Mitotic Spindle Form and Function

Abstract

The Saccharomyces cerevisiae mitotic spindle in budding yeast is exemplified by its simplicity and elegance. Microtubules are nucleated from a crystalline array of proteins organized in the nuclear envelope, known as the spindle pole body in yeast (analogous to the centrosome in larger eukaryotes). The spindle has two classes of nuclear microtubules: kinetochore microtubules and interpolar microtubules. One kinetochore microtubule attaches to a single centromere on each chromosome, while approximately four interpolar microtubules emanate from each pole and interdigitate with interpolar microtubules from the opposite spindle to provide stability to the bipolar spindle. On the cytoplasmic face, two to three microtubules extend from the spindle pole toward the cell cortex. Processes requiring microtubule function are limited to spindles in mitosis and to spindle orientation and nuclear positioning in the cytoplasm. Microtubule function is regulated in large part via products of the 6 kinesin gene family and the 1 cytoplasmic dynein gene. A single bipolar kinesin (Cin8, class Kin-5), together with a depolymerase (Kip3, class Kin-8) or minus-end-directed kinesin (Kar3, class Kin-14), can support spindle function and cell viability. The remarkable feature of yeast cells is that they can survive with microtubules and genes for just two motor proteins, thus providing an unparalleled system to dissect microtubule and motor function within the spindle machine.

Mitotic Exit and Separation of Mother and Daughter Cells

“…Loss of its function causes arrest at the M/G1 transition, and it is required for cytokinesis even when this block to G1 entry is overridden by overexpression of a CDK inhibitor (Gruneberg et al 2000;Luca et al 2001;Yoshida et al 2002). It associates with the cytoplasmic face of the SPB, which is embedded in the nuclear envelope (Winey and Bloom 2012). Nud1 recruits other components of the MEN to the SPB, and this localization is critical for MEN activation Cenamor et al 1999;Shou et al 1999;Visintin and Amon 2001;Molk et al 2004;Rock and Amon 2011;Valerio-Santiago and Monje-Casas 2011).…”

“…I mention some critical subjects largely in overview that are described extensively elsewhere. These include the late mitotic spindle and contraction of the cytokinetic apparatus, which are covered in other YeastBook chapters by Bi and Park (2012) and Winey and Bloom (2012), respectively, and are also reviewed extensively elsewhere (Tolliday et al 2001;Walther and Wendland 2003;Balasubramanian et al 2004;Moseley and Goode 2006;Moseley and Nurse 2009;Roncero and Sanchez 2010).…”

The Composition, Functions, and Regulation of the Budding Yeast Kinetochore

“…They switch stochastically between phases of assembly and disassembly, a behavior called dynamic instability (Mitchison and Kirschner 1984). Microtubules are nucleated by the centrosome, which is called the spindle pole body (SPB) in yeast (Winey and Bloom 2012). Microtubules have an inherent polarity with the minus end embedded in the SPB and the dynamic plus end distal.…”

Topology and Control of the Cell-Cycle-Regulated Transcriptional Circuitry

“…Those include dramatic changes in morphogenesis leading to the formation of the bud, the nascent daughter cell with its associated septin ring marking the future site of cytokinesis (reviewed in Bi and Park 2012;Howell and Lew 2012). Additionally, the duplication of the spindle pole body, the first structures associated with assembly of the mitotic spindle (reviewed by Winey and Bloom 2012), and the initiation of DNA replication are triggered following START (reviewed by Remus and Diffley 2009). Although the separation of function is by no means absolute, in general, many genes involved in morphogenesis, including enzymes required for cell wall biosynthesis and septin ring components, are regulated by SBF, whereas genes for DNA replication and repair, including nucleotide biosynthetic enzymes and proteins acting at the replication fork, are regulated by MBF.…”