Electron micrographs of a part of two isolated thick filaments. (A) From a trabecula that had been soaked in 1.0 mM Ca to produce dephosphorylated cMyBP-C and a band at 130 kD with anti-C5 or anti-C8C9. Arrowheads point to irregular, extended myosin heads. (B) From a trabecula that had been soaked in 7.5 mM Ca to produce phosphorylated cMyBP-C and prevent a band at 130 kD with anti-C5 or anti-C8C9. Arrows point to regular myosin heads lying along filament backbone. Line indicates 70 nm.