ACTN1

Alpha-actinin-1 (ACTN1) is a non-muscle actin-crosslinking protein that bridges the actin cytoskeleton to transmembrane adhesion complexes, functioning at focal adhesions and cell–cell junctions to organize F-actin architecture and transmit mechanical force. It directly binds the cytoplasmic domains of β1 and β3 integrins at focal contacts (PMID:2116421) and associates with α-catenin to link cadherin complexes to actin filaments (PMID:7790378); in cardiomyocytes it is non-redundantly required for Z-line formation and sarcomere assembly through stabilization of focal adhesion maturation [PMID:bio_10.1101_2025.03.28.645933]. ACTN1 protein levels are tightly controlled by Cullin-3-mediated ubiquitin-dependent degradation—essential for normal myogenesis and neuromuscular junction formation (PMID:30990797)—and by USP14-mediated deubiquitination (PMID:41291211). Missense mutations throughout its actin-binding, rod, and calmodulin-like domains cause autosomal dominant congenital macrothrombocytopenia by disrupting actin cytoskeletal organization in megakaryocytes (PMID:23434115, PMID:24069336).

1. 1990 High

   Establishing how the actin cytoskeleton connects to the plasma membrane at focal contacts: alpha-actinin was shown to bind the cytoplasmic domain of β1 integrin directly, providing the first biochemical evidence for a physical integrin–actin bridge.

   Evidence Affinity chromatography and solid-phase binding assays with purified proteins and synthetic integrin cytoplasmic peptides

   PMID:2116421

   Open questions at the time

   • Structural details of the integrin–alpha-actinin interface were not resolved
   • In vivo functional relevance of the interaction at focal adhesions was not tested
2. 1995 High

   Extending the adaptor role to cell–cell junctions: alpha-actinin was found to bind α-catenin directly and independently of actin, establishing it as a shared linker between integrin- and cadherin-based adhesion systems.

   Evidence Reciprocal co-immunoprecipitation from fibroblasts and epithelial cells with actin-independence controls

   PMID:7790378

   Open questions at the time

   • Whether the integrin-binding and catenin-binding functions of ACTN1 are mutually exclusive or simultaneous was not determined
   • Stoichiometry of the complex was not defined
3. 2002 Medium

   Revealing tissue-specific complexity: a brain-specific ACTN1 splice isoform incorporating both NM and SM exons was identified, with expression increasing postnatally in hippocampal and cortical neurons.

   Evidence RT-PCR and in situ hybridization in adult rat brain

   PMID:12099693

   Open questions at the time

   • Functional significance of the brain-specific isoform is unknown
   • Whether the BS isoform has distinct actin-binding or protein-interaction properties was not tested
4. 2011 Medium

   Identifying a cardiac protein complex: ACTN1 was found in a stable complex with FHL1, PDLIM1, and gelsolin in cardiomyocytes, suggesting a higher-order organizational unit at the Z-line.

   Evidence Tandem affinity purification/LC-MS from HEK-293 cells, validated by co-IP from mouse heart ventricles and 3D fluorescence microscopy in adult cardiomyocytes

   PMID:21246116

   Open questions at the time

   • Functional role of the complex was not demonstrated by loss-of-function
   • Whether the complex exists in non-cardiac tissues was not addressed
5. 2013 High

   Establishing disease causality: missense mutations spanning the ABD, rod, and CaM-like domains of ACTN1 were shown to cause autosomal dominant congenital macrothrombocytopenia by disorganizing the actin cytoskeleton in megakaryocytes and impairing proplatelet formation.

   Evidence Whole-exome sequencing of multiple pedigrees, Sanger validation, functional studies in CHO cells, COS-7 cells, and lentiviral transduction of fetal liver megakaryocytes with immunofluorescence and EM

   PMID:23434115 PMID:24069336

   Open questions at the time

   • Precise structural mechanism by which each domain mutation disrupts actin crosslinking was not resolved
   • Patient megakaryocytes were not studied in vivo
6. 2014 Medium

   Expanding the genotype-phenotype spectrum: additional rod-domain and other novel ACTN1 variants were confirmed to cause thrombocytopenia, with rod-domain mutations proposed to impair antiparallel dimerization rather than direct actin binding.

   Evidence Sequencing, segregation analysis, in vitro expression of variants, and actin fiber imaging across multiple families

   PMID:25361813 PMID:26453073 PMID:31237726

   Open questions at the time

   • Direct biophysical evidence for impaired dimerization was not provided
   • Genotype–phenotype correlation for platelet size (macro vs. normal) is not fully explained
7. 2019 High

   Revealing that ACTN1 protein levels must be precisely controlled during myogenesis: Cullin-3 E3-ubiquitin ligase was identified as the degradation machinery for ACTN1, and its loss caused ACTN1 accumulation, defective myoblast fusion, and neuromuscular junction malformation.

   Evidence Cullin-3 knockout mouse model with proteomic analysis, C2C12 ACTN1 overexpression phenocopying KO defects, immunostaining

   PMID:30990797

   Open questions at the time

   • The specific Cullin-3 adaptor (BTB protein) mediating ACTN1 recognition was not identified
   • Whether the same degradation pathway operates in non-muscle tissues was not tested
8. 2021 Medium

   Linking ACTN1 to oncogenic signaling: ACTN1 was shown to promote hepatocellular carcinoma growth by physically interacting with MOB1, competitively reducing LATS1/YAP phosphorylation, and thereby suppressing Hippo pathway tumor suppression.

   Evidence Co-immunoprecipitation, knockdown in HCC cells, xenograft models, pharmacological YAP inhibitor rescue

   PMID:33413564

   Open questions at the time

   • Whether the MOB1 interaction is direct or bridged by actin was not clarified
   • Relevance beyond hepatocellular carcinoma was not tested
9. 2023 Medium

   Detailing integrin-dependent oncogenic circuits: ACTN1 was found to activate FAK/PI3K/AKT and β-catenin signaling in HNSCC via integrin β1 interaction, promote MYH9-dependent GSK-3β degradation, and interact with ITGA5 to drive EMT, with a β-catenin/c-Myc positive feedback loop sustaining ACTN1 transcription.

   Evidence Co-IP/MS, dual-luciferase reporter, PDX and xenograft models, siRNA knockdown with ITGA5 rescue

   PMID:36742137 PMID:38057867

   Open questions at the time

   • Whether actin-crosslinking activity is required for these signaling functions was not tested
   • Contribution of ACTN1-ITGA5 vs. ACTN1-ITGB1 in the same tumor context was not separated
10. 2025 Medium

    Identifying opposing post-translational control: USP14 deubiquitinase was shown to stabilize ACTN1 by removing ubiquitin chains, and its pharmacological inhibition reduced ACTN1 levels and suppressed mesenchymal GBM progression, complementing the Cullin-3-dependent degradation axis.

    Evidence Co-IP, ubiquitination assays, USP14 inhibitor IU1 treatment in vitro and intracranial xenografts

    PMID:41291211

    Open questions at the time

    • Ubiquitin chain type (K48 vs. K63) on ACTN1 was not specified
    • Whether USP14 acts on ACTN1 in non-cancer contexts was not determined
11. 2025 Medium

    Demonstrating a non-redundant role in cardiac sarcomere assembly: ACTN1 depletion in iPSC-derived cardiomyocytes disrupted Z-line formation, reduced focal adhesion number and vinculin stability, and could not be rescued by ACTN2, establishing ACTN1 as essential for force transmission during early sarcomere organization.

    Evidence siRNA knockdown with isoform-specific rescue, live-cell imaging of vinculin/paxillin dynamics, focal adhesion morphometry in hiPSC-CMs (preprint)

    PMID:bio_10.1101_2025.03.28.645933

    Open questions at the time

    • Awaits peer review
    • In vivo cardiac phenotype of ACTN1 loss has not been reported
    • Mechanism distinguishing ACTN1 from ACTN2 at Z-lines is not resolved
12. 2025 Medium

    Connecting ACTN1 to innate immune homeostasis: STAT5A was identified as a transcriptional regulator of ACTN1, and loss of ACTN1-dependent actin architecture was shown to corral mitochondria, impair DRP1 recruitment, and trigger cGAS-STING/IFN-β signaling—all rescued by ectopic ACTN1 expression.

    Evidence STAT5A/B knockout cells, ACTN1 rescue, live-cell and mitochondrial imaging, cGAS-STING pathway analysis (preprint)

    PMID:bio_10.1101_2025.05.26.656095

    Open questions at the time

    • Awaits peer review
    • Whether the STAT5A-ACTN1 axis is tissue-specific is unknown
    • Direct STAT5A binding to the ACTN1 promoter was not demonstrated by ChIP
13. 2026 Medium

    Defining a functional partnership in vascular biology: PDLIM5 was shown to interact with ACTN1 via specific residues to promote F-actin bundling, filopodia formation in endothelial tip cells, and tumor angiogenesis.

    Evidence Endothelial-specific conditional Pdlim5 KO, mutagenesis of interaction residues, co-IP, actin bundling assays, in vivo tumor angiogenesis models

    PMID:41605926

    Open questions at the time

    • Relative contribution of ACTN1 vs. ACTN4 in this complex was not fully separated
    • Whether ACTN1 is required cell-autonomously in endothelial cells was not tested by ACTN1 KO

• Key unresolved questions include the structural basis for how individual disease mutations alter ACTN1 dimerization and actin bundling, the identity of the Cullin-3 BTB adaptor that recruits ACTN1 for degradation, and whether the signaling functions of ACTN1 in cancer (Hippo, β-catenin, FAK/AKT) require its actin-crosslinking activity or represent scaffolding functions independent of F-actin organization.
• No high-resolution structure of full-length ACTN1 with disease mutations
• Cullin-3 substrate adaptor for ACTN1 not identified
• Separation of crosslinking vs. scaffolding functions not achieved