The Septin AspB in Aspergillus nidulans Forms Bars and Filaments and Plays Roles in Growth Emergence and Conidiation

Abstract

In yeast, septins form rings at the mother-bud neck and function as diffusion barriers. In animals, septins form filaments that can colocalize with other cytoskeletal elements. In the filamentous fungus Aspergillus nidulans there are five septin genes, aspA (an ortholog of Saccharomyces cerevisiae CDC11), aspB (an ortholog of S. cerevisiae CDC3), aspC (an ortholog of S. cerevisiae CDC12), aspD (an ortholog of S. cerevisiae CDC10), and aspE (found only in filamentous fungi). The aspB gene was previously reported to be the most highly expressed Aspergillus nidulans septin and to be essential. Using improved gene targeting techniques, we found that deletion of aspB is not lethal but results in delayed septation, increased emergence of germ tubes and branches, and greatly reduced conidiation. We also found that AspB-green fluorescent protein (GFP) localizes as rings and collars at septa, branches, and emerging layers of the conidiophore and as bars and filaments in conidia and hyphae. Bars are found in dormant and isotropically expanding conidia and in subapical nongrowing regions of hyphae and display fast movements. Filaments form as the germ tube emerges, localize to hyphal and branch tips, and display slower movements. All visible AspB-GFP structures are retained in ⌬aspD and lost in ⌬aspA and ⌬aspC strains. Interestingly, in the ⌬aspE mutant, AspB-GFP rings, bars, and filaments are visible in early growth, but AspB-GFP rods and filaments disappear after septum formation. AspE orthologs are only found in filamentous fungi, suggesting that this class of septins might be required for stability of septin bars and filaments in highly polar cells. Septins are evolutionarily conserved GTP binding proteins that form complexes and are increasingly viewed as cytoskeletal elements (62). Septins are found in all eukaryotes except higher plants (20,52) and are involved in a variety of cellular processes including cytokinesis, vesicle trafficking, cytoskeleton organization, polarity, and formation of diffusion barriers (35,39,44,53,62). Perhaps, not surprisingly given their roles in such critical processes, septin defects have been associated with many human diseases (29).The septins were first discovered in the budding yeast Saccharomyces cerevisiae as temperature-sensitive cell cycle mutants defective in cytokinesis (31, 41). Early in the yeast cell cycle, septins localize as a cap at the future site of bud emergence. As the bud emerges, septins localize first as a ring at the mother/bud neck and later as an hourglass structure as the bud develops (25,26,37). At cytokinesis, the hourglass structure converts into two rings. Septins at the neck form diffusion barriers that keep the cellular machinery necessary for growth and cytokinesis properly localized (16,18,22,49,50,60). In addition, septins are part of the morphogenesis checkpoint that coordinates nuclear division with bud formation (37).In S. cerevisiae septins organize into hetero-oligomeric octamers that associate end to end to form nonpolar filaments (6). Electron ...

A Candida albicans Temperature-Sensitive cdc12-6 Mutant Identifies Roles for Septins in Selection of Sites of Germ Tube Formation and Hyphal Morphogenesis

“…Consistent with this, septin rings persist in the hyphal cells at sites of septation and do not disassemble quickly after septation as they do in budding cells (35,40). More frequent branching was also seen for septin in mutants of the filamentous fungus Aspergillus nidulans (19,22).…”

Forging the ring: from fungal septins' divergent roles in morphology, septation and virulence to factors contributing to their assembly into higher order structures

“…Fungal septins regulate cell morphology and the emergence of new growth foci (Barral et al, 1999;Berepiki & Read, 2013;DeMarini et al, 1997;Hartwell, 1971;Helfer & Gladfelter, 2006;Hernandez-Rodriguez et al, 2012;Lindsey et al, 2010a;Warenda & Konopka, 2002). In Can.…”

“…In the filamentous ascomycetes, N. crassa and Asp. nidulans, deletion of the core septins caused increased branching (Berepiki & Read, 2013;Hernandez-Rodriguez et al, 2012). Furthermore, heterologous expression of Asp.…”

“…In fungi, septins are well-known for their participation in septation and spore production (Hartwell, 1971;Hernandez-Rodriguez et al, 2012;Lindsey et al, 2010a;Meseroll et al, 2012;Meseroll et al, 2013). In filamentous ascomycetes, deletion of septin genes does not abolish septation but significantly reduces the number of septa in the hyphae (Berepiki & Read, 2013;Chen et al, 2016;Dagdas et al, 2012;Hernandez-Rodriguez et al, 2012;Lindsey et al, 2010a;Vargas-Muñiz et al, 2015).…”

“…In addition to septins regulating cell division, they have also been implicated in a myriad of cellular processes such as cytoskeleton organization, vesicle trafficking, morphogenesis and cell wall maintenance (Table 1) (Chen et al, 2016;Dagdas et al, 2012;Hernandez-Rodriguez et al, 2012;Kozubowski & Heitman, 2010;Li et al, 2012b;Lindsey et al, 2010a;Oh & Bi, 2011;Vargas-Muñiz et al, 2015). In light of more recent literature emerging in the field of fungal septins, here we review the divergent roles of septins in the model and pathogenic fungi, with emphasis on their assembly into higher order structures and their post-translation modification involving phosphorylation and dephosphorylation.…”