Summary
1. Thrombosthenin A, the actin-like component of the contractile protein of blood platelets, was prepared in its monomeric or globular form and compared by gel electrophoresis with actin from rabbit striated muscle. Both proteins migrate at the same speed.
The ultrastructure of the fibrillar form of thrombosthenin A was studied by negative contrast technique in the electron microscope. The pictures reveal a helicoidal, double-stranded, beaded structure similar to the fibrillar form of muscle actin. From measurements of periodicity it was concluded that the protein unit must have a diameter of approximately 35 Å.
2. The structure of thrombosthenin in its superprecipitated form, also on negative contrast pictures, shows the presence of the fibrillar form of thrombosthenin A and of large, spindle-like aggregates of thrombosthenin M.
3. Thrombosthenin in the precipitated and superprecipitated form was fixed and examined on ultra-thin sections. The precipitate is built up of fine, evenly distributed strands due mainly to the presence of the fibrillar form of thrombosthenin A; the super-precipitate shows an obvious contraction of the protein material made up of thin filaments of thrombosthenin A and spindle-like, well contrasted large needles of thrombosthenin M.
4. Glycerol-extracted normal, human platelets were exposed to conditions favoring precipitation and superprecipitation of thrombosthenin : it was possible to localize the contractile material in the platelet cytoplasm on the basis of its characteristic structure in the contracted state.
5. Normal platelets were fixed and examined by means of electron microscopy: the complex structure of the marginal region is described and possible relationships between its structure and the contractile protein are discussed.
