Cryo‐electron microscopy of tropomyosin magnesium paracrystals

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Abstract
SUMMARY: Magnesium paracrystals of tropomyosin have been examined in thin films of vitreous ice by electron microscopy. This material is radiation sensitive, with damage being located near the molecular ends. However, low‐dose images show a repeating pattern of pairs of dark lines every 40 nm which indicates that the structure has been well preserved. These dark lines are separated by about 15 nm and probably represent extra protein density at the molecular ends. It has not been possible previously to see this feature by electron microscopy. We also demonstrate that the mercurial p‐chloro‐mercuribenzoate (PCMB) attached to the tropomyosin thiol at residue 190 can be detected in unstained paracrystals embedded in vitreous ice.