Role of Cell Shape in Determination of the Division Plane in Schizosaccharomyces pombe : Random Orientation of Septa in Spherical Cells

Abstract

The establishment of growth polarity in Schizosaccharomyces pombe cells is a combined function of the cytoplasmic cytoskeleton and the shape of the cell wall inherited from the mother cell. The septum that divides the cylindrical cell into two siblings is formed midway between the growing poles and perpendicularly to the axis that connects them. Since the daughter cells also extend at their ends and form their septa at right angles to the longitudinal axis, their septal (division) planes lie parallel to those of the mother cell. To gain a better understanding of how this regularity is ensured, we investigated septation in spherical cells that do not inherit morphologically predetermined cell ends to establish poles for growth. We studied four mutants (defining four novel genes), over 95% of whose cells displayed a completely spherical morphology and a deficiency in mating and showed a random distribution of cytoplasmic microtubules, Tea1p, and F-actin, indicating that the cytoplasmic cytoskeleton was poorly polarized or apolar. Septum positioning was examined by visualizing septa and division scars by calcofluor staining and by the analysis of electron microscopic images. Freeze-substitution, freeze-etching, and scanning electron microscopy were used. We found that the elongated bipolar shape is not essential for the determination of a division plane that can separate the postmitotic nuclei. However, it seems to be necessary for the maintenance of the parallel orientation of septa over the generations. In the spherical cells, the division scars and septa usually lie at angles to each other on the cell surface. We hypothesize that the shape of the cell indirectly affects the positioning of the septum by directing the extension of the spindle.Cell polarization is a fundamental requirement of cell growth, division, and differentiation. The polar growth ensures directed cell extension, the correct organization of cytoskeletal structures, and the development of proper cell shape (for recent reviews, see references 8, 16, 23, 25, and 35) and orients the mitotic spindle (reviewed in references 22, 36, and 48). In most animal and plant species, the mitotic spindle then determines the plane of division (39,52). In the budding yeast Saccharomyces cerevisiae, the division plane is independent of spindle orientation because it is predetermined by the position of the bud (18). The division takes place at the narrow neck between the mother and bud cells. The site of bud formation is determined by the location of a previous one, and the axis of the spindle is oriented by directing cytoplasmic microtubules running from the spindle pole body into the bud (17, 18).

The use of morphomutants to investigate septum formation and cell separation in Schizosaccharomyces pombe

“…The position of the division site in S. pombe is thought to be determined by the spindle of the mitotic nucleus (Sipiczki et al 2000). According to this model, the division (septation) plane is laid down perpendicularly to the longitudinal axis of the extending spindle.…”

“…Here we show that the actin ring colocalizes with the growing edge of the septum. For this we use spherical S. pombe sph2-3 cells (Sipiczki et al 2000), which allow observation of the cytokinetic structures from all directions. The wild-type cells are cylindrical and can only be observed from the lateral direction (the cylindrical cells lie parallel to the microscope slide), which does not allow a view of the surface of the septum.…”

Cytoskeleton-Plasma Membrane-Cell Wall Continuum in Plants. Emerging Links Revisited

“…Generally, cellular structure is inherently endowed with information that can be both stored and handed over to other structures via templating processes (for plant cells, see Baluška et al, 1997Baluška et al, , 2003b. Although available data are rather scarce for higher plants, and critical linker molecules between the cytoskeleton and ECM are still missing (Pont-Lezica et al, 1993;Wyatt and Carpita, 1993;Fowler and Quatrano, 1997;Miller et al, 1997aMiller et al, , 1997bKohorn, 2000;Heath, 2001), it is increasingly obvious that cell shape and mechanical forces dictate several processes among which the control of division planes is one of the most obvious (Lintilhac and Vesecky, 1984;Lynch and Lintilhac, 1997;Green, 1999;Sipiczki et al, 2000;Smith, 2001). The integrated cytoskeleton and its dynamic adhesion to the plasma membrane allow these complex interactions between mechanical forces and chemical signals (Critchley, 2000;Sheetz, 2001).…”

Fission yeast cytoskeletons and cell polarity factors: connecting at the cortex

“…In these cells, the whole cortex overlaps the nucleus and any diagonal plane is able to define the site of septum synthesis. Although multiple sites of septum initiation are attempted, only one septum is completed (Sipiczki et al, 2000). Thus there are other factors involved in the positioning of the interphase nucleus in cell division site positioning.…”