Electron Microscopic Study of Troponin

Abstract

Troponin and its components or fragments were observed in an electron microscope by the use of the rotary shadowing technique. In freshly prepared troponin with low viscosity, globular particles were mainly observed. The size of the long axis of the particles was 13.2±1.3 nm and the size perpendicular to the long axis was 9.5±1.2 nm. The mean axial ratio was 1.4±0.3. Most of the particles observed in a stored troponin preparation, having a higher viscosity than that of fresh troponin, had a globular head with a thin tail, with the total length of 25.4±1.4 nm (head-tail type particles). The axial size of the globular portion was 8.3±1.2 nm and the tail length was 17.1±1.6 nm. Observation of various particles during the transitional stages indicated that, in the globular particles, the tail region of head-tail type particle was associated along the globular head region. Troponin T was a filamentous particle with 16.9±1.5 nm length. The 26 K fragment of troponin T, which was devoid of the N-terminal 45 residues from troponin T, was a filamentous particle with the length of 14.4±1.3 nm. Troponin T₁, one of two chymotryptic subfragments of troponin T, was a filamentous particle of 11.6±1.4 nm length. Troponin C•T in the presence of Ca²⁺ was a particle with a globular head (7 nm in size) and a tail of about 17 nm length. The Fab fragment of anti-troponin T₁ formed regular transverse striations along the thin filament of rabbit skeletal muscle with a 38 nm period. The distance between the first striation and the top of the filament was less than one period length.