Almost 50 years ago actin was identified [Straub, 1942] in an extract of muscle tissue where — together with myosin — it forms the contractile protein-complex, actomyosin. Straub recognized that actin appears in two forms: In the absence of salt it can exist as a globular protein, G-actin, whereas on addition of traces of salt it changes into a highly asymmetrical fibrous protein, F-actin. Since then actin has been intensively studied in many laboratories. It was found that actin is present in all eukaryotic cells as a major component of the cytoskeleton which defines cell shape and position of internal organelles.