MYBPC3

MYBPC3 encodes cardiac myosin-binding protein C (cMyBP-C), a sarcomeric thick-filament regulatory protein that modulates cardiac contraction and relaxation by constraining myosin head proximity to actin and tuning thin-filament activation. The N-terminal domains (C0–C7) bind both the myosin S2 segment and F-actin, positioning tropomyosin to regulate Ca²⁺-dependent cross-bridge cycling, dampen spontaneous oscillatory contractions, and mediate myofilament Ca²⁺ sensitivity, while multi-site phosphorylation by PKA, CaMKII, PKC, and PKD at Ser273/Ser282/Ser302/Ser315 relieves the myosin head tether and differentially modulates contractile kinetics and tension (PMID:10024460, PMID:18599866, PMID:32078438, PMID:26718724, PMID:22636676, PMID:24606935). The C-terminal C10 domain anchors cMyBP-C to the myosin rod (LMM), and disruption of this interaction abolishes sarcomeric incorporation (PMID:25583989). MYBPC3 is the most commonly mutated gene in hypertrophic cardiomyopathy (HCM): truncating mutations undergo nonsense-mediated mRNA decay leading to protein haploinsufficiency that reciprocally augments myosin ATPase activity, reduces the super-relaxed myosin state, increases myofilament Ca²⁺ sensitivity, and impairs relaxation, while nontruncating pathogenic variants cause haploinsufficiency through defective splicing or accelerated protein degradation (PMID:19574547, PMID:30674652, PMID:15249187, PMID:34097875, PMID:32841044).

1. 1999 High

   Establishing the primary molecular interaction: the N-terminal regulatory domain of cMyBP-C binds the myosin S2 segment, and HCM mutations in S2 disrupt this interaction, providing the first direct biochemical basis for cMyBP-C's contractile regulatory function.

   Evidence In vitro binding assays with Kd measurements using recombinant cMyBP-C motif and myosin S2 fragments, plus localization in cardiomyocytes

   PMID:10024460

   Open questions at the time

   • Whether the S2 interaction is the sole or primary mechanism of cross-bridge regulation was unclear
   • In vivo relevance of the binding affinity not yet tested
   • Phosphorylation effects on this interaction not assessed
2. 2004 High

   Genetic proof that cMyBP-C dose determines cardiac phenotype: heterozygous Mybpc3 knockout mice develop asymmetric septal hypertrophy resembling human HCM, while homozygous nulls show eccentric hypertrophy with diastolic dysfunction, establishing haploinsufficiency as sufficient for disease.

   Evidence Targeted deletion of Mybpc3 exons 1–2 in mice with echocardiographic, molecular, and morphological analysis at multiple ages

   PMID:15249187

   Open questions at the time

   • Whether human MYBPC3 mutations act purely through haploinsufficiency or also via poison-peptide effects was not resolved
   • Mechanism linking reduced cMyBP-C to hypertrophic signaling not identified
3. 2006 High

   cMyBP-C is required for normal in vivo contractile reserve: null mice lack the contractile enhancement normally elicited by β-adrenergic stimulation, showing cMyBP-C is essential for the adrenergic-mediated augmentation of cardiac function.

   Evidence Pressure-volume analysis and echocardiography in cMyBP-C null versus wild-type mice with β-adrenergic challenge and aortic banding

   PMID:17122190

   Open questions at the time

   • Whether the deficit reflects loss of cMyBP-C phosphorylation specifically or a general structural disruption was unresolved
   • Molecular basis of the blunted adrenergic response not dissected
4. 2008 High

   Structural evidence for the tether model: PKA phosphorylation of cMyBP-C displaces myosin heads radially toward actin, and this displacement is entirely absent in cMyBP-C null myocardium, directly demonstrating that cMyBP-C constrains myosin head position in a phosphorylation-dependent manner.

   Evidence Synchrotron low-angle X-ray diffraction of skinned trabeculae from wild-type and cMyBP-C null mice ± PKA

   PMID:18599866

   Open questions at the time

   • Which phosphosites mediate the tether release was not resolved
   • Contribution of thin-filament binding to the structural rearrangement not assessed
5. 2009 High

   Direct confirmation of haploinsufficiency in human HCM: no truncated cMyBP-C protein is detectable in myectomy tissue from patients with MYBPC3 truncation mutations, and total cMyBP-C is reduced to ~76% of normal, establishing that truncated transcripts are degraded rather than producing poison peptides.

   Evidence Immunoblotting and RT-PCR of human myectomy samples versus donor hearts across multiple MYBPC3 mutation types

   PMID:19574547

   Open questions at the time

   • Whether the residual ~76% protein level is sufficient to prevent disease or represents an incomplete compensatory mechanism was unknown
   • Mechanism of mRNA degradation (NMD) not directly demonstrated in these samples
6. 2011 High

   Dual binding target identified: cMyBP-C N-terminal domains bind F-actin subdomain 1, positioning them to shift tropomyosin and modulate thin-filament activation, revealing a second regulatory interface beyond myosin S2.

   Evidence Negative-stain EM and 3D helical reconstruction of F-actin decorated with N-terminal cMyBP-C fragments

   PMID:21601575

   Open questions at the time

   • Functional consequence of actin binding on thin-filament activation kinetics not directly measured
   • Relative contributions of actin versus S2 binding to contractile regulation unclear
7. 2012 High

   Increased myofilament Ca²⁺ sensitivity and diastolic dysfunction are primary consequences of MYBPC3 mutation, preceding and independent of hypertrophy, reframing these as initiating events rather than secondary consequences.

   Evidence Skinned trabeculae force measurements, intact myocyte Ca²⁺ transients, and echocardiography in pre-hypertrophic heterozygous and homozygous Mybpc3 knock-in mice

   PMID:22465693

   Open questions at the time

   • Whether correcting Ca²⁺ sensitivity alone would prevent subsequent hypertrophy was not tested
   • Specific molecular mechanism by which mutant cMyBP-C increases Ca²⁺ sensitivity not identified
8. 2012 High

   PKD phosphorylates cMyBP-C at Ser315 and requires cMyBP-C to augment maximal Ca²⁺-activated tension, expanding the kinase repertoire regulating cMyBP-C beyond PKA/CaMKII.

   Evidence Co-immunoprecipitation in contracting cardiomyocytes; force measurements in permeabilized WT versus cMyBP-C KO myocytes incubated with active PKD

   PMID:22636676

   Open questions at the time

   • Physiological stimulus triggering PKD-cMyBP-C signaling in vivo not established
   • Whether PKD phosphorylation of Ser315 alone is sufficient or requires cooperative phosphorylation at other sites
9. 2013 High

   The E258K missense mutation reveals how gain-of-function at the thin filament and loss-of-function at the thick filament converge: abolishing S2 interaction while increasing actin affinity accelerates contractile kinetics and reduces force, mechanistically linking HCM missense mutations to altered dual-binding properties.

   Evidence Yeast two-hybrid for S2 binding; force measurements in engineered cardiac tissue from cMyBP-C null mice reconstituted with WT or E258K cMyBP-C

   PMID:23980194

   Open questions at the time

   • Whether other missense mutations show the same dual-binding alteration pattern was untested
   • In vivo hemodynamic consequences not assessed
10. 2014 High

    Phosphosite hierarchy established: Ser282 non-phosphorylation combined with phosphomimetic substitutions at Ser273/302 (DAD variant) is detrimental, significantly reducing isometric tension and cross-bridge kinetics, demonstrating that site-specific phosphorylation patterns have opposing functional outcomes.

    Evidence Sinusoidal analysis in papillary fibers from cMyBP-C null mice expressing transgenic phosphosite-specific mutants (ADA, DAD, SAS)

    PMID:24606935

    Open questions at the time

    • Whether this phosphorylation hierarchy exists in vivo under physiological signaling conditions was not demonstrated
    • No structural explanation for why Ser282 state determines the functional outcome
11. 2015 High

    C10 domain integrity is essential for sarcomeric anchoring: the South Asian 25-bp deletion produces a C10 variant that cannot bind myosin LMM, fails to incorporate into sarcomeres, and causes contractile dysfunction, identifying a founder mutation mechanism.

    Evidence Adenoviral expression in adult rat cardiomyocytes; co-sedimentation, cross-linking, immunofluorescence, and subcellular fractionation

    PMID:25583989

    Open questions at the time

    • Whether soluble mislocalized cMyBP-C has toxic gain-of-function effects was not addressed
    • In vivo phenotype of this specific mutation in animal models not established
12. 2015 High

    HCM missense mutation L348P shifts tropomyosin to the open position on thin filaments beyond the effect of wild-type cMyBP-C, providing structural evidence that thin-filament activation by N-terminal domains is a gain-of-function mechanism in HCM.

    Evidence EM 3D reconstruction of thin filaments decorated with L348P versus WT and phosphorylated N-terminal cMyBP-C fragments

    PMID:26718724

    Open questions at the time

    • Whether tropomyosin shift alone accounts for the Ca²⁺ sensitization phenotype not tested in intact sarcomeres
    • How phosphorylation modulates the L348P gain-of-function not determined
13. 2019 High

    The unifying biophysical mechanism: cMyBP-C depletion shifts myosin from the super-relaxed (energy-conserving) state to a disordered-relaxed (ATP-hydrolyzing) state, and the myosin ATPase inhibitor MYK-461 rescues relaxation deficits, providing a pharmacological strategy targeting the downstream consequence of haploinsufficiency.

    Evidence Genetic and biochemical myosin conformation assays in mouse and human MYBPC3-mutant cardiomyocytes; pharmacological rescue with MYK-461

    PMID:30674652

    Open questions at the time

    • Whether super-relaxed state reduction is the sole driver of hypercontractility or whether thin-filament effects contribute independently remains unresolved
    • Long-term efficacy and safety of MYK-461 in MYBPC3 mutation carriers not established
14. 2019 High

    NMD as chronic stressor: premature termination codon mutations constitutively activate nonsense-mediated decay, and this chronic NMD activation—rather than haploinsufficiency alone—causes aberrant calcium handling; NMD inhibition normalizes calcium transients, revealing a non-sarcomeric pathogenic pathway.

    Evidence Isogenic iPSC-CMs from HCM patients; transcriptome analysis; pharmacological NMD inhibition with phenotype rescue

    PMID:30586709

    Open questions at the time

    • Mechanism by which chronic NMD activation leads to calcium handling defects not identified
    • Whether NMD inhibition is therapeutically viable in vivo unknown
15. 2020 High

    Acute reconstitution proves the N-terminal domains maintain sarcomeric stability: removal of C0–C7 from sarcomeres reduces Ca²⁺ sensitivity, speeds cross-bridge cycling, and triggers spontaneous oscillatory contractions; re-ligation rescues all phenotypes, establishing that cMyBP-C N-terminal domains actively damp contractile instability.

    Evidence TEVp cleavage and SpyCatcher/SpyTag re-ligation of N-terminal domains in permeabilized cardiomyocytes from gene-edited mice

    PMID:32078438

    Open questions at the time

    • Whether SPOC occurs in intact myocardium with partial cMyBP-C depletion not tested
    • The structural basis by which N-terminal domains suppress oscillations is unknown
16. 2020 High

    Protein-level compensation: heterozygous MYBPC3 frameshift mutations reduce WT mRNA but cardiomyocytes maintain near-normal cMyBP-C protein levels by decreasing degradation rate, revealing an innate post-translational buffering mechanism.

    Evidence Stable isotope labeling (SILAC) in patient and genome-engineered iPSC-CMs measuring synthesis and degradation rates

    PMID:31877118

    Open questions at the time

    • Whether this compensatory mechanism fails under hemodynamic stress to produce haploinsufficiency in vivo is not resolved
    • Molecular sensor/pathway controlling degradation rate adjustment unknown
17. 2020 High

    Domain-specific variant pathomechanisms: C10 missense variants fail to incorporate into myofilaments and degrade ~90% faster, while C3/C6 variants incorporate normally, demonstrating that nontruncating mutations cause loss-of-function through distinct domain-dependent mechanisms.

    Evidence Immunofluorescence and degradation rate measurements for mutant MyBP-C expressed in rat ventricular myocytes

    PMID:32841044

    Open questions at the time

    • Functional consequence of incorporated C3/C6 variants on contractility not measured
    • Whether C3/C6 variants that incorporate normally act through altered binding affinities not tested
18. 2021 High

    Systematic classification of nontruncating variants: ~50% of pathogenic missense/in-frame variants cause haploinsufficiency via RNA splicing defects or protein instability, with high specificity for pathogenicity, providing a functional framework for variant interpretation.

    Evidence Minigene splicing assays and protein stability assays for 44 nontruncating MYBPC3 variants correlated with cosegregation and population data

    PMID:34097875

    Open questions at the time

    • The other ~50% of pathogenic nontruncating variants presumably act through altered function rather than haploinsufficiency, but their mechanisms are uncharacterized
    • Whether splicing and stability assays are sufficient for clinical variant classification not validated prospectively

• Key unresolved questions: (1) the structural basis of the cMyBP-C N-terminal interaction with both myosin S2 and actin simultaneously, and how phosphorylation switches between these binding modes at atomic resolution; (2) the molecular sensor that adjusts cMyBP-C degradation rate to compensate for haploinsufficiency; (3) whether thin-filament and thick-filament regulatory mechanisms of cMyBP-C are independently sufficient for disease or must converge; (4) the non-sarcomeric role of MYBPC3 in cardiac fibroblasts and its contribution to fibrosis in HCM.
• No high-resolution structure of cMyBP-C N-terminal domains bound simultaneously to S2 and actin in the sarcomeric context
• Post-translational compensation pathway linking mRNA haploinsufficiency to reduced protein degradation unidentified
• Fibroblast role described in single study requiring independent replication