Figure 3. Capped MT minus ends purified from X. laevis egg extract are asymmetric and feature breaks in inter-protofilament and protofilament-γ-tubulin contacts.

(A) Cryo-EM reconstruction of the γ-TuRC-capped MT minus end, resulting from MT-dominated refinement. (B) Atomic models of a protofilament-wise fitted MT and the partially closed γ-TuRC (Fig. 2B) displayed in the density. Numbering is shown for γ-TuRC spokes; colour scheme is indicated at the bottom. (C) Distance from the MT axis for each protofilament for the MT at capped minus ends (green) or an ideal 13 protofilament MT (grey). (D) Molecular surfaces are shown for γ-tubulins (filled) and the first α-tubulin (outlined) of each protofilament. View along the MT axis; γ-tubulin of spoke 1 was omitted for clarity. (E) Lateral offsets (perpendicular to the MT axis) between γ-tubulin and the first α-tubulin of the respective protofilament. Parameters in (C) and (E) are measured from the respective subunit’s centre of mass. (F) Left: Surface representation of the MT seam in a typical 13 protofilament MT, where lateral contacts form between α- and β-tubulin (PDB 6EW0 (Manka and Moores, 2018)). Right: Surface representation of the seam location in the minus end of capped MTs. Dashed boxes indicate the location of the seam (left) or where the seam will form (right) further along the length of the MT or after potential release from the γ-TuRC. Colouring scheme as in panel (B). Source data are available online for this figure.