Dynamics and pharmacological perturbations of the endoplasmic reticulum in the unicellular green alga Acetabularia.

The giant unicellular green alga Acetabularia was labeled with the lipophilic fluorochrome DiOC6 (3,3'-dihexyloxacarbocyanine) and examined by confocal laser scanning microscopy to study the distribution of the endoplasmic reticulum (ER) and its dynamic changes after the application of inhibitors. In control cells, a two-dimensional polygonal network of ER sheets and tubulus is suspended between parallel, longitudinally oriented bands. These bands coincide with the main physical tracks of organelle transport. All treatments that inhibited organelle motility caused a transformation of the polygonal network into confluent large patches of lamellar ER sheets. The shape of the lamellar sheets and residual activities of the ER were dependent on the inhibitors used. The largest ER lamellae were obtained after cytochalasin D (CD) treatment which effectively stopped cytoplasmic streaming. CD also caused the formation of a network of fine tubules overlapping with the lamellar sheets. Okadaic acid, a specific inhibitor of serine/threonine-protein phosphatases, also caused inhibition of organelle movement and enlargement of lamellar areas. Tension in the cytoplasm appeared to be reduced, as judged from the convexly curved lamellar rims and wavy connecting 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, and regulatory principles are discussed that might underlie ER dynamics.