MOB4

MOB4 is a conserved, non-catalytic scaffolding subunit of the STRIPAK complex that links PP2A phosphatase activity to regulation of Hippo signaling, cytoskeletal organization, and collective cell migration. Within STRIPAK, MOB4 binds striatin family proteins at both N-terminal and C-terminal regions, and the cryo-EM structure of the STRIPAK core places one copy of MOB4 alongside PP2A-A/C, STRN3 homotetramer, and STRIP1, with interface mutations causing aberrant Hippo pathway activation (PMID:33633399, PMID:21985334). MOB4 also forms a phosphorylation-dependent complex with the GCK-III kinase MST4 that structurally mimics MST1–MOB1, and this MST4–MOB4 complex antagonizes canonical Hippo signaling by disrupting MST1–MOB1 assembly and thereby promoting YAP activity, cell growth, and migration (PMID:30072378). Conserved organismal functions include regulation of axonal transport and microtubule organization at neuromuscular junctions, neural stem cell reactivation from quiescence via Hippo pathway inhibition, spermatid individualization, and sarcomere assembly through genetic interaction with the actin-folding chaperonin TRiC, with human MOB4 transgenes rescuing Drosophila and zebrafish loss-of-function phenotypes (PMID:20392941, PMID:31167138, PMID:37259670, PMID:35737712).

1. 2008 High

   The initial question of where MOB4 operates was answered by identifying it as a stable subunit of the newly defined STRIPAK complex containing PP2A, striatins, STRIP1/2, CCM3, and GCK-III kinases, establishing MOB4 as part of a major phosphatase–kinase signaling assembly rather than functioning as an isolated Hippo pathway component.

   Evidence Iterative AP-MS of multiple STRIPAK subunits in mammalian cells

   PMID:18782753

   Open questions at the time

   • Stoichiometry and architecture of MOB4 within the complex unknown
   • Direct binding partners within STRIPAK not mapped
2. 2008 Medium

   In parallel, the Drosophila ortholog DMob4 was shown to localize to spindle poles and kinetochores and to be required for kinetochore-fiber focusing during mitosis, suggesting MOB4 controls a mitotic kinase that organizes spindle microtubules.

   Evidence RNAi screen and Mob4-GFP live imaging in Drosophila S2 cells

   PMID:18388316

   Open questions at the time

   • Identity of the regulated mitotic kinase not determined
   • Relevance of spindle function to mammalian MOB4 not tested
3. 2010 High

   Null alleles of Drosophila DMob4 revealed conserved roles in axonal transport, microtubule network organization, and neuromuscular junction growth, and cross-species rescue by human phocein demonstrated functional conservation.

   Evidence Drosophila null/hypomorphic alleles with physiological and cell-biological phenotyping; transgenic rescue with human MOB4

   PMID:20392941

   Open questions at the time

   • Mechanism by which MOB4 influences microtubule organization not resolved
   • Whether STRIPAK-dependent or -independent functions exist unclear
4. 2011 Medium

   Structure–function mapping of striatin revealed that MOB4 binds at two distinct regions (N-terminal coiled-coil and C-terminal WD-repeat domain) positioned near the PP2A and Mst3 binding sites, providing a rationale for how MOB4 could facilitate PP2A-mediated regulation of GCK-III kinases.

   Evidence Co-immunoprecipitation of striatin deletion mutants with MOB4

   PMID:21985334

   Open questions at the time

   • Direct effect of MOB4 on PP2A-mediated Mst3 dephosphorylation not demonstrated
   • Binding affinities not quantified
5. 2018 Medium

   MOB4 binding to striatin-1 was shown to be functionally essential for striatal neuron dendritic arborization, as a MOB4-binding-deficient striatin-1 construct failed to rescue dendritic and spine phenotypes caused by striatin-1 knockdown.

   Evidence shRNA knockdown with structure–function rescue constructs in primary striatal neurons

   PMID:29802198

   Open questions at the time

   • Downstream signaling targets of MOB4–striatin in neurons not identified
   • Whether MOB4 also contributes independently of striatin not addressed
6. 2018 High

   The MST4–MOB4 complex was structurally and functionally characterized, revealing a phosphorylation-dependent interaction that mimics MST1–MOB1 but instead promotes cell growth and migration by disrupting MST1–MOB1 assembly and increasing YAP activity — establishing MOB4 as a direct antagonist of canonical Hippo tumor suppression.

   Evidence Co-immunoprecipitation, structural analysis, cell growth/migration assays, and YAP activity measurements in PANC-1 cells

   PMID:30072378

   Open questions at the time

   • Whether MST4–MOB4 operates within or outside the STRIPAK complex unclear
   • In vivo relevance of this complex in tumorigenesis not tested
7. 2019 Medium

   In Drosophila neural stem cells, Mob4 together with Cka/Strip recruits PP2A to Hippo kinase, inhibiting Hippo signaling to promote reactivation from quiescence — providing a defined developmental context for STRIPAK-mediated Hippo regulation.

   Evidence Genetic loss-of-function, co-immunoprecipitation of Mob4/Cka/PP2A/Hippo complex, single-NSC transcriptomics

   PMID:31167138

   Open questions at the time

   • Direct biochemical demonstration of PP2A dephosphorylation of Hippo in this context lacking
   • Mammalian NSC relevance not established
8. 2021 High

   A 3.2-Å cryo-EM structure of the STRIPAK core resolved MOB4's precise position in the complex — one copy associating with a four-STRN3 elongated scaffold, one PP2A-A/C heterodimer, and one STRIP1 — and interface mutations disrupting STRIPAK integrity caused aberrant Hippo pathway activation, providing atomic-level understanding of how the assembly controls signaling.

   Evidence Cryo-EM structure determination, interface mutagenesis, Hippo pathway activity assays

   PMID:33633399

   Open questions at the time

   • How GCK-III kinases and CCM3 integrate into the core structure not resolved
   • Conformational dynamics and regulation of MOB4 within the complex unknown
9. 2022 Medium

   Zebrafish mob4 mutants linked STRIPAK to sarcomere assembly and actin biogenesis through genetic interaction with the chaperonin TRiC, while mob4 overexpression increased myofibril number, establishing a cytoskeletal scaffolding role beyond signaling.

   Evidence Forward genetic screen, nonsense mutant, transgenic rescue, TRiC genetic interaction, electron microscopy in zebrafish

   PMID:35737712

   Open questions at the time

   • Whether MOB4–TRiC interaction is direct or mediated through other STRIPAK subunits unknown
   • Mammalian muscle relevance not demonstrated
10. 2023 Medium

    MOB4 was shown to be essential for Drosophila spermatogenesis — controlling spermatid individualization, axonemal microtubule organization, and mitochondrial morphology — with human MOB4 rescuing all phenotypes, confirming deep functional conservation of STRIPAK-dependent cytoskeletal regulation.

    Evidence RNAi knockdown, transmission electron microscopy, human MOB4 transgenic rescue in Drosophila

    PMID:37259670

    Open questions at the time

    • Which STRIPAK-dependent kinase targets drive spermatid phenotypes not identified
    • Role in mammalian spermatogenesis not tested
11. 2025 Medium

    MOB4 was established as a coordinator of collective cell migration: knockout in MCF10A cells increased but disoriented migration, MOB4 relocalized to the leading edge during wound healing, and the phenotype required YAP1, as constitutively active YAP1 rescued orientation defects.

    Evidence CRISPR/Cas9 knockout, wound healing assay, live imaging, YAP1 epistasis rescue in MCF10A cells

    PMID:41276909

    Open questions at the time

    • How MOB4 leading-edge localization is regulated unclear
    • Whether STRIPAK-associated phosphatase or kinase activity mediates the migration phenotype not dissected
12. 2025 Medium

    MOB4 and STRN3 were identified as Rac1 interactors in Schwann cells, linking STRIPAK to lamellipodia formation, radial sorting, and YAP/TAZ-dependent extracellular matrix gene expression — broadening the signaling inputs (Rac1) and cell-type contexts of MOB4 function.

    Evidence Co-immunoprecipitation of Rac1 with STRN3/MOB4, conditional Schwann cell knockout, lamellipodia and radial sorting assays

    PMID:40056414

    Open questions at the time

    • Whether Rac1 binds MOB4 directly or via striatin not resolved
    • Relative contributions of MOB4 versus other STRIPAK subunits in Schwann cells not determined

• Key unresolved questions include: how MOB4 is regulated (post-translational modifications, conformational switching), whether MOB4 has STRIPAK-independent functions, how the MST4–MOB4 and STRIPAK-resident pools are partitioned in cells, and which direct kinase substrates mediate MOB4-dependent cytoskeletal and signaling phenotypes.
• No post-translational modification map of MOB4 exists
• No structural data for MOB4 within GCK-III kinase–containing holo-STRIPAK
• STRIPAK-independent functions of MOB4 not tested