Protein kinases required for proper segregation of chromosomes during meiosis

Abstract

Cellular division involves complex processes in which protein phosphorylation plays crucial role. As chromosome segregation is a key process in both mitosis and meiosis, its proper progression is essential for cell growth and viability. Although numerous studies investigating the role of protein kinases on chromosome segregation have been performed,1-3 no systematic analysis of their involvement in meiosis has been performed so far. The fission yeast Schizosaccharomyces pombe, a widely used model organism for studying eukaryotic biology, possesses more then one hundred protein kinases. From these, 96 are non-essential and can be deleted from the yeast genome without affecting cell viability. Kovacikova et al. analyzed chromosome segregation during meiosis in yeast strains carrying the S. pombe non-essential kinase knockout alleles and found seven mutants with apparent defect in meiosis.4 Strong defect in meiotic chromosome segregation was observed in bub1∆ and mph1∆ strains, while the remaining strains, hhp2∆, ppk24∆, mug27∆, spo4∆ and atg1∆, showed various alterations, such as weak missegregation phenotype, asci with more than four DNA masses or lagging chromosomes. In their study, Kovacikova et al. focused on the analysis of mph1∆ and spo4∆/spo6∆ mutations, as phenotypes of remaining strains, except for ppk24∆, have previously been described.5,6 First, authors analyzed cells lacking the Mph1 protein kinase. They marked chromosome II in mph1∆ knockout cells with GFP dots and tested chromosome segregation. Analysis of mph1∆ mutant cells carrying homozygous cen2-GFP dots revealed 10% homolog non-disjunction during first meiotic division. When GFP dots were present only on one copy of the chromosome II (heterozygous cen2-GFP), no major defect in sister chromatid segregation was observed when compared with wild-type cells. This indicates that the fission yeast Mph1, similar to other components of the spindle assembly checkpoint, plays a crucial role in homolog disjunction during meiosis I. The other question was if Spo4/Spo6, a meiosis-specific kinase complex that was shown to be important for progression of the second meiotic division,7 and its ortholog Dbf4-dependent Cdc7 kinase, required for DNA replication in most eukaryotes,8 is also required for proper sister chromatid segregation during meiotic divisions. Kovacikova and co-authors analyzed spo4∆ and spo6∆ mutant cells, where one copy of the first chromosome was marked with GFP (lys1-GFP). Although, most of cells arrested at the binucleate stage, a small fraction went through both meiotic divisions. These cells were able to produce asci with four nuclei. Scoring GFP dots in such asci showed that more than 40% contained GFP dots in both halves, suggesting missegregation of sister centromeres during meiosis I. Unexpectedly, missegregation of sister chromatids was not observed when the authors analyzed anaphase I cells. Importantly, Kovacikova et al. noticed that anaphase II spindles in spo4∆ and spo6∆ cells were significantly longer as compared with wild-type spindles. Live-cell imaging showed that these spindles pushed sister nuclei apart, leading to abnormal position of spore in mutant asci. This defect could be rescued only with the wild-type allele and not the “kinase dead” allele of Spo4 (Spo4K95A), which indicates that altered Spo4 kinase activity is responsible for abnormal meiosis II spindle elongation. Moreover, duration of anaphase II in spo4∆ cells was more than twice as long as compared with wild-type cells. This implies that Spo4 is required for proper timing of anaphase II. Interestingly, similar results were observed with another sporulation-defective strain, spo5∆, which points out the possible link between sporulation and proceeding of anaphase II. Concomitant inhibition of the S. pombe CDK kinase, Cdc2, together with spo4 deletion caused reduction of the spo4∆ mutant phenotype to approximately one-half, suggesting that altered Cdc2 activity is responsible for abnormal spindle elongation in cells lacking Spo4. Taken together, Kovacikova et al. showed that protein kinases such as Mph1 or Spo4 play important roles during meiosis, and detailed studies of protein kinases and reversible protein phosphorylation are essential for our understanding of processes ensuring proper chromosome segregation.