Microfilaments and cytoplasmic streaming: inhibition of streaming with cytochalasin.

Cytochalasin B reversibly inhibits cytoplasmic streaming in both Nitella and Avena cells. Colchicine, on the other hand, has no effect on streaming in either plant; nor does colchicine prevent the recovery of streaming after cytochalasin is withdrawn. The inhibition of protein synthesis by cycloheximide has no effect on either streaming itself or on the recovery of streaming after cytochalasin withdrawal. All this suggests that microfilaments may provide one component of the structure that generates the streaming force and that microtubules play little, if any, role in the process. Ultrastructural studies of Nitella demonstrate that microfilaments are localized at the boundary of the streaming endoplasm and the stationary ectoplasm. Microfilaments are organized in discrete bundles, with possible cross-bridges between individual filaments in each bundle. These bundles are closely associated with the extensive endoplasmic reticulum. Cytochalasin B does not cause ultrastructural changes in Nitella microfilaments as it does in some animal-cell filaments. Since the molecular mechanism of cytochalasin's action is unknown, there may be no necessary correlation between functional inhibition by the drug and altered microfilament morphology. A model is advanced which proposes that streaming is generated by an interaction between microfilaments and the endoplasmic reticulum.