Septum Formation and Cytokinesis in Ascomycete Fungi

Abstract

Cell division is a fundamental cellular process that is essential for proliferation of unicellular as well as multicellular organisms. It reflects the final stage ofthe cell cycle, during which a cell is physically divided into two daughter cells that contain a full set of chromosomes and other cellular organelles. Cytokinesis can be divided into several general steps that apply to most eukaryotic cells (Barr and Gruneberg 2007; Eggert et al. 2006): (1) the selection ofthe future cell division plane based on spatial as well as temporal cues, (2) the assembly of a cortical actomyosin ring (CAR) at this site, and (3) its constriction coupled with membrane invagi- nation. In general, the formation of the CAR and its subsequent constriction is tightly cou- pled to the cell cycle to ensure that cell separ- ation does not occur prior to chromosome segregation. (4) The formation of an extra- cellular cell wall, the septum, composed of glu- cans, chitin, and other polysaccharides in fungi further requires coordination of CAR constric- tion with secretion of cell wall biosynthetic and remodeling enzymes to build the extracellular septum. (5) This primary septum is covered by additional layers of cell wall material that form the secondary septum and is finally degraded by secreted hydrolytic enzymes in the uni- cellular yeasts and during sexual development of filament-forming molds to allow detachment of the two cells.