A combination of immunocytochemical and electron microscopic methods was used to study the effects of okadaic acid, a specific inhibitor of protein phosphatase types 1 and 2A, on the Golgi complex and the microtubule system of interphase CHO cells. At a concentration of 0.25 microM and within 2–3 h of exposure, okadaic acid caused a reversible disorganization of the Golgi complex, observed as a disintegration of the stacks of cisternae and formation of clusters of tubules and vesicles dispersed in the cytoplasm. At the same time, staining for mannosidase II was shifted from the Golgi stacks to the endoplasmic reticulum, whereas the clusters of tubules and vesicles for the main part were negative. This change in localization of the enzyme was not blocked by cycloheximide and thus not dependent on ongoing protein synthesis. The changes in the morphology of the Golgi complex were coordinated in time with a remodelling of the microtubule system, observed as a reduction in the number of microtubules, a tendency of the remaining microtubules to arrange in an aster-like pattern, and an increased sensitivity to low concentrations of the microtubule-disruptive drug nocodazole. After removal of the drug, the microtubule system was rapidly normalized (1-2 h) and subsequently also the Golgi complex (4-8 h). The results suggest that okadaic acid induces a redistribution of the Golgi stacks into the endoplasmic reticulum, leaving the trans-most elements behind as tubules and vesicles.(ABSTRACT TRUNCATED AT 250 WORDS)

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