Characterization of the Aspergillus nidulans septin AspB and its interactions
Rodriguez, Yainitza Hernandez
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Septins are evolutionarily conserved GTP binding proteins that form heteroligomeric complexes, are considered novel cytoskeletal elements and control diverse cellular functions and processes. Septins are found in some protists, fungi and animals, and though present in some algae they are absent in higher plants. Septins have been shown to interact with and organize actin and tubulin and have been suggested to bridge interactions between both cytoskeleton systems. We explored these connections by characterizing the septin AspB and its interaction with actin and microtubules in the filamentous fungus Aspergillus nidulans. AspB is key for conidiophore morphology and sporulation as ΔaspB shows conidiophores with fused abnormal layers, very few attached conidia and poor conidiation. ΔaspB also shows early germination, multiple emergence of polarity axes during germ tube and branch formation and delayed septation. AspB localizes to sites of germ tube emergence, septation and branching and emerging layers of the conidiophore. AspB forms filaments and bars that are lost in the absence of septins AspA and AspC, suggesting that septin-septin interactions are necessary for filament formation. AspB filaments are extremely reduced in hyphae of ΔaspE, a septin exclusive to filamentous fungi. AspB seems to influence polarity by interacting with microtubules as they colocalize, ΔaspB hyperemergence of growth can be reduced by depolymerization of microtubules, fewer microtubule bundles were observed in newly formed branches of ΔaspB and depolymerization of microtubules results in abnormal AspB filaments. AspB might also influence polarity by interacting with actin as they colocalize, polarity defects were observed in ΔaspB after depolymerization of actin, tropomyosin TpmA was lost from actin filaments and depolymerization of actin resulted in abnormal AspB cytoplasmic ring localization. AspB might play roles in endocytosis as AspB colocalized with endocytic vesicles and the endocytic zone is absent or of abnormal length and abnormally positioned in ΔaspB. We show that AspB is essential for normal growth, development and conidiation, it localizes throughout fungal growth and development, it may play roles in nuclear dynamics and might control polar growth by its interactions with the endocytic machinery, actin and microtubules and/or their associated proteins.