Abstract
Septum formation in Neurospora crassa was studied by fluorescent tagging of actin, myosin, tropomyosin, formin, fimbrin, BUD-4, and CHS-1. In chronological order, we recognized three septum development stages: 1) septal actomyosin tangle (SAT) assembly, 2) contractile actomyosin ring (CAR) formation, 3) CAR constriction together with plasma membrane ingrowth and cell wall construction. Septation began with the assembly of a conspicuous tangle of cortical actin cables (SAT) in the septation site >5 min before plasma membrane ingrowth. Tropomyosin and myosin were detected as components of the SAT from the outset. The SAT gradually condensed to form a proto-CAR that preceded CAR formation. During septum development, the contractile actomyosin ring remained associated with the advancing edge of the septum. Formin and BUD-4 were recruited during the transition from SAT to CAR and CHS-1 appeared two min before CAR constriction. Actin patches containing fimbrin were observed surrounding the ingrowing septum, an indication of endocytic activity. Although the trigger of SAT assembly remains unclear, the regularity of septation both in space and time gives us reason to believe that the initiation of the septation process is integrated with the mechanisms that control both the cell cycle and the overall growth of hyphae, despite the asynchronous nature of mitosis in N. crassa.
Highlights
Filamentous fungi such as the ascomycete model fungi, Neurospora crassa and Aspergillus nidulans, grow by apical extension of tubular cells called hyphae
A similar correlation between mitosis and septation may apply to N. crassa [22,23,24,25,26], the asynchronous mitosis in this fungus makes the connection between cell cycle and septum formation difficult to establish
Septa are Formed at Regular Intervals in Mature Leading N. crassa Hyphae
Summary
Filamentous fungi such as the ascomycete model fungi, Neurospora crassa and Aspergillus nidulans, grow by apical extension of tubular cells called hyphae. These cells are compartmentalized by cross walls that retain a central pore through which cytoplasm and organelles including nuclei flow freely. Hyphal septation and yeast cytokinesis are alike but without the final step of cell separation These two processes share key features such as the selection of the cross wall formation site, the assembly of a contractile actomyosin ring (CAR) and the coupled processes of plasma membrane ingrowth and cell wall construction [11,12,13,14,15,16]. A similar correlation between mitosis and septation may apply to N. crassa [22,23,24,25,26], the asynchronous mitosis in this fungus makes the connection between cell cycle and septum formation difficult to establish
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