The microtubule lattice--20 years on

In 1974, optical diffraction and image analysis indicated that tubulin dimers in the cylindrically complete A-tubule of flagellar doublet microtubules are arranged with helical symmetry, while those in the incomplete B-tubule associate differently. Recently, electron micrographs of reassembled brain microtubules decorated with kinesin heads have shown that the tubulin dimers there are arranged as in the B-tubule. The lack of symmetry of microtubules assembled in vitro prompts Linda Amos to speculate here that the assembly process in vitro may differ from that occurring in the cell.