Myosin binding protein C, a phosphorylation-dependent force regulator in muscle that controls the attachment of myosin heads by its interaction with myosin S2.
Myosin binding protein C (MyBP-C) is one of the major sarcomeric proteins involved in the pathophysiology of familial hypertrophic cardiomyopathy (FHC). The cardiac isoform is tris-phosphorylated by cAMP-dependent protein kinase (cAPK) on beta-adrenergic stimulation at a conserved N-terminal domain (MyBP-C motif), suggesting a role in regulating positive inotropy mediated by cAPK. Recent data show that the MyBP-C motif binds to a conserved segment of sarcomeric myosin S2 in a phosphorylation-regulated way. Given that most MyBP-C mutations that cause FHC are predicted to result in N-terminal fragments of the protein, we investigated the specific effects of the MyBP-C motif on contractility and its modulation by cAPK phosphorylation. The diffusion of proteins into skinned fibers allows the investigation of effects of defined molecular regions of MyBP-C, because the endogenous MyBP-C is associated with few myosin heads. Furthermore, the effect of phosphorylation of cardiac MyBP-C can be studied in a defined unphosphorylated background in skeletal muscle fibers only. Triton skinned fibers were tested for maximal isometric force, Ca(2+)/force relation, rigor force, and stiffness in the absence and presence of the recombinant cardiac MyBP-C motif. The presence of unphosphorylated MyBP-C motif resulted in a significant (1) depression of Ca(2+)-activated maximal force with no effect on dynamic stiffness, (2) increase of the Ca(2+) sensitivity of active force (leftward shift of the Ca(2+)/force relation), (3) increase of maximal rigor force, and (4) an acceleration of rigor force and rigor stiffness development. Tris-phosphorylation of the MyBP-C motif by cAPK abolished these effects. This is the first demonstration that the S2 binding domain of MyBP-C is a modulator of contractility. The anchorage of the MyBP-C motif to the myosin filament is not needed for the observed effects, arguing that the mechanism of MyBP-C regulation is at least partly independent of a "tether," in agreement with a modulation of the head-tail mobility. Soluble fragments occurring in FHC, lacking the spatial specificity, might therefore lead to altered contraction regulation without affecting sarcomere structure directly.
Kunst
G
G
Department of Anaesthesiology, University of Heidelberg, Heidelberg, Germany.
Kress
K R
KR
Gruen
M
M
Uttenweiler
D
D
Gautel
M
M
Fink
R H
RH
D002118:Calcium / Q000502:physiology; D002352:Carrier Proteins / Q000378:metabolism / Q000502:physiology*; D004548:Elasticity; D006652:Histological Techniques; D007537:Isometric Contraction / Q000502:physiology; D007700:Kinetics; D018485:Muscle Fibers, Skeletal / Q000502:physiology; D018482:Muscle, Skeletal / Q000378:metabolism*; D009200:Myocardial Contraction / Q000502:physiology*; D009218:Myosins / Q000502:physiology*; D010446:Peptide Fragments / Q000502:physiology*; D010766:Phosphorylation; D011994:Recombinant Proteins / Q000378:metabolism; D012518:Sarcomeres / Q000378:metabolism*; D012995:Solubility; D000099163:Myosin Binding Protein C
D002118:Calcium; D002352:Carrier Proteins; D009218:Myosins; D010446:Peptide Fragments; D011994:Recombinant Proteins; D000099163:Myosin Binding Protein C