William A. Voter

Electron microscopy of MAP 2 (microtubule-associated protein 2).

In this study we demonstrate by the platinum-replication technique that MAP 2 (microtubuleassociated protein 2) is a long, thin, flexible structure. The contour length of the molecule is variable ranging up to a maximum of about 185 nm. The mass per unit length is about 1630 daltons/ nm, approximately the same as a double helix. Circular dichroism of purified MAP 2 showed, however, that the alpha helix content is actually very low. The MAP 2 strands sometimes have either one or two terminal knobs. There was very little difference in the appearance of the MAP 2 prepared either with or without the inclusion of a 100°C treatment. We also present images of taxol-stabilized microtubules decorated with MAP 2 molecules showing that the latter often extend outward from the microtubule wall as thin filaments up to 80 or 90 nm in length.

[4] Image reconstruction in electron microscopy: Enhancement of periodic structure by optical filtering

Publisher Summary This chapter provides information on the image reconstruction in electron microscopy. Electron microscopy can routinely provide images of negatively stained biological specimens at a resolution of 1.5–3 nm, and of embedded and sectioned specimens at a resolution of 3–5 nm. This resolution, in particular with negative stain, is sufficient for the visualization of individual protein molecules and to show some details of subunit arrangement and shape. One approach to understanding the results of image reconstruction is to ask what each diffraction spot contributes to the image. The answer to this question follows directly from the theory of Fourier analysis and physical optics, but without going into the theory, the chapter presents a series of images reconstructed from different combinations of diffraction spots, in which the interpretation is illustrated. In addition, the optical diffraction pattern provides a direct criterion for distinguishing between the desired periodic features and the granular noise: all the information related to the periodic structures is contained in the discrete diffraction spots, whereas the noise produces a background of weak spots spread over the whole diffraction plane.