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.
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