ABSTRACT
Microfilaments consisting of the protein actin form contractile assemblies with the protein myosin to cause cytoplasmic motion. Actin and myosin are also major components of muscle fibers. The intermediate filaments of the cytoskeleton contain a variety of proteins including keratin and have various functions, including strengthening cell structures. In all cases, the energy for motion is provided by the coupled hydrolysis of ATP or guanosine 5-triphosphate (GTP).
