The Control of Septum Formation in Fission Yeast

Downloaded from genesdev.cshlp.org on September 28, 2021 - Published by Cold Spring Harbor Laboratory Press REVIEW The control of septum formation in fission yeast Kathleen L. Gould1,3 and Viesturs Simanis2,3 1Howard Hughes Medical Institute and Department of Cell Biology, Vanderbilt University School of Medicine, Nashville, Tennessee 37232 USA; 2Swiss Institute for Experimental Cancer Research, 1066 Epalinges, Switzerland If cell division is to produce two daughter cells that are chromosome separation, a structure called the medial viable, faithful copies of the parent cell, the landmark ring forms at the center of the cell, overlying the events of the cell division cycle, S phase, mitosis, and nucleus. The position of this ring anticipates the site of cytokinesis, must be executed in the correct order and septum formation. This ring is composed of F-actin and with high fidelity. Though considerable advances have many other components (see below). F-actin patches are been made toward understanding the mechanisms that subsequently polarized to the medial ring, and the cell is control the onset of S phase and mitosis, regulation of thus primed for septation. At the end of anaphase, when the events that occur at the end of the cell cycle, such as the spindle begins to break down, biosynthesis of the the reorganization of the cytoskeleton and the initiation septum is initiated, and the primary septum grows in- of cell separation or cytokinesis, are less well under- ward from the cell cortex (Johnson et al. 1973). As its stood. The fission yeast Schizosaccharomyces pombe biosynthesis proceeds, it is surrounded by F-actin provides a simple eukaryotic model for the study of cy- patches (Marks and Hyams 1985), and a ring of F-actin is tokinesis. S. pombe cells are rod shaped, grow mainly by seen at its leading edge (Jochova´et al. 1991). In electron elongation at their ends, and divide by binary fission af- micrographs, F-actin filaments are seen between the ter forming a centrally placed division septum. Study of edges of the developing septum (Kanbe et al. 1989). The S. pombe mutants has begun to shed light on how sep- completed septum has a three-layered structure, com- tum formation and cytokinesis are regulated both spa- posed of the inner, primary septum, flanked by the two tially and temporally, to achieve proper co-ordination secondary septa. The primary septum can be visualized with mitosis. Some of the genes defined by these mu- by Calcuofluor staining. Fission begins by degradation of tants have been functionally conserved through eukary- the outer cell wall opposite the site of the completed otic evolution, suggesting that aspects of this control septum and is completed by centripetal degradation of will be common to all eukaryotic cells. the primary septum to bring about cell separation (Johnson et al. 1973). At this time, the F-actin patches are relocated to the old (pre-existing) end of the cell, from Septum formation and cytokinesis in the fission yeast where growth will resume. S. pombe The main morphological events of septum formation S. pombe mutants defective in septum formation and cytokinesis in S. pombe are shown in Figure 1. As in or cytokinesis higher eukaryotes, the fission yeast cytoskeleton under- goes a characteristic series of rearrangements during mi- One of the considerable advantages of a simple model tosis and cytokinesis (for review, see Robinow and Hy- system, such as yeast, is that genes that encode proteins ams 1989). The distribution of F-actin in S. pombe cells important for the regulation or execution of complex is linked intimately with sites of growth and septum processes can be identified through the study of mutants. formation. During interphase, F-actin is observed mainly A small number of heat-sensitive, cell-division-defective as cortical patches at the growing ends of the cell, though mutants were identified in early screens (e.g., Nurse et actin cables can also be seen (Marks and Hyams 1985; al. 1976). Recently, the mutant set has been expanded Balasubramanian et al. 1996). Structural studies of cor- considerably by the use of a method involving selection tical actin have demonstrated that it is composed of spi- for cells that increase ploidy (e.g., Chang et al. 1996). ral actin filaments that are associated with an invagina- Originally designed to enrich for mutants that rerepli- tion of the plasma membrane (Mulholland et al. 1994). cated their DNA, this elegant screen is based on the use During the early stages of mitosis, before any visible of cells that cannot mate but are able to undergo meiosis if they are able to diploidize by other means (Broek et al. 1991). This can occur by rereplication of DNA, by failure 3Correspondence may be directed to either author. E-MAIL [email protected]; FAX (615) 343-4539. to separate the daughter chromosomes before septation, E-MAIL [email protected]; FAX +41-21-652-6933. or by failure to synthesize, or correctly position, the di- GENES & DEVELOPMENT 11:2939–2951 © 1997 by Cold Spring Harbor Laboratory Press ISSN 0890-9369/97 $5.00 2939 Downloaded from genesdev.cshlp.org on September 28, 2021 - Published by Cold Spring Harbor Laboratory Press Gould and Simanis vision septum. The two latter classes increase in ploidy include actin (act1: Ishiguro and Kobayashi 1996; Mc- by having two nuclei in the same cytoplasm. Thus, the Collum et al. 1996), type II myosin (myo2: Kitayama et screen enriches for mutants defective in septum forma- al. 1997), profilin (cdc3: Balasubramanian et al. 1994), tion. The following sections describe some S. pombe cell tropomyosin (cdc8: Balasubramanian et al. 1992), a myo- cycle mutants that show terminal phenotypes indicating sin light chain-like protein (cdc4: McCollum et al. 1995), that they are unable to execute events important for one a formin (cdc12: Chang et al. 1997), an SH3 domain- or more of the stages in septum formation and cell cleav- containing protein (cdc15: Fankhauser et al. 1995; Chang age described in Figure 1. They are listed in Table 1, and et al. 1996), and the product of the rng2 gene (Chang et al. a schematic representation of some of the phenotypes 1996), which has not been cloned to date. All of these produced by loss-of-function mutations in these genes is genes are necessary for medial ring assembly. However, shown in Figure 2. in addition to the lack of medial ring formation, mutant alleles of several of these genes retain residual activity at Nomenclature the restrictive temperature and display other aberrant The original report of fission yeast cell cycle mutants phenotypes (see below). These additional abnormalities described two categories of septation mutants, called most likely reflect the protein’s involvement in other ‘‘early’’ and ‘‘late’’ (Nurse et al. 1976). Early mutants actin-requiring processes. were those that failed to make a septum at all, whereas Profilin is an ubiquitous actin monomer-binding pro- late mutants arrested after forming a disorganized sep- tein. It was first purified in a 1:1 complex with actin and tum. This terminology assumed (logically enough) that can sequester actin monomers under some conditions. early mutants were defective in an early event of septum However, profilin is now considered to be a critical me- formation, whereas late mutants were defective in an diator of actin filament polymerization (for review, see event after the initiation of septum biosynthesis. Subse- Theriot and Mitchison 1993; Sohn and Goldschmidt- quent analysis of the mutants and cloning of the genes Clermont 1994). The inability of Cdc3p mutants to form they defined have demonstrated that this is not the case a medial ring (Balasubramanian et al. 1994) is consistent (see below). Therefore, this terminology is not used. Fur- with a primary role for profilin in actin filament assem- thermore, throughout this review, ‘‘medial ring,’’ rather bly. Profilin also binds tightly to polyproline and to pro- than ‘‘actin ring,’’ is used to describe the structure teins containing stretches of proline residues. Among formed at the cell cortex early in mitosis, because it con- the polyproline-containing proteins found to bind pro- tains many proteins in addition to actin. filin in vitro is Cdc12p (Chang et al. 1997). During interphase, Cdc3p localizes to the growing tips The medial ring of the cells, where actin patches are concentrated, whereas Cdc8p is seen in discrete patches at the cell Mutants defective in assembly of the medial ring cortex. At mitosis, Cdc8p is found as a component of the Analysis of mutants has shown that many components medial ring, joining the ring after F-actin (Balasubrama- are involved in the assembly of the medial ring. These nian et al. 1992), whereas Cdc3p forms a broad ring at the Figure 1. Schematic representation of the main events of septum formation and cytokinesis in S. pombe. The nucleus is shown as a gray circle, whereas the small red dots represent F-actin patches. NETO is New End Take-Off (Mitchison and Nurse 1985): Cells switch from one-end to two-end growth. The genes implicated in each of the stages are indicated. Grouping of genes does not necessarily imply that their products interact physically. 2940 GENES & DEVELOPMENT Downloaded from genesdev.cshlp.org on September 28, 2021 - Published by Cold Spring Harbor Laboratory Press Fission yeast cytokinesis cell equator (Balasubramanian et al. 1994). Consistent cdc8 exhibit delocalized actin patches at all stages of the with these localization patterns, mutants in cdc3

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