Filaments of Amoeba Proteus

Amoeba proteus has two types of cytoplasmic filaments that are thought to be important in cell motility, cytoplasmic streaming, and changes in cytoplasmic consistency : thin, 50-70 A wide filaments, and short, thick, 160 A wide rods (Pollard and Ito, 1970) . The thin filaments resemble the actin filaments of muscle (Hanson and Lowy, 1963), Acanthamoeba (Weihing and Korn, 1969 ; Pollard et al ., 1970), and Physarum (Hatano and Oosawa, 1966) . Cell-free extracts of Amoeba proteus show characteristic patterns of streaming when they are warmed to room temperature in the presence of 3 mm adenosine triphosphate (ATP) (Thompson and Wolpert, 1963 ; Pollard and Ito, 1970) . Fibrils become visible by phase-contrast microscopy after about 10 min, and continue to increase in size and number for about 1 hr . These fibrils are birefringent when viewed in a polarizing microscope. Extracts treated with 0 .5 mm [ethylenebis(oxyethylenenitrilo)]tetraacetic acid (EGTA) instead of ATP also form fibrils at room temperature but do not stream . The fibrils have been identified by electron microscopy as bundles of thin filaments (Morgan et al ., 1967) that are apparently identical with the thin filaments of the intact amebas (Pollard and Ito, 1970) . We have now demonstrated that these thin filaments form specific ATP-dissociable complexes with muscle heavy meromyosin (HMM) that are indistinguishable from the complexes of HMM with the F actins from muscle (Huxley, 1963), Acanthamoeba (Pollard et al., 1970), and Physarum (Nachmias et al ., 1970) . This indirect method provides tentative, but reasonable, identification of the Amoeba proteus thin filaments as F actin .