Structural Changes in Myosin Binding Protein C in Different Biochemical Conditions

Myosin binding protein C (MyBP-C) is an accessory protein of the thick filaments of vertebrate striated muscle. Its function in skeletal muscle is not yet understood, but it is known to modulate contractility in the heart. Defects in MyBP-C lead to both skeletal and cardiac muscle disease. MyBP-C is an elongated molecule consisting primarily of a linear array of 10 (skeletal) or 11 (cardiac) immunoglobulin- and fibronectin-like 10 kDa domains. In addition to binding to the thick filament via its C-terminus, MyBP-C can also interact with myosin subfragment-2 and with actin via its N-terminus. MyBP-C is observed in EM images of sarcomeres as a set of up to nine transverse stripes, 43 nm apart, in each half of the A-band. Electron tomography of skeletal muscle in the relaxed state suggests that these stripes are due to extension of MyBP-C from the surface of the thick filament to the thin filaments, to which it binds. To investigate whether MyBP-C might change structurally in different physiological states, we have carried out experiments on isolated myofibrils and A-segments (A-bands free of thin filaments) to determine the appearance of the stripes under different biochemical conditions. Preliminary results suggest that in myofibrils (where thin filaments overlap thick filaments), stripes are present in both relaxed and rigor states, although they are less prominent in rigor. In A-segments (lacking thin filaments), stripes are common in the relaxed and activated (ATP/high Ca2+) states, but weak or absent in rigor. The results are consistent with a model in which the N-terminus of MyBP-C binds to actin filaments when they are available, and to myosin subfragment-2 when they are not.