Cell shape and cell division in fission yeast

Abstract

The fission yeast Schizosaccharomyces pombe has served as an important model organism for investigating cellular morphogenesis. This unicellular rod-shaped fission yeast grows by tip extension and divides by medial fission. In particular, microtubules appear to define sites of polarized cell growth by delivering cell polarity factors to the cell tips. Microtubules also position the cell nucleus at the cell middle, marking sites of cell division. Here, we review the microtubule-dependent mechanisms that regulate cell shape and cell division in fission yeast.

Figure 1

Microtubule organization in fission yeast. (A) A typical fission yeast cell has between three and five dynamic microtubule bundles organized along the long axis of the cell that are organized by iMTOCs into antiparallel bundles with minus ends overlapping at the middle of the cell and plus ends facing and interacting with the cell tips. Two complementary modes of microtubule organization are presented in (B) and (C). (B) In the first model, iMTOCs are tethered to the nuclear membrane. The Mto1p–Mto2p complex, a component of the iMTOC, recruits γ-TuRCs which nucleate microtubules. Microtubule polymers are then bundled into an antiparallel configuration by Ase1p. (C) In the second model, new microtubules nucleate on pre-existing microtubules. The Mto1p–Mto2p complex recruits γ-TuRCs to the lattice of a pre-existing microtubule. Ase1p stabilizes the antiparallel configuration between new and old microtubules. The kinesin Klp2p slides the new microtubule to the minus end of the old microtubule (marked by the arrow), establishing an antiparallel bundle. Microtubule length is regulated by +TIP proteins and the rescue factor Cls1p/Peg1p. A growing microtubule can exhibit catastrophe and shrinkage (red arrow). It can then be rescued by Cls1p/Peg1p at the iMTOC and re-grow (green arrow).

Figure 2

Microtubule pushing centers the nucleus and cortical medial nodes to define the site of cell division. The iMTOC tethers the microtubule bundle to the nucleus. During microtubule–cortex contact, sustained polymerization at the microtubule plus end produces a pushing force (shown by the large arrowhead) that displaces the nucleus in the opposite direction (nuclear movement depicted by blue arrows). The antiparallel configuration of the microtubule bundle ensures that, over time, the nucleus oscillates back and forth toward the geometrical center of a growing cell. Coupling between the nucleus and the medial cortical nodes ensures that the nodes, over time, are also positioned at the cell middle. The medial nodes subsequently organize the actomyosin ring for cell division.

Figure 3

Cell-shape formation in fission yeast. (A) A scheme of microtubule plus ends delivering polarity factors to the cell tip. The microtubule plus end delivers the +TIP complex carrying the Tea1p–Tea4p complex to the tip of the cell (1), where it is docked on the Mod5p receptor (2). The Tea1p–Tea4p complex recruits the polarisome complex containing Bud6p and For3p (3). For3p nucleates F-actin cables, which serve as tracks for the vesicular transport of the growth machinery to the cell tip (4). (B) A model for how the balance between the rate of delivery (R_Delivery) and rate of dispersion (R_Dispersion) of polarity factors defines cell shape. (C) Consequences of changes in the delivery of polarity factors (i.e., R_Delivery) or changes in the dispersion of polarity factors (i.e., R_Dispersion) on cell shape. When R_Delivery is equal or greater than R_Dispersion, cells maintain linear growth. When R_Delivery and R_Dispersion are displaced from the cell long axis, cells grow bent. When R_Delivery does not reach the old cell tips, new cell tips are initiated and cells grow T-shaped. Finally, when R_Delivery is less than R_Dispersion, cells grow oval or round.