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Category: Microbial Genetics and Molecular Biology; Fungi and Fungal Pathogenesis
The Cytoskeleton in Filamentous Fungi, Page 1 of 2
< Previous page | Next page > /docserver/preview/fulltext/10.1128/9781555816636/9781555814731_Chap16-1.gif /docserver/preview/fulltext/10.1128/9781555816636/9781555814731_Chap16-2.gifAbstract:
This chapter focuses on the microtubule and actin cytoskeletons of filamentous fungi, including the motor proteins that are integral to cytoskeletal function. Relevant results from yeasts are discussed to provide background and context. The emphasis is on more recent data from live-cell imaging as well as genetic and molecular genetic studies. It has been shown that both the microtubule and the actin cytoskeletons play roles in polarized growth of hyphae, and how these cytoskeletal elements function to support hyphal growth and organelle distribution in elongated hyphae is a topic of great interest. Dynein in filamentous fungi also participates in organizing the microtubule network by regulating microtubule dynamics and by providing force for transporting microtubules. In filamentous fungi, the actin cytoskeleton and its myosin motors are important for the delivery of cell membrane and cell wall components to the growing hyphal tip and to the septum. Myosins are a diverse superfamily of actin motor proteins that play various cellular roles. In filamentous fungi such as A. nidulans and N. crassa, four families of myosins have been found, including myosin-I, myosin-II, myosin-V, and the fungus-specific chitin synthases with myosin motor domains. Hyphal growth in filamentous fungi needs both microtubule and actin cytoskeletons, and thus, it would be important to understand how these two systems interact to coordinate vesicle transport towards the hyphal tip.
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Microtubule plus end localization of GFP-labeled cytoplasmic dynein heavy chain (NUDA) and NUDF/LIS1. Microtubules (MT) are stained by an anti-α -tubulin antibody. This figure is a modified version of Fig. 2 from Han et al., 2001 , with permission from Elsevier Ltd.
Diagram showing that early endosomes move bidirectionally along a microtubule. While the anterograde movement towards the microtubule plus end is driven by kinesin-3, the retrograde movement away from the plus end is driven by dynein ( Lenz et al., 2006 ; Abenza et al., 2009 ; Zekert and Fischer, 2009 ).
Model for tip growth in A. nidulans. The Spitzenkörper is not specifically labeled but includes (but is not necessarily limited to) the vesicle cluster and the apical actin cluster as well as the apical SSOA patch, SECC, and apical SEPA, which are not shown. Secretory vesicles containing components necessary for tip growth are transported toward the tip along microtubules powered by kinesin molecules (data not shown). The plus ends of microtubules are extremely dynamic. In some cases, they transiently contact the vesicle cluster. In such cases, the secretory vesicles could be transferred directly from microtubules to the cluster. In other cases, secretory vesicles presumably fall off the microtubule as the plus end disassembles, and they are transported to the vesicle cluster by myosin molecules (not shown) on actin cables. Vesicles fuse with the plasma membrane, releasing their contents, and the components of the membranes of the secretory vesicles (here represented by SYNA) become incorporated into the plasma membrane. As the tip grows, the ring of actin/ABPA endocytic patches moves forward, removing SYNA and other vesicle membrane components from the plasma membrane and incorporating them into endocytic vesicles for recycling. Although we do not have direct evidence, information from other systems and from M. Peñalva, J. Rodríguez, and J. Abenza (personal communication) indicates that these vesicles move to the post-Golgi sorting endosome by mechanisms that are not yet defined. From this compartment, SYNA-containing membranes move away from the tip on microtubules, powered by dynein, to be eventually incorporated into Golgi body-derived secretory vesicles containing cell wall biosynthetic enzymes and wall precursors. This figure and its legend are from Taheri-Talesh et al., 2008 , with permission from the American Society of Cell Biology.