Mass determination of native smooth muscle myosin filaments by scanning transmission electron microscopy

The thick filaments of vertebrate smooth muscle have a fundamentally different arrangement of myosin molecules from the bipolar, helical organization present in striated muscle filaments. This side-polar, non-helical structure is probably critical to the ability of smooth muscles to shorten by large amounts; however, details of myosin organization beyond this general description are unknown. The non-helical arrangement of myosin precludes the use of helical reconstruction methods for structural determination, and a tomographic approach is required. As a first step towards this goal we have determined the number of myosin molecules present at each 14.5 nm repeat in native smooth muscle myosin filaments by scanning transmission electron microscopy. The mass-per-length of myosin filaments was 159 kDa/nm, corresponding to 4.38(+/-0.11) (mean+/-s.e.m.) myosin molecules at each 14.5 nm level. The mass of thin filaments in the preparation (intrinsic control) was 21 kDa/nm, consistent with current models of smooth muscle thin filament structure, and the mass of tobacco mosaic virus (mass standard) was within 5% of the known value. We conclude that native smooth muscle myosin filaments contain four myosin molecules at each 14.5 nm level, two on each side of the side-polar structure.