The Schizosaccharomyces pombe cdc5+ gene was identified in the first screen for cell division cycle mutants in this yeast. The cdc5+ gene was reported to be required for nuclear division but because of its modest elongation and leaky nature at the non‐permissive temperature, it was not investigated further. Here, we report the characterization of the single allele of this gene, cdc5‐120, in more detail. The mutant arrests with a 2N DNA content and a single interphase nucleus. Further genetic analyses suggest that cdc5+ gene function is essential in the G2 phase of the cell cycle. We have cloned and sequenced the cdc5+ gene. The deduced protein sequence predicts that Cdc5 is an 87 kDa protein and contains a region sharing significant homology with the DNA binding domain of the Myb family of transcription factors. Deletion mapping of the cdc5+ gene has shown that the N‐terminal 232 amino acids of the protein, which contain the Myb‐related region, are sufficient to complement the cdc5ts strain. A cdc5 null mutant was generated by homologous recombination. Haploid cells lacking cdc5+ are inviable, indicating that cdc5+ is an essential gene. A fusion protein consisting of bacterial glutathione S‐transferase joined in‐frame to the N‐terminal 127 amino acids of the Cdc5 protein is able to bind to DNA cellulose at low salt concentrations. This evidence suggests that cdc5+ might encode a transcription factor whose activity is required for cell cycle progression and growth during G2.

The conserved CDC5 family of Myb-related proteins performs an essential function in cell cycle control at G2͞M. Although c-Myb and many Myb-related proteins act as transcription factors, herein, we implicate CDC5 proteins in pre-mRNA splicing. Mammalian CDC5 colocalizes with pre-mRNA splicing factors in the nuclei of mammalian cells, associates with core components of the splicing machinery in nuclear extracts, and interacts with the spliceosome throughout the splicing reaction in vitro. Furthermore, genetic depletion of the homolog of CDC5 in Saccharomyces cerevisiae, CEF1, blocks the first step of pre-mRNA processing in vivo. These data provide evidence that eukaryotic cells require CDC5 proteins for pre-mRNA splicing. The Schizosaccharomyces pombe cdc5-120 mutant was isolated in a screen for mutants defective in cell cycle progression (1). At the restrictive temperature, cdc5-120 cells arrest growth in G 2 (1, 2), indicating that cdc5 ϩ function is required for G 2 ͞M progression. CDC5 has been conserved throughout evolution, and related genes have been cloned from Saccharomyces cerevisiae (termed CEF1; ref.3), Arabidopsis thaliana (4), Drosophila melanogaster (3), Caenorhabditis elegans (3), Xenopus laevis (5), and Homo sapiens (3, 6, 7). We conclude that these proteins are conserved functionally, because D. melanogaster and human CDC5 (hCDC5) complement the cdc5-120 mutant, S. cerevisiae CEF1 is essential during G 2 ͞M in this evolutionarily distinct yeast (3), and overexpression of dominant negative forms of hCDC5 slows G 2 progression in mammalian cells (8).In their N termini, CDC5 proteins are highly related to the DNA-binding domain of human c-Myb (2, 3, 9). Whereas human c-Myb contains three Myb repeats, Ϸ50-amino acid motifs with characteristic spacing of tryptophan residues (9), CDC5 proteins contain two Myb repeats (R1 and R2) followed by a Myb-likerepeat (MLR3) that contains some, but not all, of the hallmarks of a typical Myb repeat (3). Based on their homologies to c-Myb, CDC5 proteins were hypothesized to carry out their essential function in cell cycle control through transcriptional regulation, a notion supported by the following observations: (i) the Myb repeats of S. pombe cdc5p fused to glutathione S-transferasebound DNA cellulose (2); (ii) the Myb repeats of A. thaliana cdc5p selected a specific DNA sequence in a cyclic amplification and selection of targets protocol (4); and (iii) the C terminus of hCDC5 fused to the GAL4-DNA binding domain activated transcription in a reporter assay (8). To date, however, no downstream transcriptional targets for any of the CDC5 proteins have been identified. hCDC5 was identified recently in a biochemical purification of the mammalian spliceosome assembled in vitro (10), indicating that CDC5 proteins may be involved in pre-mRNA splicing rather than transcriptional regulation. Herein, we extend this observation by showing that mammalian CDC5 colocalizes with splicing factors in the nuclei of mammalian cells, coimmunoprecipitates with core components of th...

SUMMARYChromosome alignment at the equator of the mitotic spindle is a highly conserved step during cell division, however, its importance to genomic stability and cellular fitness are not understood. Normal mammalian somatic cells lacking Kif18A function complete cell division without aligning chromosomes. These alignment-deficient cells display normal chromosome copy numbers in vitro and in vivo, suggesting that chromosome alignment is largely dispensable for maintenance of euploidy. However, we find that loss of chromosome alignment leads to interchromosomal compaction defects during anaphase, abnormal organization of chromosomes into a single nucleus at mitotic exit, and the formation of micronuclei in vitro and in vivo. These defects slow cell proliferation and reduce postnatal growth and survival with variable penetrance in mice. Our studies support a model in which the alignment of mitotic chromosomes promotes proper nuclear envelope reassembly and continued proliferation by ensuring that chromosomes segregate as a compact mass during anaphase.