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Category: Microbial Genetics and Molecular Biology; Fungi and Fungal Pathogenesis
Mitotic Cell Cycle Control, Page 1 of 2
< Previous page | Next page > /docserver/preview/fulltext/10.1128/9781555816636/9781555814731_Chap06-1.gif /docserver/preview/fulltext/10.1128/9781555816636/9781555814731_Chap06-2.gifAbstract:
This chapter focuses on the current understanding of how control of the mitotic phase of the cell cycle is achieved in filamentous fungi. Survival of filamentous fungi requires exploration by rapid polarized growth to find nutritional requirements, as well as the production of large numbers of asexual and/or sexual spores that can lie dormant until suitable conditions trigger germination. In fungi, the mitotic microtubule organizing center is the spindle pole body, while in mammalian cells the centrosomes perform this function. As suggested by the fact that filamentous fungi often maintain many nuclei in a common cytoplasm, cytokinesis does not accompany every nuclear division. The process of cytokinesis during filamentous growth is generally achieved by the formation of a cross wall called a septum at a specific point in hyphae. The chapter talks about the biochemical activities that regulate mitotic entry, which are conserved in different filamentous fungi. It discusses some of the genes identified in the extragenic suppressor screens and highlights one particular extragenic suppressor screen. This screen, performed by Berl Oakley’s lab, was for extragenic suppressors of the benA33β-tubulin mutant and led to the discovery of mipB, which encoded a new type of tubulin now known by its universal name, γ-tubulin. The chapter also discusses in detail how NIMA regulates changes in mitotic nuclear transport. Filamentous fungi provide rich and varied experimental opportunities to further our understanding of mitosis and its regulation. Until relatively recently, A. nidulans has been the workhorse of the filamentous fungi for studies of mitosis.
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Schematic showing mammalian open mitosis and fungal closed mitosis. The NE breaks down during open mitosis so that microtubules nucleated from the cytoplasmic centrosomes can attach to kinetochores. During closed mitosis the spindle pole bodies are in the NE and nucleate spindle microtubules that attach to kinetochores within nuclei.
Regulation of Cdk1/cyclin B by tyrosine dephosphorylation. Cdk1/cyclin B activation requires dephosphorylation carried out by the Cdc25 phosphatase.
Mitotic entry requires both Cdk1/cyclin B and NIMA kinase activities in A. nidulans. Active Cdk1/cyclin B cannot enter the nucleus without NIMA activity. NIMA needs Cdk1/cyclin B-dependent phosphorylations to become fully active. Activation of both Cdk1/cyclin B and NIMA allows them to enter the nucleus and phosphorylate their nuclear substrates, thereby triggering mitosis.
The APC regulates mitotic exit by triggering sister chromatid segregation. Cdc20 binds to and activates the APC. This allows the APC to ubiquitinate securin, targeting it for degradation by the proteasome. Securin degradation relieves inhibition of separase, which then degrades the Scc1 component of the securin ring complex. Spindle defects activate the SAC, which prevents Cdc20 binding to APC.
Schematic of interphase and mitotic A. nidulans nuclei. NPCs are embedded within the NE. During interphase, Nups occupy the central channel of NPCs, restricting diffusion. In mitosis, the NPCs partially disassemble such that the central channel is now open and the NE is permeable. This facilitates nuclear entry of tubulin, allowing spindle formation from the spindle pole bodies.
Septation is not always linked to nuclear division in A. nidulans. (A) A germling containing eight nuclei, which underwent its first asymmetric septation following the third mitosis. (B) Diagram of a conidiophore that has formed by growth of a multinucleate cell containing a stalk and vesicle. This cell originally formed by growth of a specialized aerial hypha termed the vesicle from the foot cell, in which multiple nuclear divisions occur without septation. Cytokinesis is linked to nuclear division during the formation of metulae and phialides as well as when uninucleate conidiospores form by budding from the phialide. Cytokinesis occurs through septation in hyphae, but by budding to form metulae, phialides and conidia.
Schematic showing how development of a clamp and specialized septa help maintain the dikaryotic state of basidiomycete hyphae (adapted from Iwasa et al., 1998 ).
Cytokinesis and nuclear division in different A. nidulans cell types