6 Septation and Cytokinesis in FungiJ. Wendland 1 , A. Walther 1CONTENTSI. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 105II. Selecting Sites of Septation . . . . . . . . . . . . . . 106A. Bud Site Selectionin Saccharomyces cerevisiae . . . . . . . . . . . 106B. Placement of the Site of Cell Divisionin Schizosaccharomyces pombe . . . . . . . . . 108C. Septation in Filamentous Fungi . . . . . . . . 109III. Protein Complexes at Septal Sites . . . . . . . . . 110A. The Septin Ring . . . . . . . . . . . . . . . . . . . . 111B. The Acto-Myosin Ring . . . . . . . . . . . . . . . 112IV. Dynamic Contraction of the Acto-MyosinRing and Chitin Synthesis Lead to SeptumFormation . . . . . . . . . . . . . . . . . . . . . . . . . . . 113V. Coordination of the End of Mitosiswith Cytokinesis . . . . . . . . . . . . . . . . . . . . . . 116VI. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . 116References . . . . . . . . . . . . . . . . . . . . . . . . . . . 117I. IntroductionFungi display either of two growth modes:yeast-like or f i lamentous. Yeast-like growth, asexemplif i edbytheunicellularbaker’syeastSaccha-romycescerevisiae,ischaracterizedbycytokinesis,a process that results in the partial degradationof the chitin-rich septum to allow separation ofdaughter cells from their mother cells. Polarizedgrowth in f i lamentous fungi produces hyphae thatare compartmentalized by septation. Septation inf i lamentous fungi is carried out by proteins whosehomologs act during cytokinesis in yeast cells.However, septa do not separate the cytoplasms ofadjacent compartments, due to the presence ofa septal pore. Chitin hydroloysis is absent at septalsites, which allows the formation of a multicellularmycelium. Cytokinesis and septation mark theultimate step of a cell cycle and, therefore, need to1 Junior Research Group, Growth Control of Fungal Pathogens,Leibniz Institute for Natural Products Research and InfectionBiology, Hans-Knöll Institute and Department of Microbiology,Friedrich-Schiller University Jena, Beutenbergstr. 11a, 07745 Jena,Germanybe tightly controlled with other processes such asdaughter cell growth/tip growth, DNA replicationand mitosis. Although the concept of cell divisionis a fundamental problem of all cells, differingsolutions have been developed in bacteria, fungi,plants, and animals. Cell division has to deal withthe following tasks:1. a site has to be chosen as division plane,2. proteincomplexesassembleatthischosensite,3. infungal,aswellasinanimalcells,acontractileacto-myosin ring that has been formed earlierbecomes dynamic at the end of mitosis, which4. is accompanied by chitin deposition.Very elaborate control mechanisms, called eithermitotic exit network (MEN) or septation initiationnetwork (SIN), have evolved to link mitosis withcytokinesisinyeast-likeorganisms,althoughinf i l-amentousascomycetesthatgenerate multinucleatecellular compartments, a strict coupling of mitoticexitandseptationseemsquestionable.Synthesisofa chitin-rich septum occurs both in yeast-like andf i lamentous fungi. In yeast, daughter cells activelyseparate from their mother cells by dissolving thechitin ring structure, using specif i c chitinases. Inf i lamentous fungi, however, the septum remainsintact, and the cytoplasm of apical and subapicalcompartments is connected via septal pores.These differences between septation andcytokinesis, as well as similarities between fungiand animal cells have provided the basis for theuse of tractable fungal model systems to elucidatethe underlying mechanisms. In this chapter,we will discuss the key events leading to septa-tion/cytokinesis. We start with the yeast modelsystems S. cerevisiae and Schizosaccharomycespombe, and then compare knowledge acquiredin these organisms with results from f i lamentousfungi, particularly Neurospora crassa, Aspergillusnidulans, and Ashbya gossypii. Analysis of con-served features is facilitated by the availability ofgenome sequences for these fungi (see Table 6.1).The Mycota IGrowth, Differentation and SexualityKües/Fischer (Eds.)© Springer-Verlag Berlin Heidelberg 2006
