The motif of human cardiac myosin-binding protein C is required for its Ca2+-dependent interaction with calmodulin

The N-terminal modules of cardiac myosin-binding protein C (cMyBP-C) play a regulatory role in mediating interactions between myosin and actin during heart muscle contraction. The so-called "motif," located between the second and third immunoglobulin modules of the cardiac isoform, is believed to modulate contractility via an "on-off" phosphorylation-dependent tether to myosin ΔS2. Here we report a novel Ca(2+)-dependent interaction between the motif and calmodulin (CaM) based on the results of a combined fluorescence, NMR, and light and x-ray scattering study. We show that constructs of cMyBP-C containing the motif bind to Ca(2+)/CaM with a moderate affinity (K(D) ∼10 μM), which is similar to the affinity previously determined for myosin ΔS2. However, unlike the interaction with myosin ΔS2, the Ca(2+)/CaM interaction is unaffected by substitution with a triphosphorylated motif mimic. Further, Ca(2+)/CaM interacts with the highly conserved residues (Glu(319)-Lys(341)) toward the C-terminal end of the motif. Consistent with the Ca(2+) dependence, the binding of CaM to the motif is mediated via the hydrophobic clefts within the N- and C-lobes that are known to become more exposed upon Ca(2+) binding. Overall, Ca(2+)/CaM engages with the motif in an extended clamp configuration as opposed to the collapsed binding mode often observed in other CaM-protein interactions. Our results suggest that CaM may act as a structural conduit that links cMyBP-C with Ca(2+) signaling pathways to help coordinate phosphorylation events and synchronize the multiple interactions between cMyBP-C, myosin, and actin during the heart muscle contraction.