Figure 3.

Scale drawings of the actin filament, intermediate filament, and microtubule. The actin filament and microtubule models are based on reconstructions of cryo-electron micrographs. The model of the intermediate filament is based on measurements of mass per unit length and characterization of the oligomeric intermediates that anneal to form the filament. Actin filaments and microtubules are polar; the names of their ends are indicated. Intermediate filaments are not polar because they assemble from bipolar tetramers of helical polypeptides. The dark green represents zones where pairs of coiled coils overlap to make antiparallel tetramers. (The actin filament and microtubule are courtesy of Graham Johnson. The intermediate filament model is reprinted from Herrmann and Aebi 2016.)