The regulation of the cell cycle during early development is an important and complex biological process. We have cloned a cDNA, XCS-1, that may play an important role in regulating mitosis during early embryogenesis in Xenopus laevis. XCS-1 is a maternally expressed gene product that is the Xenopus homologue of the human cleavage signal protein (CS-1). XCS-1 transcripts were detected in oocytes with the titer decreasing just prior to the MBT. During development the XCS-1 protein was detected on the membrane and in the nucleus of blastomeres. It was also detected on the mitotic spindle in mitotic cells and on the centrosomes in interphase cells. Overexpression of myc-XCS-1 in Xenopus embryos resulted in abnormal mitoses with increased numbers of centrosomes, multipolar spindles, and abnormal distribution of chromosomes. Also, we observed incomplete cytokinesis resulting in multiple nuclei residing in the same cytoplasm with the daughter nuclei in different phases of the cell cycle. The phenotype depended on the presence of the N terminus of XCS-1 (aa 1–73) and a consensus NIMA kinase phosphorylation site (aa159-167). Mutations in this site affected the ability of the overexpressed XCS-1 protein to produce the phenotype. These results suggest that XCS-1 is a maternal factor playing an important role in the regulation of the cell cycle during early embryogenesis and that its function depends on its state of phosphorylation.

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