Chapter 26 : Fungal Cell Cycle: A Unicellular versus Multicellular Comparison

Mitosis, septation, and cytokinesis in Saccharomyces cerevisiae and Aspergillus nidulans. Three rounds of mitosis in S. cerevisiae beginning from vegetatively growing yeast cell (top) compared to A. nidulans beginning from dormant conidium (bottom). Black dots are nuclei. M1, M2, and M3 denote the indicated mitotic divisions.

Cell cycle landmarks in Saccharomyces cerevisiae and Aspergillus nidulans. Major morphological landmarks for each cell cycle stage are shown for S. cerevisiae (left) and A. nidulans (right). Gray circle, nucleus; solid border, intact nuclear envelope; broken-line border, partial nuclear pore complex disassembly; small black dots, spindle pole bodies; blue dots, nucleolus; lines, microtubules. (Data from references 117 and 120 .)

Cyclin-dependent kinase (CDK)/cyclin B regulation. Comparison of CDK and cyclin activity at G2/M phases in Schizosaccharomyces pombe (canonical), Saccharomyces cerevisiae, and Aspergillus nidulans. (Data from reference 52 .)

Model of the Cdc fourteen early anaphase release (FEAR) network and mitotic exit network (MEN) at the end of mitosis. In early anaphase in Saccharomyces cerevisiae the FEAR network promotes the release of Cdc14 from the nucleolus. In late anaphase the MEN releases Cdc14 from the nucleus into the cytoplasm, where it activates both Swi5 and Hct1. Swi5 triggers the transcription of the cyclin-dependent kinase (CDK) inhibitor Sic1. Hct1 binds the anaphase-promoting complex (APC), and this complex marks Cdk1 for degradation. These events cause the cell to exit mitosis and enter into a new cell division cycle. Gray circles represent the nucleus. (Modified from reference 121 with permission.)

Cell cycle checkpoints in Saccharomyces cerevisiae and Aspergillus nidulans. Sensors (depicted as rectangles) monitor the completion of specific cell cycle events (depicted as diamonds) and forward the information to effectors that halt or reboot the cell cycle. G1: Reboot regulation (not reported in S. cerevisiae). S phase: DNA damage network includes branches that are responsible for DNA repair and damage-induced transcription. Depending on when the checkpoint is triggered, cell cycle progression can be delayed or halted at G1/S or G2/M boundaries or DNA replication can be slowed (shown as dotted line). S/G2: Morphogenesis checkpoint (not reported in A. nidulans). M phase: Spindle assembly checkpoint (SAC) and spindle position checkpoint (SPOC) SAC arrests at metaphase, preventing anaphase. SPOC not reported in A. nidulans. (Data from reference 122 .)

DNA damage checkpoint in Saccharomyces cerevisiae and Aspergillus nidulans. Sensor proteins monitor DNA damage and transmit the information to transducers and effectors that ultimately modulate the cell cycle. Only proteins discussed in text are shown. DSB, double-strand break; RPA, replication protein A; ATRIP, ATR interacting protein. (Data from reference 80 .)

Spindle assembly checkpoint (SAC) in Saccharomyces cerevisiae. Unattached kinetochores activate the SAC response by recruiting MPS1, which phosphorylates Knl1. The KMN (Knl1-Mis12-Ndc80) network serves as a scaffold for the recruitment of checkpoint proteins. Binding of Bub1/3 promotes the recruitment of the Mad1/Mad2 core complex. Kinetochore-bound Mad1/Mad2 catalyzes the conformational change of the open Mad2-O to the closed Mad2-C form. Mad2-C interacts with Cdc20 and forms the mitotic checkpoint complex (MCC) consisting of Cdc20, Mad2-C, BubR1, and Bub3. This inhibits the anaphase-promoting complex activator Cdc20 and blocks the metaphase to anaphase transition. KT, kinetochore. (Data from reference 92 .)

Spindle position checkpoint (SPOC) controls mitotic exit in Saccharomyces cerevisiae. When the spindle is misaligned, Bub2/Bfa1 keeps Tem1 in its inactive form. Kin4 protects Bub2/Bfa1 itself from an inhibitory modification. Bub2/Bfa1 disappears from the mother cell once one spindle pole body (SPB) is correctly positioned in the daughter cell. At the same time the amount of Tem1 and Bub2/Bfa1 increases at the SPB of the daughter cell. Lte1, which is located at the cortex of the daughter cell, activates Tem1, and Bub2/Bfa1 is phosphorylated by the polo kinase Cdc5. Bub2/Bfa1 disappears from the daughter cell, whereas Kin4 is removed from the SPB of the mother cell. Simultaneously, Lte1 diffuses into the cytoplasm of the mother and daughter cells. It is possible that all these events contribute equally to the activation of the mitotic exit network. The nucleus is depicted as a gray circle, and the pink highlighted part of the cell shows Lte1 in the cytoplasm. The blue empty circle represents the GTPase Tem1, and the blue filled circle is Tem1 associated with the dimer complex Bub2/Bfa1. (Data from reference 123 .)

Summary of the morphogenesis checkpoint in Saccharomyces cerevisiae. In the nucleus the kinase Swe1 inhibits Cdc28 via phosphorylation. When the bud emerges, Elm1, Hsl1, and Hsl7 are recruited to the bud side of the septin collar and in turn recruit a subpopulation of Swe1 from the nucleus to the bud side of the septin collar, where it is degraded. This depletes nuclear Swe1 and allows progression into mitosis. Actin depolymerization and delayed budding activate the morphogenesis checkpoint, which stabilizes Swe1. Dotted lines represent the septin collar between mother and bud. The gray circle represents the nucleus. Swe1 with a solid border is stable and active. Swe1 with a broken line border is destabilized. The arrow shows recruitment of Swe1 from the nucleus.