ALPK3

ALPK3 is a cardiac-enriched pseudokinase that functions as a structural scaffold required for sarcomere integrity and cardiomyocyte architecture, with loss of function producing cardiomyopathy (PMID:21441111, PMID:36321451). Phosphoproteomic analysis upon kinase-domain inhibition and overexpression detected no catalytic phosphotransferase activity, establishing ALPK3 as a pseudokinase that instead acts by co-localizing with the myomesin force-buffering proteins MYOM1 and MYOM2 at the sarcomere M-band and nuclear envelope; loss-of-function variants mislocalize myomesins and dysregulate additional M-band proteins controlling sarcomere protein turnover (PMID:36321451). Through this scaffolding role ALPK3 maintains sarcomeric proteostasis, anchoring MuRF E3 ubiquitin ligases and SQSTM1/p62 at the M-band, and its loss displaces myomesins and drives thick-filament protein aggregation (PMID:41221624). ALPK3 deficiency disorders sarcomeres and intercalated discs and causes abnormal calcium handling in both knockout mice and isogenic human stem cell-derived cardiomyocytes (PMID:21441111, PMID:27106955). A truncating knock-in model links ALPK3 to contractile regulation, showing prolonged relaxation, elevated diastolic calcium, reduced protein kinase A-mediated phosphorylation, and a decreased myosin super-relaxed state that is partially corrected by the myosin inhibitor mavacamten (PMID:40128237). AAV-mediated delivery of full-length ALPK3 restores contractile function in human cardiac organoids and mouse models carrying ALPK3 truncating mutations [PMID:bio_10.1101_2025.07.31.667858].

1. 2001 Medium

   Established the first cellular context for ALPK3 (Midori), asking where the protein acts and whether it influences cardiac cell fate.

   Evidence Differential display, immunolocalization, and gain/loss-of-function in P19CL6 cells

   PMID:11418590

   Open questions at the time

   • Proposed transcriptional regulator role rests on overexpression autoinduction, not direct DNA binding
   • Performed in a single cell-line differentiation model
2. 2011 High

   Tested whether ALPK3 is required in vivo, showing its loss causes cardiomyopathy with intercalated disc and myofibrillar defects.

   Evidence Knockout mouse model with MRI, histology, and electron microscopy

   PMID:21441111

   Open questions at the time

   • Did not define the molecular function underlying the structural defects
   • Mixed hypertrophic/dilated phenotype mechanism unresolved
3. 2016 High

   Confirmed loss of function as the disease mechanism in human cells and identified calcium handling as an affected process.

   Evidence Patient iPSC-derived and CRISPR-engineered isogenic human ESC cardiomyocytes with EM, multi-electrode array, and calcium imaging

   PMID:27106955

   Open questions at the time

   • Did not identify molecular partners or the basis of calcium dysregulation
   • Catalytic status of ALPK3 not addressed
4. 2022 High

   Resolved the long-standing assumption that ALPK3 is an active kinase, establishing it as a pseudokinase that scaffolds myomesins at the M-band and nuclear envelope.

   Evidence Alpha-kinase domain alignment, phosphoproteomics under kinase inhibition/overexpression, and co-localization in isogenic iPSC-CMs, mouse models, and patient tissue

   PMID:36321451

   Open questions at the time

   • Structural basis of myomesin anchoring not defined
   • Whether scaffolding is direct binding or indirect not established
5. 2022 Low

   Placed ALPK3 in a hypertrophy regulatory circuit as a downstream target whose suppression mitigates pressure-overload hypertrophy.

   Evidence ISO/Ang-II hypertrophy models with miR-384-5p overexpression, RT-qPCR, and echocardiography

   PMID:35510648

   Open questions at the time

   • No direct miR-384-5p:ALPK3 binding or reporter assay shown
   • Inferred from expression correlation in a single lab
6. 2025 High

   Linked ALPK3 to active contractile regulation, connecting its loss to PKA signaling, calcium handling, and myosin super-relaxed state, and demonstrated pharmacological correction.

   Evidence K201X knock-in mice with sarcomere length/relaxation, calcium imaging, phosphorylation assays, myosin SRX measurement, and mavacamten rescue

   PMID:40128237

   Open questions at the time

   • Mechanism connecting ALPK3 scaffolding to reduced PKA phosphorylation unresolved
   • Whether mavacamten rescue addresses cause or downstream contractility only
7. 2025 Medium

   Demonstrated therapeutic restoration and broadened the model to a generalizable M-band quality-control scaffold relevant beyond ALPK3 mutations.

   Evidence AAV-ALPK3 delivery in human cardiac organoids and mouse models, including TTN-truncated organoids (preprint)

   PMID:bio_10.1101_2025.07.31.667858

   Open questions at the time

   • Preprint, not peer-reviewed
   • Cross-rescue of TTN organoids mechanistically uncharacterized
   • Durability and in vivo translation untested

• How ALPK3 mechanistically couples its M-band scaffolding to PKA-dependent phosphorylation and myosin regulation, and the structural basis of myomesin/MuRF/p62 anchoring, remain unresolved.
• No structural model of the scaffold complex
• Direct binding interfaces with MYOM1/MYOM2, MuRF, and SQSTM1 not mapped
• Link between scaffolding and reduced PKA signaling undefined