Septin Scaffolds and Cleavage Planes in Saccharomyces

John Chant toward the bud orient the nucleus and spindle for mitosis Figure 1 illustrates the key point that, once a yeast cell has initiated a bud early in the cell cycle, it has Cambridge, Massachusetts 02138 committed to its cleavage plane. The molecular manifestation of commitment is the formation of a ring of septin proteins (Cdc3p, Cdc10p, Cdc11p, and Cdc12p) be-For one cell to divide into two, it must solve the problem neath the cell membrane at the future division site (Kim of positioning its cleavage plane to bisect the spindle et al., 1991; Figure 1; Figure 2a). The bud grows from axis such that one genome is segregated to each prog-within the septin ring, which remains as a collar encir-eny cell. Many eukaryotic cells rely on a mechanism in cling the mother–bud neck. As the bud appears, DNA which the spindle apparatus dictates the position of replication begins and continues during bud growth (By-the cleavage plane, thereby ensuring this arrangement. ers, 1981). Once the genome is replicated and the bud Saccharomyces cells solve the problem of cleavage has grown to nearly the size of its mother, the spindle plane placement by a different mechanism: both the aligns parallel to the mother–bud axis and elongates, spindle and the cleavage plane respond independently depositing one nucleus each to mother and daughter to the budding axis of the cell such that, by the time (Figures 1C and 1D; Byers, 1981; Kilmartin and Adams, cytokinesis occurs, the spindle and cleavage plane are 1984). Hence, in Saccharomyces, polarization toward positioned in the appropriate arrangement. A ring of the bud site in G1 defines the spatial axis for all that will proteins called septins defines the cleavage plane, be-follow: actin microfilaments target growth, microtubules ginning in G1, and persists, encircling the mother–bud position the spindle, and septins mark the cleavage junction for the duration of the cell cycle, where it acts plane. as a scaffold for the proteins that ultimately execute the The Septin Ring, a Scaffold Defining mechanics of cytokinesis. During mitosis, the spindle the Cleavage Plane aligns through the opening of the septin ring and elon-Four Saccharomyces septin proteins are known: Cdc3p, gates; cytokinesis then occurs in the plane of the septin Cdc10p, Cdc11p, and Cdc12p (individual proteins are ring. Despite differences between how budding yeast named for their genes). In cells singly defective for any of cells and many symmetrically dividing …