Nuclear bundle/cable containing actin during yeast meiosis

Actin polymerizes to form filaments/cables for motility, transport, and structural framework in a cell. Recent studies show that actin polymers are present not only in cytoplasm, but also in nuclei of vertebrate cells, and their formation is induced in response to stress. Here, by electron microscopic observation with rapid freezing and high-pressure freezing, we found a unique bundled structure containing actin in nuclei of budding yeast cells undergoing meiosis. The nuclear bundle/cable during meiosis consists of multiple filaments with a rectangular lattice arrangement often showing "feather-like" appearance. The bundle is immuno-labeled with anti-actin antibody and sensitive to an actin-depolymerizing drug. Like cytoplasmic bundles, nuclear bundles with actin are rarely seen in pre-meiotic cells and spores, and are induced during meiotic prophase-I. The formation of the nuclear bundles/cables is independent of meiotic DNA double-stranded breaks. We speculate that nuclear bundles/cables containing actin play a role in nuclear events during meiotic prophase I.