Both skeletal and cardiac muscles of vertebrates are of great interest because of their relevance to human physiology and disease. However, vertebrate filaments are more difficult to study than invertebrates, as they contain proteins in addition to myosin (titin, myosin binding protein C (MyBP-C)) and are much more labile. We have used single particle techniques to analyze negatively stained cardiac myosin filaments and have defined the organization of the myosin heads, titin, and MyBP-C (Zoghbi et al., 2008). Two of every three levels of heads in the filament have a similar interacting-head structure to that in the invertebrates (fitting, in left figure), although the interaction is less stable. In addition, the reconstruction shows the layout of the template protein, titin (blue beads in right figure), running along the filament surface between the myosin heads (green), and the likely organization of MyBP-C’s C-terminal domains (orange), binding to the filament backbone and to titin. The rest of MyBP-C probably extends from the thick filament towards the thin filaments. We are currently investigating the structure skeletal filaments using negative staining, cryo-EM and 3D reconstruction.
3D reconstruction of portion of vertebrate cardiac muscle thick filament showing fitting of interacting myosin heads into repeating motif (left) and myosin heads (green), titin (blue) and part of MyBP-C (orange). From Zoghbi et al., 2008.