Subunit Structure and Conformations of Tubulin Protofilaments

Microtubules are structural elements of eukaryotic cells which are important for cell motility, cell shape, and cytoplasmic transport (for a review see Kirschner, 1978). One of their characteristic features is the rapid assembly when the cell needs them for a specific purpose such as mitosis, and their disappearance after use. To understand their function one would therefore like to know not only the structure of microtubules but also that of microtubule precursors which might shed light on the process of assembly. The overall shape of flagellar microtubules has been studies by X-ray diffraction, electron microscopy and image proeessing techniques (Grimstone and Klug, 1966; Cohen et al., 1971; Tilney et al., 1973; Arnos and Klug, 1974; and others). They are hollow cylinders of 11 nm mean radius, consisting of 13 protofilaments parallel to the tubule axis. The protofilaments are made up of tubulin monomers of molecular weight 55,000, paired into heterodimers of alpha and beta tubulin. The axial repeat of monomers is 4 nm, their stagger in adjacent protofilaments is about 1 nm