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...

The identification of novel cardiomyocyte-intrinsic factors that support ventricular function will expand the number of candidate genes and therapeutic options for heart failure, a leading cause of death worldwide. Here, we demonstrate that a conserved RNA-binding protein RBPMS2 is required for ventricular function in zebrafish and for myofibril organization and the regulation of intracellular calcium dynamics in zebrafish and human cardiomyocytes. A differential expression screen uncovered co-expression of rbpms2a and rbpms2b in the zebrafish myocardium. Double knockout embryos suffer from compromised ventricular filling during the relaxation phase of the cardiac cycle, which significantly reduces cardiac output. RNA-sequencing analysis and validation studies revealed differential alternative splicing of the cardiomyopathy genes myosin binding protein C3 (mybpc3) and phospholamban (pln) in rbpms2-null embryos. Cardiomyocytes in double mutant ventricles and those derived from RBPMS2-null human induced pluripotent stem cells exhibit myofibril disarray and calcium handling abnormalities. Taken together, our data suggest that RBPMS2 performs a conserved role in regulating alternative splicing in cardiomyocytes, which is required for myofibrillar organization, optimal calcium handling, and cardiac function.