The novel murine calmodulin-binding protein Sha1 disrupts mitotic spindle and replication checkpoint functions in fission yeast

Entry into mitosis is normally blocked in eukaryotic cells that have not co mpleted replicative DNA synthesis; this "S-M' checkpoint control is fundame ntal to the maintenance of genomic integrity. Mutants of the fission yeast Schizosaccharomyces pombe defective in the S-M checkpoint fail to arrest th e cell cycle when DNA replication is inhibited and hence attempt mitosis an d cell division with unreplicated chromosomes, resulting in the 'cut' pheno type. In an attempt to identify conserved molecules involved in the S-M che ckpoint we have screened a regulatable murine cDNA library in S. pombe and have identified cDNAs that induce the cut phenotype in cells arrested in S phase by hydroxyurea. One such cDNA encodes a novel protein with multiple c almodulin-binding motifs that, in addition to its effects on the S-M checkp oint, perturbed mitotic spindle functions, although spindle pole duplicatio n was apparently normal. Both aspects of the phenotype induced by this cDNA product, which we term Sha1 (for Spindle and hydroxyurea checkpoint abnorm al), were suppressed by simultaneous overexpression of calmodulin. Sha1 is structurally related to the product of the Drosophila gene abnormal spindle (asp), These data suggest that calmodulin-binding protein(s) are important in the co-ordination of mitotic spindle functions with mitotic entry in fi ssion yeast, and probably also in multicellular eukaryotes.