Grand average of the ventral disc microtubule-microribbon complex.

<p>Selected 0.776 nm XY-slices from the grand average (inset shows the location of each slice of the grand average). <b>A</b>) Microribbons are made of three sheets; axis-facing (A), inner sheet (I), and margin-facing (M). The overall microribbon structure shows a distinct 16 nm repeat over two successive αβ-tubulin dimers. Crossbridges (CB) are best visible on the margin-facing sheet but most likely contact the axis-facing sheet on the other side. <b>B</b>) Slice through the dorsal-facing microtubule wall near the microribbon attachment point showing the α- and β-tubulin densities and the regions of the side-arms (SA) forming an axial 8 nm repeat. The white line shows the characteristic pseudo-helix of neighboring protofilaments (PF6–9), which are composed of α- and β-tubulin subunits at a 4 nm spacing. <b>C</b>) The lumen (L) of the microtubule is empty. The associated side-arms are connected laterally (arrow). <b>D</b>) A slice through the most ventral microtubule-associated densities (gMAP1, gMAP2, and P) reveals the position of the microtubule seam—the offset is between gMAP1 and the paddle (P), but there is no offset between gMAP2 and gMAP1. Scale bar = 5 nm.