Phosphorylation

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A knockout screen for protein kinases required for the proper meiotic segregation of chromosomes in the fission yeast Schizosaccharomyces pombe

A characteristic feature of meiosis is that chromosome replication in S-phase is followed by two consecutive cell divisions to produce haploid cells. Orchestrated regulation of meiotic divisions is critical for proper segregation of chromosomes. Identification and characterization of proteins involved in this process is essential for our understanding of how chromosome number is reduced during meiosis.

One of the main cell division control mechanisms operating during both mitosis and meiosis is the spindle assembly checkpoint (SAC), which monitors the proper attachment of chromosomes to spindle fibers and prevents anaphase until all kinetochores are properly attached.^1,2 SAC proteins play a particularly important role in preventing degradation of cyclin B and securin until completion of the process of attachment of chromosomes to spindle microtubules in metaphase. The SAC inhibits anaphase-promoting complex/cyclosome, and the anaphase onset is delayed until all chromosomes are properly attached to microtubules.² Protein kinases are known to play important roles in SAC regulation and other processes required for proper segregation of chromosomes during both mitosis and meiosis.^3-6

A recent paper by Kovacikova et al.⁷ systematically analyzes the role of non-essential S. pombe protein kinases in meiotic chromosome segregation. Interestingly, the new role for protein kinases Mph1 and Spo4 was discovered. First, they found that Mph1 protein kinase, member of Mps1 family of SAC kinases, is required for proper segregation of recombined homologous chromosomes during meiosis I. This chromosome segregation defect caused by mph1Δ is probably due to precocious start of anaphase I, as is the case in other SAC-defective mutant cells. Second, a new role for Spo4 protein kinase, the fission yeast ortholog of Dbf4-dependent Cdc7 kinase,⁸ was discovered. In S. cerevisiae Cdc7 kinase plays role in setting up mono-orientation of sister kinetochores during the first meiotic division.⁹ Is this function conserved, and are Spo4 kinase and its regulatory subunit Spo6 required for proper segregation of sister centromeres during meiosis in S. pombe? The advantage of S. pombe as a model object is in production of linear asci in which the order of spores reflects the descent of nuclei from the two meiotic divisions. Kovacikova et al. scored the segregation of sister centromeres in a strain with only one copy of chromosome I marked with GFP (lys1-GFP). They observed that in 40% of spo4Δ and spo6Δ asci with four nuclei, lys1-GFP dots occupied both halves of the ascus, which indicated possible missegregation of sister centromeres during meiosis I. However, using more direct method based on staining with antibodies against tubulin and GFP, in spo4Δ and spo6Δ mutants, surprisingly no missegregation in anaphase I and anaphase II was detected. Unexpectedly, formation of extremely elongated anaphase II spindles was observed. These elongated spindles overlapped, and, as a result, corresponding nuclei separated during meiosis II were no longer adjacent (Fig. 1). This observation explained the abnormal pattern of lys1-GFP dots in spores of spo4Δ and spo6Δ mutants and suggested that Spo4 and Spo6 are important for keeping proper length of anaphase II spindles. Although Kovacikova et al. suggested that dysregulation of the activity of the cyclin-dependent kinase may cause abnormal elongation of anaphase II spindles in spo4Δ mutant cells, more work will be needed to understand the mechanism how Spo4 regulates timely anaphase II completion.

Figure 1. A comparison of anaphase II in wild-type (wt) and in spo4Δ mutant (spo4Δ) shows that anaphase II spindles are abnormally expanded in spo4Δ mutant cells.⁷ Tubulin is in red, DNA in blue.

Kovacikova et al. provide another piece of evidence that reversible phosphorylation and protein kinases play an important role in ensuring complete and proper chromosome segregation during meiosis.