Thin Filament Detail

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Thin filament detail within the global average of repeat subvolumes from an electron tomogram of isometrically contracting insect flight muscle after alignment and averaging. The specimen used for the tomography has been smash frozen against a liquid helium cooled copper block, freeze substituted, embedded, sectioned and stained. (A) Global average of originally extracted repeats prior to subvolume alignment. From left to right are a central section of the initial global average, a surface view in the same direction as the original tomogram and a surface view from a direction along the inter-thick-filament axis showing the paired bulges due to troponin. Since this is a stained specimen, regions of protein (stain rich and thus low electron translucency) are dark in this representation (low electron counts) and regions rich in embedding medium, and thus high electron translucency are bright (high electron counts). (B) Global average after subvolume alignment is completed. The 2.75 nm stagger on different sides of the actin filament is now visible as is the different spacing separating troponin densities on opposite sides of the actin filament. The negative stain effect in the specimen is now pronounced. In the surface view, middle, both the troponin densities and the myosin heads have increased definition. The appearance suggests the presence of four myosin heads, two on each actin long pitch helical strand. Viewed along the filament axis, the difference in spacing of the troponin densities on the front and back surfaces (left, red, and right, blue, in this view) is easily visible. From Wu et al., 2009, J Struct Biol 168: 485-502.

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