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 i nvolved in the pathophysiology of familial hypertrophic cardiomyopathy (FHC ). The cardiac isoform is tris-phosphorylated by cAMP-dependent protein kin ase (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 segmen t of sarcomeric myosin S2 in a phosphorylation-regulated way. Given that mo st MyBP-C mutations that cause FHC are predicted to result in N-terminal fr agments of the protein, we investigated the specific effects of the MyBP-C motif on contractility and its modulation by cAPK phosphorylation. The diff usion of proteins into skinned fibers allows the investigation of effects o f defined molecular regions of MyBP-C, because the endogenous MyBP-C is ass ociated with few myosin heads. Furthermore, the effect of phosphorylation o f 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, Ca2+/force relation, rigor force, and stiffness in the ab sence and presence of the recombinant cardiac MyBP-C motif. The presence of unphosphorylated MyBP-C motif resulted in a significant (1) depression of Ca2+-activated maximal force with no effect on dynamic stiffness, (2) incre ase of the Ca2+ sensitivity of active force (leftward shift of the Ca2+/for ce 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 demonstrat ion that the S2 binding domain of MyBP-C is a modulator of contractility. T he anchorage of the MyBP-C motif to the myosin filament is not needed for t he observed effects, arguing that the mechanism of MyBP-C regulation is at least partly independent of a "tether," in agreement with a modulation of t he head-tail mobility. Soluble fragments occurring in FHC, lacking the spat ial specificity, might therefore lead to altered contraction regulation wit hout affecting sarcomere structure directly.