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Electron Tomography of Insect Flight Muscle in Isometric Contraction and Rigor States

Electron microscopy is a technology fully capable of bridging the light microscopic world with the world of protein structure at atomic resolution. There are numerous examples of cases where cryoelectron microscopy has been used to fit an atomic structure into a structure that has been difficult to crystallize for high resolution structure analysis. More difficult is the fitting of atomic structures into more conventionally prepared tissue like that used in freeze substitution. We have been using electron tomography to obtain 3-D images of insect flight muscle in various states, including those produced in an active isometric contraction as well as in rigor. These tomograms were obtained using non-uniform tilt angles, cross correlation alignment, area matching and Whittaker-Shannon interpolation of the 3-D transform. The fitting of atomic structures into raw tomograms is a difficult proposition because of the relatively highly noisy feature of the images. Better results are obtained when averages are produced. We have used two method for producing averages. One of these, the so-called column average is used when the muscle filaments are relatively straight with good axial order, or can be straightened in 3-D but their lateral ordering is poor. Column averaging improved considerably reduced the noise in tomograms of contracting muscle enabling the four major crossbridge conformation to be observed (Fig. 1).