The Role of the Cytoskeleton in Positioning of the Nucleus in Premitotic Tobacco BY-2 Cells

Abstract

In premitotic tobacco BY-2 cells, the nucleus moves from the periphery to the central region of the cell and settles there. This premitotic migration of the nucleus occurs in the presence of aphidicolin, an inhibitor of DNA polymerase α, which suppresses the formation of the preprophase band of microtubules (PPB). Thus, the formation of the PPB is not a prerequisite for this premitotic nuclear migration. Tethered by transvacuolar cytoplasmic strands, which contain both microtubules and actin filaments, the nucleus migrates to the site of future division. This migration is not prevented by disruption of actin filaments caused by treatment with cytochalasin, suggesting that microtubules play an important role in the premitotic migration of the nucleus. In the preprophase cell, microtubules were present in the PPB and in cytoplasmic strands, which extended from the nucleus to the periphery of the cell near the PPB and near the poles, and which were associated with actin filaments. At the end of prophase, microtubules in the cytoplasmic strands and the PPB disappeared, leaving actin filaments in the transvacuolar cytoplasmic strands. Thereafter, the position of the mitotic apparatus (or that of the cytokinetic phragmoplast) was maintained by cytoplasmic strands that contained only actin filaments. The disruption of microtubules by treatment with propyzamide prevented the formation of the PPBlike band of actin filaments, as well as that of the PPB itself. From these results, we propose the following hypothesis. In premitotic tobacco BY-2 cells, microtubules change their arrangement irrespective of the presence of actin filaments. The resultant premitotic network of microtubules serves as a scaffold for building up the mitotic network of actin filaments, which plays an important role in maintaining the position of the mitotic apparatus and in controlling the direction of expansion of the phragmoplast.