Unshadowed myosin molecules: STEM mass-maps of myosin heads.

Myosin molecules were directly visualized without heavy metal shadowing by scanning transmission electron microscopy (STEM) under low dose conditions. The general appearance and dimensions of heavy metal-free molecules were similar to those of shadowed myosin, either after freeze-drying without or air-drying with glycerol. Two characteristic configurations of myosin head regions were found, a first type showing two pear-shaped heads with narrow necks and a second type showing two heads connected by an extra mass in the central regulatory domain where the light chains are located. The mass of the latter type (mol. wt. = 265 +/- 39 kd) is in excellent accordance with biochemical data whereas the mass of the first type is somewhat lower (mol. wt. 219 +/- 44 kd).