[The structure of microtubules]

Abstract

Cytoplasmic microtubules, plane monolayers of tubulin filaments and single filaments have been investigated by electron microscopy. The methods of optical diffraction, filtering and three-dimensional reconstruction have been used to analyse the micrographs and to construct a model of the dimeric molecule of tubulin. When viewed in the direction perpendicular to the longitudinal axes the molecule bear resemblance to the figure "8". When rotated by 90 degrees it looks as if composed of two globules. Two variants of the quoternary structure are considered: 1) each of two subunits is a deformed ring; 2) each rings of the "8"-like particles consists of two halves of different subunits. The second protein layer which is formed on microtubules in the presence of protamine consists of tubulin and is characterized by the crystal lattice similar to that of the microtubule itself, but rotated through the angle of about 95 degrees. In the presence of protamine we also detected flat spirals and stacks of such spirals forming short tubules.