Myosin thick filaments from adult rabbit skeletal muscles

Abstract

Myosin subfragment 1 (S1) forms dimers in the presence of Mg 2+ or MgADP or MgATP. The entire myosin molecule forms head–head dimers in the presence of MgATP. The angle between the two subunits in the S1 dimer is 95°. Assuming that the length of the globular part of S1 is ≈12 nm and that the S1/S2 joint (lever arm ≈7 nm) is clearly bent, the cylinder tangent to this dimer should have a diameter of ≈18 nm, close to the ≈16–20 nm suggested by many studies for the diameter of thick filaments in situ. These conclusions led us to re-examine our previous model, according to which two heads from two opposite myosin molecules are inserted into the filament core and interact as dimers. We studied synthetic filaments by electron microscopy, enzyme activity assays, controlled digestion and filament–filament interaction analysis. Synthetic filaments formed by rapid dilution in the presence of 1 mM EDTA at room temperature (≈22°C) had all their myosin heads outside the backbone. These filaments are called superfilaments (SF). Synthetic filaments formed by slow dilution, in the presence of either 2 mM Mg 2+ or 0.5 mM MgATP and at low temperature (≈0°C) had one myosin head outside the backbone and one head inside. These filaments are called filaments (F). Synthetic filaments formed by slow dilution, in the presence of 4 mM MgATP at low temperature (≈0°C) had most of their heads inserted in the filament core. These filaments are called antifilaments (AF). These experimental results provide important new information about myosin synthetic filaments. In particular, we found that myosin heads were involved in filament assembly and that filament–filament interactions can occur via the external heads. Native filaments (NF) from rabbit psoas muscle were also studied by enzyme assays. Their structure depended on the age of the rabbit. NF from 4-month-old rabbits were three-stranded, i.e. six myosin heads per crown, two of which were inside the core and four outside. NF from 18-month-old rabbits were two-stranded (similar to F).