ABSTRACT
In S. cerevisiae it is possible to judge the stage o f the cell division cycle o f any particular cell by its morphology. Cells in late G l phase o f the cell cycle have no bud. Upon entry into S phase, a bud starts to emerge from the surface o f the mother cell. Throughout S phase, the bud grows rapidly at its tip. In G2 phase the bud switches from polarized tip growth to isotropic growth and becomes ovoid in shape. In mitosis (M), the bud approaches the size o f the mother cell and the process o f nuclear division provides the bud with a nucleus. Upon exit from M phase, the cells undergo cytokinesis, the process by which the cytoplasmic contents o f the mother and daughter cells are divided following the completion o f nuclear division.1,7,8
Septins are important proteins in polarity. The position o f the bud neck in S. cerevisiae is determined by septins. They are believed to comprise 10 nm diameter filaments seen to line the bud neck in electron micrographs. Septins are the first proteins to assemble at the site on the mother cell where the bud will emerge. They form a small ring at this site and the new bud grows out from within the septin ring. During M phase when the bud approaches the size o f the mother cell the single septin ring splits and separates to form two rings, one on the mother cell side and one on the daughter cell side o f the neck. During cytokinesis the two septin rings move further apart. The mother and daughter cell each inherit one septin ring. After cell separation the septin ring becomes diffuse and the septins appear to migrate around the cortex and concentrate at the new bud site.1,7,9
The Saccharomyces cerevisiae Actin Cytoskeleton Like mammalian cells S. cerevisiae cells have both actin-based microfilament and tubulin-based
microtubule cytoskeletons and many components are equivalent. There are three major F (filamentous)-actin structures in S. cerevisiae: cortical actin patches, cytoplasmic actin cables, the cytokinetic actomyosin ring. Cortical actin patches are highly dynamic and short-lived spots o f F-actin that continually form at the cortex, move very rapidly over short distances and then disassemble. Cortical actin patches are made up o f highly branched actin filaments nucleated by a seven-subunit protein complex called the Arp2/3 complex. The Arp2/3 complex has weak activity unless stimulated by a Nucleation Promoting Factor (NPF) that in S. cerevisiae include Abplp, Panlp, the type I myosins (МуоЗр and Myo5p) and Lasl7p.1,1012 Lasl7p is the unique yeast ortholog o f the mammalian Wiskott-Aldrich syndrome Protein (WASP) family and the only activator to bind monomeric actin. Lasl7p incorporates signals from upstream Rho family GTPases (Cdc42p, Rho3p and Rho4p) and interacts with a number o f proteins including verprolin (Vrplp) (the human WASP-Interacting Protein, or WIP, homolog) and the type I myosins in a module that signals to the Arp2/3 nucleator complex.13'15 C DC 42 activates the Arp2/3 complex indirectly in mammalian cells by alleviating auto-inhibition in WASP and its ubiquitously expressed homolog N-(neuronal) WASP. In contrast, other WASP-related proteins such as mammalian WAVE (WASP-familv Verprolin-homologous protein) and S. cerevisiae Las 17p do not have GTPase binding domains. Lasl7p does not appear to be auto-inhibited, at least in vitro.16
