An electron microscopic and optical diffraction analysis of the structure of Limulus telson muscle thick filaments.

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Abstract
Long, thick filaments (> 4.0 .mu.m) rapidly and gently isolated from fresh, unstimulated Limulus muscle by an improved procedure were examined by EM and optical diffraction. Images of negatively stained filaments appear highly periodic with a well-preserved myosin cross-bridge array. Optical diffraction patterns of the EM show a wealth of detail and are consistent with a myosin helical repeat of 43.8 nm, similar to that observed by X-ray diffraction. Analysis of the optical diffraction patterns, in conjunction with the appearance in which the myosin cross-bridges are arranged on a 4-stranded helix, with 12 cross-bridges per turn of each helix, thus giving an axial repeat every 3rd level of cross-bridges (43.8 nm).