Printed in Great Britain CYTOPLASMIC STREAMING IN CHARA: A CELL MODEL ACTIVATED BY ATP AND INHIBITED BY CYTOCHALASIN B

After vacuolar perfusion of Cliara internode cells, the cytoplasm remaining in situ can be reactivated by ATP to give full rates of streaming. Observations during both perfusion and reactivation indicated that the generation of the motive force was associated with fibres con-sisting of bundles of microfilaments. In the absence of ATP, the remaining endoplasmic organelles were immobilized along such fibres. When ATP was introduced, organelles moved along the fibres at speeds up to 50 /tm s~', but were progressively released from contact to leave the fibres in a conspicuously clean state. Inorganic pyrophosphate freed the organelles from the fibres without supporting movements. Motility required millimolar Mg2+ levels, free Ca*+ at io~7 M or less and was inhibited by high levels of Cl ~ and by pH's on either side of 7-0. The reactivated movements were rapidly and completely inhibited by 25 fig ml"1 cytochalasin B. The results are interpreted in terms of actin filaments in the stationary cortex interacting with a myosin-like protein which is able to link to endoplasmic organelles. Movement results from an active shear type of mechanism