Overview of the Cytoskeleton from an Evolutionary Perspective

Abstract

Organisms in the three domains of life depend on protein polymers to form a cytoskeleton that helps to establish their shapes, maintain their mechanical integrity, divide, and, in many cases, move. Eukaryotes have the most complex cytoskeletons, comprising three cytoskeletal polymers-actin filaments, intermediate filaments, and microtubules-acted on by three families of motor proteins (myosin, kinesin, and dynein). Prokaryotes have polymers of proteins homologous to actin and tubulin but no motors, and a few bacteria have a protein related to intermediate filament proteins.

Figure 1.

Fluorescence micrograph of a cultured fibroblast stained with fluorescent phalloidin (blue) and antibodies to microtubules (green) and intermediate filaments (red). Individual microtubules are large enough to be resolved by light microscopy, but actin filaments (blue) and intermediate filaments are too small and too densely packed to be resolved. In cells with desmosomes or hemidesmosomes, intermediate filaments extend to the periphery of the cell and anchor these adhesive structures. (Courtesy of Harald Herrmann, University of Heidelberg, Germany.)

Figure 2.

Structures of (A) an actin monomer, (B) a tubulin dimer, and (C) a keratin heterodimer. Ribbon diagrams compare the building blocks of the three cytoskeletal polymers. The scale of the keratin is 20% that of actin and tubulin to fit this long molecule into the figure. (A,B, Reprinted from Pollard and Earnshaw 2008; C, reprinted from Herrmann and Aebi 2016.)

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.)