Fluorescence studies on modes of cytochalasin B and phallotoxin action on cytoplasmic streaming in Chara.

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Abstract
Cytoplasmic streaming in characean algae may be driven by interaction between subcortical actin bundles and endoplasmic myosin. Cytotoxic actin-binding drugs and fluorescent actin labels were perfused into the cytoplasm of streaming Chara cells. Cytochalasin B (CB) reversibly inhibits streaming. Phalloidin did not inhibit streaming in Chara but did modify the inhibitory effect of CB. Use of 2 fluorescent actin probes, fluorescein isothiocyanate-heavy meromyosin (FITC-HMM) and nitrobenzoxadiazole-phallacidin (NBD-Ph), permitted visualization of the effects of CB and phalloidin on the actin bundles. FITC-HMM labeling in perfused but nonstreaming cells revealed alteration of the actin bundles by CB. Phalloidin alone does not perceptibly alter the actin bundles but does block the alteration by CB if applied as a pretreatment. NBD-Ph perfused into the cytoplasm of streaming cells stains actin bundles without inhibiting streaming. NBD-Ph staining of actin bundles is not initially observed in cells inhibited by CB but does appear simultaneously with the recovery of streaming as CB leaks from the cells. The established effects of phallotoxins and CB on actin in vitro were supported; apparently streaming is generated by actin-myosin interactions.