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Tubulin Structure and the Development of Electron Crystallography

The identification of tubulin as the major protein component of microtubules (1) occurred at about the same time, a little over thirty years ago, as the first demonstration of the ability to obtain 3- dimensional data from electron micrographs. DeRosier and Klug's paper on image reconstruction of the bacteriophage tail (2) marks what many consider the beginning of electron crystallography - determining protein structures by electron microscopy. Microtubules have long been the target of active study by light and electron microscopy, and even before the discovery of tubulin played a significant role in driving technical developments in EM. The demonstration that tubulin could form two-dimensional crystalline sheets (3) seemed to pave the way for high resolution .structural studies. The discovery of the sheet polymer of tubulin came shortly after Unwin and Henderson's first publications on bacteriorhodopsin showing secondary structure resolved by EM (4). Application of their methods to tubulin in the late 1970's resolved low resolution features of the protein, but technical problems prevented achieving much more than showing the molecular envelope. Many developments in both tubulin studies and electron crystallography followed this work over the next 15 years. As techniques were refined for purifying, labeling and visualizing tubulin and microtubules, the dynamic properties of microtubules and their role in the cell cycle were characterized, and some hints about the mechanism of polymerization and disassembly were derived from biophysical and microscopic investigations.