Evidence for the lack of actin involvement in mitosis and in the contractile process in Spirostomum teres.

Studies using actin decoration techniques and electron microscopy have failed to show the presence of actin in the ciliate Spirostomum teres. The internal and external membranes of dividing cells were made permeable by treatment with Triton. The myosin subfragment S-1 was introduced into the cells for incubation under conditions suitable for actin decoration. Arrowhead decoration of microfilaments was not observed within dividing micronuclei or in cytoplasmic filament bundles. However, using similar procedures, the brush border of mouse small intestine yielded clearly decorated microfilaments. A spectrophotometric DNase I inhibition assay specific for G-actin demonstrated that the level of actin in S. teres extracts was less than 0.06% of the total soluble protein and confirms the observations made using the decoration procedures. On the basis of this work, it appears that actin does not play a significant part in either mitotic chromosome movement or contractility in S. teres.