Characterization of Septins ASPA and ASPC in Aspergillus nidulans
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To divide each cell must determine a division site and form an organizational scaffold to recruit the proper cellular machinery. This machinery works to accurately replicate and distribute cell contents. Septin proteins are important members of this division scaffold, they affect cell growth and morphology in fungi and animals. Fungi grow in one of two forms: hyphae or yeasts. Saccharomyces cerevisiae grows as yeast under most conditions. Here we show: introduction of the septin AspC from the filamentous fungus A. nidulans caused S. cerevisiae to make elongated cells similar to those of hyphal fungi. AspC localizes to bud and filament necks, while its S. cerevisiae ortholog, Cdc12, localizes only to bud necks. Our results suggest that AspC competes with Cdc12 for incorporation into the yeast septin scaffold and once there alters cell shape. In A. nidulans there are five septins AspA, AspB, AspC, AspD, and AspE. We characterized A. nidulans septins AspA and AspC in an attempt to learn more. In S. cerevisiea, Cdc11 and Cdc12, orthologs of AspA and AspC are essential so results from our studies shed new light on septin function. Deletion mutants show early germination, increased germ tube and branch emergence, abnormal septation and disorganized conidiophores. The unexpected increased growth phenotypes in deletion mutants suggest septins may function as negative regulators or interact with proteins that are negative regulators. AspA-GFP and AspC-GFP show novel fungal septin localization; it is dynamic and found at all stages of growth examined so far. Both septins show different types of animal-like septin localization which is punctate or as micro-filaments. In mammalian cells this type of localization is associated with membrane trafficking or the cytoskeleton. In other organisms septins form multi-septin scaffolds. In S. cerevisiae when Cdc11 or Cdc12 is mutated the other septins do not localize. AspA-GFP localizes in ûaspC and therefore septin structure may still be intact. However, AspC-GFP does not show localization in ûaspA suggesting that discrete septin structures are not formed. Our results suggest that in A. nidulans AspA and AspC may have different roles in septin complex formation, membrane trafficking and cytoskeletal interactions.