Understanding cytokinesis: lessons from fission yeast

Abstract

For decades after the discovery that a contractile ring made of actin filaments and myosin II produces the force to constrict the cleavage furrow of animal cells, the complexity of cytokinesis has slowed progress in understanding the mechanism. Mechanistic insights, however, have been obtained by genetic, biochemical, microscopic and mathematical modelling approaches in the fission yeast Schizosaccharomyces pombe. Many features that have been identified in fission yeast are probably shared with animal cells, as both inherited many cytokinesis genes from their common ancestor about one billion years ago.

Figure 1. Strategies for cytokinesis used by plant, fission yeast and animal cells

Plants, amoebas, fungi and animals all arose from a common ancestor, branching off as shown. In plants, the cell division plane is selected by the nucleus specifying the position of a preprophase band of microtubules around the equator. Plants lack key proteins to make a contractile ring, so they depend on membrane fusion to separate the two daughter cells. Phragmoplast microtubules transport Golgi vesicles to the midplane to form the new plasma membrane. Amoebas divide much like animal cells and are not illustrated. In fission yeast, the cell division plane is selected by the nucleus specifying the position of nodes around the equator, whereas in animals, spindle and astral microtubules specify the position of the contractile ring. Fission yeast and animal cells assemble a contractile ring of actin filaments and myosin II around the equator of the cell between the chromosomes, which are separated by microtubules of the mitotic apparatus. The ring constricts and the daughter cells separate by membrane fusion. See also Fig. 2 for more details on the process in fission yeast.

Figure 2. Time course of cytokinesis in fission yeast

Time zero is defined as the time when the spindle pole bodies separate. Starting at time −60 minutes, interphase nodes containing the anillin-like protein Mid1 (also known as Dmf1) and cell cycle kinases form near the plasma membrane. Negative signals from the ends of the cell position these nodes around the equator. Dynamic microtubules push the nucleus to the centre of the cell. At time −10 minutes, interphase nodes begin to mature into cytokinesis nodes by the addition of myosin II (Myo2), ring assembly protein 2 (Rng2; a member of the IQGAP family), the F-BAR domain-containing protein cell division control protein 15 (Cdc15) and the formin Cdc12. Following spindle pole body separation, Cdc12 and Cdc3 (also known as profilin) stimulate the polymerization of actin filaments that bind tropomyosin and cross-linking proteins. During anaphase A (at time +5 minutes), interactions of myosin II with actin filaments condense nodes into a contractile ring, which matures by adding more Cdc15, capping protein, unconventional myosin II (Myp2; also known as Myo3) and other proteins. In anaphase B (at time +10 to +30 minutes) the mitotic spindle elongates and the anillin-like protein Mid2 and septins form double rings adjacent to the contractile ring. Mid1 disappears from the ring at the onset of its constriction. At the end of anaphase, a signalling pathway consisting of a GTPase and three protein kinases (the septation initiation network (SIN)) triggers constriction of the contractile ring (at time +40 minutes), membrane invagination and synthesis of a new cell wall to form a septum. Constriction ends at time +70 minutes. After another 30 minutes, scission of the plasma membrane separates the cytoplasm and septum remodelling separates the cells.

Figure 3. Mechanism of contractile ring assembly in fission yeast

a. | the cycle of reactions hypothesized in the search, capture, pull and release mechanism of contractile ring assembly. Cytokinesis nodes contain the anillin-like protein Mid1 (also known as Dmf1), myosin II (Myo2) and the formin cell division control protein 12 (Cdc12). Actin filaments grow in random directions from nodes by adding subunits at rate V_pol (the rate of actin polymerization). If a filament approaches a node within distance r, less than a defined distance r_c (capture radius), myosin II can capture the filament. Myosin II applies force on attached filaments and moves nodes at velocity V. Actin filament connections between nodes break owing to severing or other reactions, with a time constant of τ_break, after which the cycle is repeated. b | An overview of contractile ring assembly and constriction in fission yeast. During interphase, Cdc12 is distributed throughout the cell in small clusters called speckles. At time zero minutes, Cdc12 joins nodes around the equator and nucleates actin filaments. The actin filaments form bundles as myosin II pulls the cytokinesis nodes into a ring. As the contractile ring constricts, the septum forms between the daughter cells, which separate by membrane fusion.