Scanning transmission electron microscopic mass determination of in vitro self-assembled smooth muscle myosin filaments.

We have measured the mass per unit length of in vitro self-assembled smooth muscle myosin filaments, using scanning transmission electron microscope darkfield images of freeze-dried samples. The measured values were integral multiples, usually 1, 2, 3 or 4, of 75 kDa per nm. The data corroborate an earlier proposal, that these filaments are built from monolayer sheets of molecules, each sheet having two antiparallel myosin molecules per 14.3 nm of its length.