Arrangement of subunits in flagellar microtubules

Abstract

Electron micrographs of outer doublet tubules from flagella were analysed by methods which make use of the computed diffraction patterns of electron microscope images. Analysis of singlet A tubules in the tips of flagella has led to a determination of the helical surface lattice of the A subfibre, confirming that there are 13 longitudinal protofilaments in the tubule wall and that dimers in neighbouring protofilaments form a staggered arrangement, equivalent to the lattice with an axial periodicity of 8.0 nm predicted in earlier work. A low resolution 3 dimensional image of the A tubule was reconstructed, which supports the evidence for an 8.0 nm long heterodimer oriented along the protofilaments. The heterodimer is identified as a pair of 4.0 nm morphological units, which appear to be globular at this resolution. Filtered images were obtained from doublet tubules which show that the B subfibre is also made up of 8.0 nm dimers, but it differs from the A tubules in that dimers in adjacent filaments are not in a staggered arrangement but are lined up obliquely at a shallow angle. Using the additional information about the hands of the lattices in the 2 subfibres which is presented, a model for the whole doublet is proposed.