Pseudo-atomic structural models docked into density maps. Panels (a) - (f) show external views of the structures for MT 11-3, MT 12-3, MT 13-3, MT 14-3, MT 15-4, and MT 16-4, respectively. Helices H12, H11′ and H3′ are marked in (e). Panels (g) - (l) show the same structures as in the (a) - (f), but from the lumen side. Density features in the inter-PF regions are consistent across the six structures, indicating a structural conservation of interactions. A ring-shaped density feature for the inter-PF interaction is indicated by red arrows in (h). A homology model of the missing H1′-S2 loop has been included in the fitting for α-tubulin.