Aspergillus nidulans Septin AspB Plays Pre- and Postmitotic Roles in Septum, Branch, and Conidiophore Development

Members of the septin family of proteins act as organizational scaffolds in areas of cell division and new growth in a variety of organisms. Herein, we show that in the filamentous fungus Aspergillus nidulans, the septin AspB is important for cellular division, branching, and conidiation both pre- and postmitotically. AspB localizes postmitotically to the septation site with an underlying polarity that is evident as cytokinesis progresses. This localization at the septation site is dependent on actin and occurs before the cross-wall is visible. AspB localizes premitotically as a ring at sites of branching and secondary germ tube emergence. It is the only known branch site marker. In addition, AspB is found at several stages during the development of the asexual reproductive structure, the conidiophore. It localizes transiently to the vesicle/metula and metula/phialide interfaces, and persistently to the phialide/conidiospore interface. A temperature-sensitive mutant of AspB shows phenotypic abnormalities, including irregular septa, high numbers of branches, and immature asexual reproductive structures

Septin Function in Candida albicans Morphogenesis

The septin proteins function in the formation of septa, mating projections, and spores in Saccharomyces cerevisiae, as well as in cell division and other processes in animal cells. Candida albicans septins were examined in this study for their roles in morphogenesis of this multimorphic, opportunistically pathogenic fungus, which can range from round budding yeast to elongated hyphae. C. albicans green fluorescent protein labeled septin proteins localized to a tight ring at the bud and pseudohyphae necks and as a more diffuse array in emerging germ tubes of hyphae. Deletion analysis demonstrated that the C. albicans homologs of the S. cerevisiae CDC3 and CDC12 septins are essential for viability. In contrast, the C. albicans cdc10Δ and cdc11Δ mutants were viable but displayed conditional defects in cytokinesis, localization of cell wall chitin, and bud morphology. The mutant phenotypes were not identical, however, indicating that these septins carry out distinct functions. The viable septin mutants could be stimulated to undergo hyphal morphogenesis but formed hyphae with abnormal curvature, and they differed from wild type in the selection of sites for subsequent rounds of hyphal formation. The cdc11Δ mutants were also defective for invasive growth when embedded in agar. These results further extend the known roles of the septins by demonstrating that they are essential for the proper morphogenesis of C. albicans during both budding and filamentous growth