Characterization of novel mutations at the Schizosaccharomyces pombe cdc2 regulatory phosphorylation site, tyrosine 15

Consequences of abnormal CDK activity in S phase

Cyclin Dependent Kinases (CDKs) are important regulators of DNA replication. In this work we have investigated the consequences of increasing or decreasing the CDK activity in S phase. To this end we identified S-phase regulators of the fission yeast CDK, Cdc2, and used appropriate mutants to modulate Cdc2 activity. In fission yeast Mik1 has been thought to be the main regulator of Cdc2 activity in S phase. However, we find that Wee1 has a major function in S phase and thus we used wee1 mutants to investigate the consequences of increased Cdc2 activity. These wee1 mutants display increased replication stress and, particularly in the absence of the S-phase checkpoint, accumulate DNA damage. Notably, more cells incorporate EdU in a wee1(-) strain as compared to wildtype, suggesting altered regulation of DNA replication. In addition, a higher number of cells contain chromatin-bound Cdc45, an indicator of active replication forks. In addition, we found that Cdc25 is required to activate Cdc2 in S phase and used a cdc25 mutant to explore a situation where Cdc2 activity is reduced. Interestingly, a cdc25 mutant has a higher tolerance for replication stress than wild-type cells, suggesting that reduced CDK activity in S phase confers resistance to at least some forms of replication stress.

Localized activation of Src-family protein kinases in the mouse egg

Recent studies in species that fertilize externally have demonstrated that fertilization triggers localized activation of Src-family protein kinases in the egg cortex. However, the requirement for Src-family kinases in activation of the mammalian egg is different from lower species and the objective of this study was to characterize changes in the distribution and activity of Src-family protein tyrosine kinases (PTKs) during zygotic development in the mouse. Immunofluorescence analysis of mouse oocytes and zygotes with an anti-phosphotyrosine antibody revealed that fertilization stimulated accumulation of P-Tyr-containing proteins in the egg cortex and that their abundance was elevated in the region overlying the MII spindle. In addition, the poles of the MII spindle exhibited elevated P-Tyr levels. As polar body extrusion progressed, P-Tyr-containing proteins were especially concentrated in the region of cortex adjacent to the maternal chromatin and the forming polar body. In contrast, P-Tyr labeling of the spindle poles eventually disappeared as meiosis II progressed to anaphase II. In approximately 24% of cases, the fertilizing sperm nucleus was associated with increased P-Tyr labeling in the overlying cortex and oolemma. To determine whether Src-family protein tyrosine kinases could be responsible for the observed changes in the distribution of P-Tyr containing proteins, an antibody to the activated form of Src-family PTKs was used to localize activated Src, Fyn or Yes. Activated Src-family kinases were found to be strongly associated with the meiotic spindle at all stages of meiosis II; however, no concentration of labeling was evident at the egg cortex. The absence of cortical Src-family PTK activity continued until the blastocyst stage when strong cortical activity became evident. At the pronuclear stage, activated Src-family PTKs became concentrated around the pronuclei in close association with the nuclear envelope. This pattern was unique to the earliest stages of development and disappeared by the eight cell stage. Functional studies using chemical inhibitors and a dominant-negative Fyn construct demonstrated that Src-family PTKs play an essential role in completion of meiosis II following fertilization and progression from the pronuclear stage into mitosis. These data suggest that while Src-family PTKs are not required for fertilization-induced calcium oscillations, they do play a critical role in development of the zygote. Furthermore, activation of these kinases in the mouse egg is limited to distinct regions and occurs at specific times after fertilization.

Relationship of actin, microtubules, and crosswall synthesis during septation in Aspergillus nidulans

Studies of cytokinesis in animal cells demonstrate that microtubules play an important role in signaling the position of the actin-containing contractile ring and subsequent formation of the cleavage furrow. Septation in several fungi closely resembles animal cell cytokinesis in that a circumferential ring of actin is visible at the incipient division site. However, this does not necessarily mean that division is contractile since actin may also serve to localize septal wall synthesis. In addition, several studies in fission yeast have suggested that microtubules are dispensable for actin ring formation. We have used synchronized cells and fluorescence microscopy to follow actin structures, nuclear division and septal wall synthesis during septation in Aspergillus nidulans. Our data suggest that actin first appears at the septum site as a circumferential ring and that it later broadens and invaginates, forming an hourglass-shaped structure coincident with septal cell wall synthesis. Depolymerization of microtubules early in septation prevents circumferential actin ring formation. Depolymerization of microtubules after circumferential actin ring formation blocks both the progression to invaginating bands and septal wall synthesis. In contrast to studies in yeast cells, our data suggest that microtubules are required for both the initiation and progression of septation in A. nidulans.

The fission yeast SPB component Cut12 links bipolar spindle formation to mitotic control

During fission yeast mitosis, the duplicated spindle pole bodies (SPBs) nucleate microtubule arrays that interdigitate to form the mitotic spindle. cut12.1 mutants form a monopolar mitotic spindle, chromosome segregation fails, and the mutant undergoes a lethal cytokinesis. The cut12(+) gene encodes a novel 62-kD protein with two predicted coiled coil regions, and one consensus phosphorylation site for p34(cdc2) and two for MAP kinase. Cut12 is localized to the SPB throughout the cell cycle, predominantly around the inner face of the interphase SPB, adjacent to the nucleus. cut12(+) is allelic to stf1(+); stf1.1 is a gain-of-function mutation bypassing the requirement for the Cdc25 tyrosine phosphatase, which normally dephosphorylates and activates the p34(cdc2)/cyclin B kinase to promote the onset of mitosis. Expressing a cut12(+) cDNA carrying the stf1.1 mutation also suppressed cdc25.22. The spindle defect in cut12.1 is exacerbated by the cdc25.22 mutation, and stf1.1 cells formed defective spindles in a cdc25.22 background at high temperatures. We propose that Cut12 may be a regulator or substrate of the p34(cdc2) mitotic kinase.