Figure 1.

Illustration of rotary sectioning and the different views of microtubule end structure. (A) Rotary sectioning. Left: Slices through 3D tomographs of microtubule ends are taken at various angles. The αβ-tubulin subunits of the microtubule are represented as pink and green circles, and the gray plane represents one such slice through the volume. Bright subunits are in front of the plane, faint subunits are behind it, and intermediate shaded subunits are in the slice. Right: View of the resulting slice, showing the end structure. The vertical head-to-tail assemblies of αβ-tubulin are called protofilaments. (B) Conflicting models for how microtubules grow. Left: Sheet-like, partially curved extensions on a subset of protofilaments. Right: All protofilaments elongate independently and are fully curved. (C) Cartoon of a shrinking microtubule end, with the ends of protofilaments fully curved.