Evidence for active interactions between microfilaments and microtubules in myxomycete flagellates

We have previously observed the apparent displacement of microfilaments over microtubules in the backbone structure of permeabilized flagellates of Physarum polycephalum upon addition of ATP (Uyeda, T.Q.P., and M. Furuya, 1987. Protoplasma. 140: 190-192). We now report that disrupting the microtubular cytoskeleton by treatment with 0.2 mM Ca²⁺ for 3-30 s inhibits the movement of the microfilaments induced by subsequent treatment with 1 mM Mg-ATP and 10 mM EGTA. Stabilization of microtubules by pretreatment with 50 μM taxol retarded both the disintegrative effect of Ca²⁺ on the subsequent, ATP-induced movement of the microfilaments. These results suggest that the movement of the microfilaments depends on the integrity of the microtubular cytoskeleton. EM observation showed that the backbone structure in control permeabilized flagellates consists of two arrays of microtubules closely aligned with bundles of microfilaments of uniform polarity. The microtubular arrays after ATP treatment were no longer associated with microfilaments, yet their alignment was not affected by the ATP treatment. These results imply that the ATP treatment induces reciprocal sliding between the microfilaments and the microtubules, rather than between the microfilaments themselves or between the microtubules themselves. While sliding was best stimulated by ATP, the movement was partially induced by GTP or ATPγS, but not by ADP or adenylyl-imidodiphosphate (AMP-PNP). AMP-PNP added in excess to ATP, 50 μM vanadate, or 2 mM erythro-9-[3-(2-hydroxynonyl)]adenine (EHNA) inhibited the sliding. Thus, the pharmacological characteristics of this motility were partly similar to, although not the same as, those of the known microtubule-dependent motilities.