Abstract
Actin polymerises to form filaments/cables for motility, transport, and the structural framework in a cell. Recent studies show that actin polymers are present not only in the cytoplasm but also in the nuclei of vertebrate cells. Here, we show, by electron microscopic observation with rapid freezing and high-pressure freezing, a unique bundled structure containing actin in the nuclei of budding yeast cells undergoing meiosis. The nuclear bundle during meiosis consists of multiple filaments with a rectangular lattice arrangement, often showing a feather-like appearance. The bundle was immunolabelled with an anti-actin antibody and was sensitive to an actin-depolymerising drug. Similar to cytoplasmic bundles, nuclear bundles are rarely seen in premeiotic cells and spores and are induced during meiotic prophase-I. The formation of the nuclear bundle is independent of DNA double-stranded breaks. We speculate that nuclear bundles containing actin play a role in nuclear events during meiotic prophase I.
Highlights
Actin polymerises to form filaments/cables for motility, transport, and the structural framework in a cell
The nucleus was surrounded by double-layered nuclear membranes and contained electron-dense regions corresponding to the nucleolus (Fig. 2a, b, c)
In this study, by using transmission electron microscopy (TEM) with rapid freezing-fixation, we found nuclear bundles containing actin in budding yeast cells undergoing the physiological programme of meiosis
Summary
Actin polymerises to form filaments/cables for motility, transport, and the structural framework in a cell. In budding and fission yeasts, actin is present in the cytoplasm in a polymerised form, such as rings, patches and cables[21,22,23], as well as the filasome, which is a less well-defined cytoplasmic amorphous structure containing F-actin[24]. Previous electron microscopic (EM) analysis of the cytoplasm in fixed mitotic cells revealed a linear actin bundle/cable containing multiple actin filaments[25]. Actin polymers are visualised by using a green fluorescent protein (GFP) fused with an actin-binding protein, Abp[140], or by staining with an actin-specific peptide with a fluorescent dye such as phalloidin[26] This staining confirmed the presence of filaments/cables and patches containing actin polymers in the yeast cytoplasm
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