Submitted to: Eukaryotic Cell
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/23/2009
Publication Date: 1/2/2010
Citation: Lindsey, R.L., Cowden, S., Hernandez-Rodriguez, Y., Momany, M. 2010. Septins AspA and AspC are important for normal development and limit the emergence of new growth foci in the multicellular fungus Aspergillus nidulans. Eukaryotic Cell. 9(1):155-163. Interpretive Summary: Septins are cytoskeletal proteins found in fungi, animals and microsporidia where they form multi-septin complexes that act as scaffolds recruiting and organizing other proteins to ensure normal cell division and development. There are five septins found in the filamentous fungus Aspergillus nidulans (AspA, AspB, AspC, AspD and AspE). Here we characterize the septins AspA and AspC in A. nidulans. Mutants isolates in which the septins AspA or AspC or both AspA and AspC have been removed show early abnormalities in cell growth and an increased number of germ tubes and branches. Localization studies using green fluorescent protein (GFP) attached to the septins AspA and AspC show identical patterns when cells are observed under a microscope. AspA with GFP forms abnormal structures in strains without AspC while AspC with GFP does not localize in strains without AspA. Our results show that AspA and AspC interact with each other and they are both important for normal cell development, especially for preventing the inappropriate emergence of germ tubes and branches. To our knowledge, this is a first report of an increase in the number of new growth foci associated with loss of septin function in any organism and raises the intriguing possibility that septins might limit new growth foci in other multicellular organisms.
Technical Abstract: Septins are cytoskeletal proteins found in fungi, animals and microsporidia where they form multi-septin heteropolymeric complexes that act as scaffolds recruiting and organizing other proteins to ensure normal cell division and development. Here we characterize AspA and AspC, two of the five septins found in the multicellular, filamentous fungus A. nidulans. Mutants with deletions of aspA, aspC or both aspA and aspC show early and increased emergence of germ tubes and branches, abnormal septation and disorganized conidiophores. Strains in which the native aspA has been replaced with a single copy of aspA-GFP driven by the native septin promoter or in which aspC has been replaced with a single copy of aspC-GFP driven by the native promoter show wild type phenotypes. AspA-GFP and AspC-GFP show identical localization patterns as discrete spots or bars in dormant and expanding conidia, rings at forming septa and at the bases of emerging germ tubes and branches, and as punctate spots and filaments in the cytoplasm. In conidiophores, AspA-GFP and AspC-GFP localize as diffuse bands or rings at the bases of emerging phialide layers and conidial chains and as discrete spots or bars in newly formed conidia. AspA-GFP forms abnormal structures in 'aspC strains while AspC-GFP does not localize in 'aspA strains. Our results show that AspA and AspC interact with each other and are important for normal development, especially for preventing the inappropriate emergence of germ tubes and branches. This is the first report of a septin limiting the emergence of new growth foci in any organism.