Optical Diffraction Studies on the Structure of Troponin-Tropomyosin-Actin Paracrystals*

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Abstract
1. In order to clarify the structure of paracrystals of actin (Hanson, 1967) and two types of paracrystals made of troponin-tropomyosin-actin complex, electron micrographs of negatively stained specimens of these paracrystals were studied by optical diffraction and optical filtering methods. 2. The actin paracrystal consists of parallel arrays of actin filaments. The filaments align in register along the filament axis, being in contact with each other at the portions between the crossover points of double strands. 3. The first type of paracrystal made of troponin-tropomyosin-actin complex shows essentially the same arrangement of actin filaments as that in the actin paracrystal. 4. The second type of troponin-tropomyosin-actin paracrystal shows clear regular transverse striations of about 380 A period, which seem to be made of troponin molecules. The period corresponds to the length of seven actin molecules. The actin subunits of neighboring filaments are not in register. 5. The structure of troponin-tropomyosin-action filament was discussed.

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