A. Representative low-pass filtered cryo-EM micrographs of MTs decorated with wild-type DCX (WT) or NDC-NDC construct (NN) after 30 sec or 1 h incubation at 37 °C. Additional DCX proteins were added in excess after MT deposition on EM grids to maximise MT decoration. The final ratio of WT or NN to tubulin was 50:5 μM, giving dense protein background. WT co-polymerized exclusively 13-PF MTs, while NN both 13- and 14-PF MTs. Rapid (30 sec) MT polymerization in excess of WT produces MTs in an intermediate GTP/GDP.Pi state, whereas long-lived WT-MTs and NN-MTs are all in the GDP state. 