Okadaic acid affects spindle organization in metaphase II-arrested rat oocytes

We used okadaic acid (OA), a potent phosphatase inhibitor, to study the role of phosphatases in the organization of the spindle microtubules in metaphase II-arrested rat oocytes. OA induced a dramatic elongation of the spindle and disorganization of the metaphase plate. Biochemical analysis revealed some dramatic changes after OA treatment: increased histone H1 kinase activity, a burst of protein phosphorylation, and inhibition of protein synthesis. The latter effect alone cannot be responsible for the changes in the spindle structure since inhibition of protein synthesis with puromycin had no such effect. To identify the targets of OA among the proteins associated with the spindle, meiotic spindles of OA-treated and control oocytes were purified. After OA treatment some proteins gained and others lost their association with the spindle. A few proteins that lost their affinity for the spindle also underwent hyperphosphorylation after OA treatment. These results suggest that hyperphosphorylation of some proteins associated with the spindle alters their affinity to microtubules and thus stabilizes them.