Myosin Binding Proteins of the Thick Filament: In Positions of Power
Myosin is a hexameric protein that consists of two heavy chains (MHC) and two pairs of light chains. The heavy chains dimerize forming a coiled-coil helix that makes up the light meromyosin or “rod” segment of myosin. Each MHC unwinds from its partner near its N-terminus and folds separately into one of the two catalytic S1 cross-bridge “heads” of a myosin molecule. At the tail end of each S1 head the MHC forms a long 8.5 nm α-helical segment, the lever arm domain, whose movement transduces energy from the hydrolysis of ATP into motion and force. Two light chains, i.e., one each essential light chain (ELC) and regulatory light chain (RLC), bind in tandem to the lever arm domain and thus occupy key positions that not only stabilize the lever arm8 but also act to position the myosin heads relative to the myosin rod and the actin filament9. By orienting the myosin heads at this juncture, the light chains thus can modulate the speed and force of contraction as in cardiac muscle10 or, in the case of regulated myosins such as those of smooth and invertebrate muscle, to turn contraction “on” or “off” altogether in response to a rise in intracellular Ca2+ as Ca2+ either binds directly to a trimeric complex formed by the ELC, RLC, and MHC (invertebrate muscles) or activates a Ca2+/calmodulin-dependent myosin light chain kinase that phosphorylates RLC (smooth muscle).
A single thick filament consists of ~300 myosin molecules (each consisting of 2 MHC, 2 ELC, and 2 RLC) that assemble with their rods facing inward to form a bipolar filament. Multiple thick filaments are aligned to create A (anisotropic)-bands such that thick filament rod segments delineate a bare zone devoid of cross-bridges (the H-zone) centered at the M-line in sarcomeres (Figure 1). Additional myosin binding proteins bind along the length of the thick filament starting at the M-line and repeat outward toward the Z-lines at intervals every ~42 nm, or roughly coincident with every third crown of myosin heads emerging from the thick filament11. Myosin binding protein-C (MyBP-C)12, the best known of these myosin binding proteins, is localized to a series of 9 of these positions on each side of the A-band11, 13. Thus, cMyBP-C is present at a limited stoichiometry with respect to myosin with ~1 MyBP-C for every 9-10 myosin molecules (~37 MyBP-C molecules per thick filament) or 2-4 MyBP-C molecules at each position12. At each position MyBP-C is anchored to the thick filament through three domains at its C-terminus that bind to both myosin and titin14-16. Titin, the giant protein that spans each half sarcomere from Z-disk to M-line, also binds to myosin along the length of the thick filament16.
While the positions of ELC and RLC relative to myosin are known from the S1 crystal structure8, the precise arrangement of cMyBP-C with respect to myosin is still uncertain. The C-terminus of cMyBP-C has alternately been proposed to encircle the thick filament in a trimeric collar arrangement17 or to extend linearly along the thick filament backbone18. In addition, a second binding site near the N-terminus of MyBP-C can also bind to myosin S2, the hinge segment that joins the catalytic heads to the myosin rod19. If so, then MyBP-C, like ELC and RLC, binds to myosin near the critical junction where the lever arm domain meets the myosin rod. Binding of cMyBP-C at this position has been proposed to limit the extension of myosin heads away from the thick filament backbone, thereby limiting interactions of MyBP-C with actin20. On the other hand, the same region of cMyBP-C can also bind reversibly to actin21, consistent with previous observations that MyBP-C binds to actin and thin filaments22, 23 and with conclusions from modeling of X-ray diffraction data suggesting that MyBP-C interacts with thin filaments in muscle18. If so, then cMyBP-C interactions with actin could be similar to those proposed for myosin ELC since interactions of the ELC N-terminus with the thin filament may also limit cross-bridge cycling and shortening velocity24. In either case, it is clear that thick filament associated proteins are well positioned to influence myosin cross-bridge kinetics.
