DYNAMICS AND PHARMACOLOGICAL PERTURBATIONS OF THE ENDOPLASMIC-RETICULUM IN THE UNICELLULAR GREEN-ALGA ACETABULARIA

The giant unicellular green alga Acetabularia was labeled with the lip ophilic fluorochrome DiOC(6) (3,3'-dihexyloxacarbocyanine) and examine d by confocal laser scanning microscopy to study the distribution of t he endoplasmic reticulum (ER) and its dynamic changes after the applic ation of inhibitors. In control cells, a two-dimensional polygonal net work of ER sheets and tubulus is suspended between parallel, longitudi nally oriented bands. These bands coincide with the main physical trac ks of organelle transport. All treatments that inhibited organelle mot ility caused a transformation of the polygonal network into confluent large patches of lamellar ER sheets. The shape of the lamellar sheets and residual acitivities of the ER were dependent on the inhibitors us ed. The largest ER lamellae were obtained after cytochalasin D (CD) tr eatment which effectively stopped cytoplasmic streaming. CD also cause d the formation of a network of fine tubules overlapping with the lame llar sheets. Okadaic acid, a specific inhibitor of serine/threonine pr otein phosphatases, also caused inhibition of organelle movement and e nlargement of lamellar areas. Tension in the cytoplasm appeared to be reduced, as judged from the convexly curved lamellar rims and wavy con necting ER tubules. In contrast, N-ethylmaleimide, a sulfhydryl group blocking reagent, rapidly stopped streaming and halted all activities of the ER in a rigor-like state. These effects are interpreted in the context of actin based motility phenomena prevalent in Acetabularia, a nd regulatory principles are discussed that might underlie ER dynamics .