Septin function in yeast model systems and pathogenic fungi

The septins were first discovered in the budding yeast Saccharomyces cerevisiae and were named for their role in cytokinesis and septum formation (69). Septins are now known to be highly conserved in fungi and animals, although absent in plants and many protozoans (e.g., Plasmodium fasciculatum and Dictyostelium discoideum). The septin proteins are characterized by presence of a distinct type of GTPase domain and by their ability to form filaments. The S. cerevisiae septin proteins form a series of 10-nm filaments that assemble into a ring on the inner surface of the plasma membrane at the bud neck. Septin rings are thought to function as a scaffold to recruit proteins to the bud neck and to act as a boundary domain to restrict diffusion during budding and cytokinesis (30, 39, 66, 68). However, septins are now implicated in a broad range of dynamic membrane events. In S. cerevisiae, septins have been found to also play a role in conjugation and sporulation. Moreover, analyses of septin function in other organisms, including fungi that undergo different patterns of growth and differentiation, are revealing new aspects of septin function. Therefore, we will provide an overview of the current understanding of the relatively well-studied roles of septins during S. cerevisiae budding and then use this as a context to review studies of septin function during other developmental pathways in S. cerevisiae and other fungi. The other fungi will include the model fission yeast Schizosaccharomyces pombe and two opportunistic fungal pathogens: the multimorphic Candida albicans and the filamentous fungus Aspergillus nidulans.