X‐RAY PATTERNS FROM MICROTUBULES *

Electron microscopy and x-ray diffraction look at structures under different conditions. In general, there has been good agreement in relating images obtained with these methods. In the case of microtubules, however, the interpretation of the x-ray diffraction patterns in terms of the image seen in the electron microscope has not been straightforward. The results f rom the two methods have led to the proposal of different arrangements of subunits (i.e., different surface lattices) making up the microtubule. In this paper we consider this problem in the light of new experimental results. Most of our knowledge of microtubule structure has come from electron microscope observations. Microtubules from different systems appear to have certain common structural features, although they differ in stability. The tubule consists of a cylindrical shell about 250 A in diameter, with a wall thickness of about 50 A and a hollow center about 140 A in width. The wall of the tubule is made up of 13 longitudinal filaments about 50 A apart.', T h e filaments are built from globular subunits about 40 A in width which are revealed in negatively stained preparations. Fraying of the tubules into the filaments readily occurs. The surface lattice of microtubules was first described by Grimstone and Klug 3 in 1966. They used optical diffraction patterns from negatively stained protozoan flagella. A basic feature of the pattern is a 40 A layer line showing a strong maximum near the meridian. Klug and Grimstone were able to define the general features of the microtubule lattice, which was shown to be a lowpitch helix with a small vertical displacement between neighboring subunits (FIGURE l a ) . A similar structure has been found in negatively stained preparations of a variety of ciliary and flagellar microtubules.'