During interphase in all eukaryotic cells the double lipid bilayer of the nuclear envelope (NE) physically separates the chromosomes, and chromosome-related processes, from the cytoplasm and increases in area by 59% (Lim et al., 2007) as the nuclear volume doubles in preparation for mitosis (reviewed by Hetzer et al., 2005;Lim et al., 2007;Winey et al., 1997). In the open mitosis of animal cells, NE breakdown allows the spindle microtubules that are nucleated by the cytoplasmic centrosomes to attach to and then separate the chromosomes. In the closed mitosis of yeast, the centrosome equivalents, called spindle pole bodies (SPBs), are embedded in the NE and nucleate the formation of an intranuclear spindle (Ding et al., 1997). As the spindle elongates in anaphase B, nuclear volume remains constant but division of the roughly spherical nucleus into two smaller spheres, which occurs in less than 5 minutes, requires a rapid increase of 26% in NE area (Lim et al., 2007).The nucleus, often thought of as a freestanding organelle, is actually a specialized region of the endoplasmic reticulum (ER) (Voeltz et al., 2002): the outer NE is continuous with both the ER and the inner NE (Hetzer et al., 2005), providing a means by which lipids and membrane proteins can move between the sheet form of the ER at the nuclear periphery and the tubular form of the ER, which, in animal cells, extends throughout the cytoplasm and, in yeast, is primarily at the cell periphery (Pidoux and Armstrong, 1993;Voeltz et al., 2002).The Ran GTPase influences many cellular functions (Quimby and Dasso, 2003), including nucleocytoplasmic transport, NE reformation after mitosis in animal cells (Hetzer et al., 2005), and NE structure in fission (Demeter et al., 1995) and budding (Ryan et al., 2003) yeast. However, the mechanism by which the Ran GTPase influences mitosis-specific NE changes during both open and closed mitosis remains unknown. We have previously shown that, when the Ran system is mis-regulated in the fission yeast Schizosaccharomyces pombe, cells arrest with NEs that have lost their integrity, and that this defect is concomitant with and depends on the passage through mitosis (Demeter et al., 1995).The predictions of a biophysical model of the fission-yeast nucleus (Lim et al., 2007) and experimental observations of abnormal nuclear shapes seen when microtubule bundles lacking SPBs at their ends cause thin tethers to protrude from the spherical nucleus (Khodjakov et al., 2004;Tange et al., 2002;Toya et al., 2007;West et al., 1998;Zheng et al., 2007) raise the possibility that the SPB influences the mechanical properties of the NE to prevent tether formation during normal mitosis and ensure symmetric nuclear division. Because of limitations on the mechanical strength of lipid bilayers and their ability to stretch in response to pressure exerted by the elongating spindle, our computational model incorporates a lipid reservoir (Lim et al.,The double lipid bilayer of the nuclear envelope (NE) remains intact during closed mitosis. In the ...

It has long been known that during the closed mitosis of many unicellular eukaryotes, including the fission yeast (Schizosaccharomyces pombe), the nuclear envelope remains intact while the nucleus undergoes a remarkable sequence of shape transformations driven by elongation of an intranuclear mitotic spindle whose ends are capped by spindle pole bodies embedded in the nuclear envelope. However, the mechanical basis of these normal cell cycle transformations, and abnormal nuclear shapes caused by intranuclear elongation of microtubules lacking spindle pole bodies, remain unknown. Although there are models describing the shapes of lipid vesicles deformed by elongation of microtubule bundles, there are no models describing normal or abnormal shape changes in the nucleus. We describe here a novel biophysical model of interphase nuclear geometry in fission yeast that accounts for critical aspects of the mechanics of the fission yeast nucleus, including the biophysical properties of lipid bilayers, forces exerted on the nuclear envelope by elongating microtubules, and access to a lipid reservoir, essential for the large increase in nuclear surface area during the cell cycle. We present experimental confirmation of the novel and non-trivial geometries predicted by our model, which has no free parameters. We also use the model to provide insight into the mechanical basis of previously described defects in nuclear division, including abnormal nuclear shapes and loss of nuclear envelope integrity. The model predicts that (i) despite differences in structure and composition, fission yeast nuclei and vesicles with fluid lipid bilayers have common mechanical properties; (ii) the S. pombe nucleus is not lined with any structure with shear resistance, comparable to the nuclear lamina of higher eukaryotes. We validate the model and its predictions by analyzing wild type cells in which ned1 gene overexpression causes elongation of an intranuclear microtubule bundle that deforms the nucleus of interphase cells.