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(1) Within the low viscous flowing endoplasm of Physarum polycephalum a considerable amount of actin is in the non-filamentous state. This can be demonstrated by applying poly-L-lysin to surface spreads of native protoplasm. (2) It has been shown that in protoplasmic drops the endoplasm-ectoplasm transformation is accompanied by an actin polymerization from the non-filamentous state to F-actin. (3) The actual state of the labile G-F-actin equilibrium determines the varying consistency (viscosity) of the cytoplasm. (4) Increasing viscosity can be interpreted as being brought about by a) shifting of the G-F-actin equilibrium to the filamentous side, and (b) increased myosin-mediated binding sites between actin filaments. (5) Polymerization and depolymerization processes are involved in the rhythmically occurring contraction-relaxation cycle of cytoplasmic actomyosin in Physarum. (6) Cytoplasmic actin and myosin represent the architectural proteins of the contractile gel reticulum in eukaryotic cells. (7) The importance of the regulation of actin polymerization as a basic control mechanism of the eukaryotic cell is discussed.

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A special cell line derived from a rat mammary adenocarcinoma (RMCD cells) displays a distinct pattern of actomyosin fibrils (AM fibrils) visible with phase contrast, Nomarski interference and polarized light optics. It was shown that the cytoplasmic AM fibrils are arranged as bundles of highly parallel F-actin filaments. The chimical nature of the filaments was identified by incubation with heavy meromyosin from rabbit skeletal muscle. These cytoplasmic actomyosin fibrils actively contract under isotonic conditions. This was shown by contraction experiments under polarized light optics, by cinematographic analysis and by direct proof of the contractility of AM fibrils isolated by laser micro-dissection. Thus, cytoplasmic AM fibrils can be assumed to represent structures essential for motive force generation in contraction processes in non-muscle cells.