Abundance and classification of two types of actin-like filament. (A) Examples of lumenal filaments extracted from sub-volumes of 3D tomograms and projected in Z, chosen from straight sections of microtubule so that lumenal filaments can be shown over an extended length without being obscured by surrounding microtubule lattice. Images show projected sections of central regions of microtubules containing actin-like filaments and are separated into two groups (Class I and Class II) based on their morphology. Class II filaments were thicker and typically better defined. Scale bar is 25 nm. (B) Graph shows the proportion of microtubule lumen occupied by actin-like filaments (of either class) in 11 tomograms. (C) Proportion of actin-like filament lengths classified as Class I or Class II from the dataset. (D and E) Graph showing the average length of actin-like filaments (error bars represent SEM) of either class and their frequency distribution.