling, showing PKA inhibits MAST3 and the two phosphorylation events mutually antagonize to create a switch-like control of PP2A activity.

   Evidence In vitro kinase assays, mass-spectrometry phosphosite mapping, and mathematical modeling

   PMID:28613156

   Open questions at the time

   • In vivo dynamics of the proposed switch in intact neurons not directly measured
   • Functional identity of all PKA phosphosites on MAST3 not fully characterized
6. 2018 Low

   Raised the possibility of functional interdependence among MAST family members, observing reduced MAST3 protein in Mast1-mutant animals.

   Evidence Mast1 mutant mouse model with western blot quantification of Mast3

   PMID:30449657

   Open questions at the time

   • Mechanism of MAST3 level reduction not directly probed
   • Observation incidental to a MAST1-focused study; no MAST3-specific manipulation
7. 2019 Low

   Connected MAST3 to a disease-relevant inflammatory phenotype, showing overexpression drives synoviocyte proliferation and inflammation via NF-κB.

   Evidence MAST3 overexpression in rat adjuvant-arthritis fibroblast-like synoviocytes with proliferation and NF-κB pathway analysis

   PMID:31644963

   Open questions at the time

   • No direct enzyme-substrate link to the NF-κB pathway established
   • Overexpression-only design without loss-of-function corroboration
8. 2021 Medium

   Linked MAST3 to a human neurodevelopmental disorder, showing kinase-domain de novo variants act through gain-of-function on the ARPP-16 substrate in cells restricted to excitatory cortical neurons.

   Evidence Patient-variant cDNA transfection in HEK293T with ARPP-16 phosphorylation immunoblotting, single-nuclei RNA-seq, and IHC

   PMID:34185323

   Open questions at the time

   • In vivo neuronal consequence of variant gain-of-function not modeled
   • How elevated ARPP-16 phosphorylation with reduced protein produces net gain-of-function unresolved
9. 2025 Medium

   Identified a non-canonical growth-suppressive function, showing MAST3 directly phosphorylates YAP to trigger its proteasomal degradation independently of the MST-LATS Hippo cascade.

   Evidence Co-IP, immunoblotting, shRNA knockdown/overexpression with proliferation and invasion assays, and Hippo target luciferase reporters in breast cancer cells

   PMID:40312366

   Open questions at the time

   • Specific YAP phosphosites targeted by MAST3 not mapped
   • How MAST3 substrate choice is partitioned between ARPP-16, PTEN, and YAP in different tissues unknown
10. 2025 Low

    Generated a systems-level hypothesis that MAST3 acts in cytoskeletal regulation, nominating candidate substrates and interactors from large-scale phosphoproteomics.

    Evidence Phosphoproteomic co-regulation analysis across 562 datasets identifying MAST3 phosphosites and predicted partners (KIF15, EPB41L1, CP110, HNRNPU, LMNA, CKAP4, CAMSAP2)

    PMID:41203242

    Open questions at the time

    • No direct experimental validation of any predicted substrate or interactor for MAST3
    • Cytoskeletal role inferred from co-regulation, not functional assay

• How MAST3 partitions its kinase activity among ARPP-16, PTEN, YAP, and inflammatory targets across tissues, and whether a unifying recruitment or substrate-selection mechanism governs these distinct roles, remains unresolved.
• No structural model of MAST3 substrate selectivity
• Tissue-specific determinants of which substrate MAST3 engages are unknown
• Direct demonstration of a microtubule-associated cytoskeletal function still lacking