The Effect of Cardiac Myosin-Binding Protein C on Calcium Regulation of the Actin–Myosin Interaction Depends on Myosin Light Chain Isoforms

Abstract—In addition to troponin and tropomyosin, cardiac myosin-binding protein C (cMyBP-C), which has an effect on the function of myosin and thin filament activation, is involved in regulation of the actin–myosin interaction in the myocardium. The β-isoform of myosin heavy chain expressed in slow skeletal muscles is identical to that in the myocardium; however, myosin isoforms in slow skeletal muscles and in cardiac muscle differ in the composition of the myosin light chain isoforms. We investigated the effect of cMyBP-C on the calcium regulation of the interaction of the myosin of slow skeletal muscle (m. soleus) with actin, using an in vitro motility assay and an optical trap. It was found that the physiological concentration of cMyBP-C resulted in increased calcium sensitivity of the sliding velocity of regulated thin filaments over myosin extracted from the slow soleus muscle and increased the velocity of thin filaments, as opposed to cardiac myosin. In the optical trap, cMyBP-C did not affect the step size of myosin but reduced the duration of a single actin–myosin interaction, thus explaining the increase in the velocity of filaments in the in vitro motility assay. Thus, the regulatory properties of cMyBP-C are exhibited in different ways depending on the composition of myosin light chain isoforms.