Two Saccharomyces cerevisiae genes were isolated based upon their dosage-dependent rescue of a temperature-sensitive mutation of the gene CDC28, which encodes a protein kinase involved in control of cell division. CLNI and CLN2 encode closely related proteins that also share homology with cyclins. Cyclins, characterized by a dramatic periodicity of abundance through the cell cycle, are thought to be involved in mitotic induction in animal cells. A dominant mutation in the CLN2 gene, CLN2-1, advances the G1-to S-phase transition in cycling cells and impairs the ability of cells to arrest in G1 phase in response to external signals, suggesting that the encoded protein is involved in G, control of the cell cycle in Saccharomyces.Cyclins were initially identified in the embryos of a number of marine invertebrates as proteins that undergo dramatic fluctuations in abundance as a function of cell cycle progression (1, 2). In clams and sea urchins, where early embryonic cleavages occur with a high degree of synchrony, cyclin levels can be seen to peak at the onset of mitosis. During mitosis, cyclins are rapidly degraded but begin to accumulate during the subsequent interphase. Periodicity of cyclin levels results entirely from posttranslational regulation, as the cyclin mRNAs are abundant maternal species that are translated at a constant rate during early development. Based on the kinetics of cyclin accumulation and turnover, it was proposed that these proteins might be involved in triggering mitosis. The first evidence, however, that cyclins are rate limiting for mitotic induction was the demonstration that injection of cyclin-encoding mRNA could induce maturation ofXenopus oocytes (3), a process analogous to mitosis in that cells resting in meiotic prophase are induced to enter into meiotic divisions.In this report we describe a class of proteins homologous to cyclins from the budding yeast, Saccharomyces cerevisiae. The genes encoding these cyclin homologs were identified by using a genetic screen in which elevated expression of a heterologous sequence was required for rescue of a temperature-sensitive mutation in the gene CDC28 (4). CDC28 encodes the catalytic subunit of a protein kinase complex required for cell cycle initiation in Saccharomyces (5-8). The observation that cyclin hyperexpression can rescue temperature-sensitive cdc28 mutations implies a role for these yeast cyclins in the maintenance or regulation of the Cdc28 protein kinase complex. Furthermore, we show that a mutation in one of the cyclin genes confers a dominant pleiotropic phenotype characterized by advance of the G1-to S-phase transition in cycling cells as well as by loss of G, control of cell division in response to nutrient limitation. These results suggest that in budding yeast, a class of cyclins is rate-limiting for G1-to S-phase transition rather than for the induction of mitosis. MATERIALS AND METHODSStrains and Medium. Suppressor plasmids were initially isolated and subcloned by using S. cerevisiae strain JF210-92: MATa, trpi...

The Saccharomyces cerevisiae gene CDC28 encodes a protein kinase required for cell cycle initiation. In an attempt to identify genes encoding proteins that interact with the Cdc28 protein kinase, high-copy plasmid suppressors of a temperature-sensitive cdc28 mutation were isolated. One such suppressor, CKS1, was found to encode an 18-kilodalton protein that shared a high degree of homology with the suc1+ protein (p13) of Schizosaccharomyces pombe (67% amino acid sequence identity). Disruption of the chromosomal CKS1 gene conferred a G1 arrest phenotype similar to that of cdc28 mutants. The presence of the 18-kilodalton Cks1 protein in yeast lysates was demonstrated by using Cks-1 specific antiserum. Furthermore, the Cks1 protein was shown to be physically associated with active forms of the Cdc28 protein kinase. These data suggest that Cks1 is an essential component of the Cdc28 protein kinase complex.

Antibodies raised against the protein encoded by a lacZ-CDC28 in-frame fusion were shown to immunoprecipitate the CDC28 product from yeast cell lysates. The polypeptide p36CDC28 is a phosphoprotein of apparent Mr 36,000. Immune complexes prepared from yeast cell lysates by using anti-CDC28 antibody were found to possess a protein kinase activity, as determined by the transfer of label from [gamma-32P]ATP to a coprecipitated Mr 40,000 protein of unknown identity or function (p40). This activity was absent or thermolabile when extracts were prepared from several different cdc28 temperature-sensitive strains. The protein kinase activity was dependent on Zn2+ and transferred phosphate specifically to serine and threonine residues.

The Dictyostelium G^4 gene encodes a G-protein a subunit that is primarily expressed during the muhicellular stages of development. g^4 null mutants, created by gene disruption, show aberrant morphological differentiation, reduced levels of prespore gene expression, and a loss of the ability to produce spores. These developmental phenotypes can be rescued by complementation with the wild-type gene. Cells that overexpress the G"4 gene {0^4^^*^) also show reduced spore production but display an aberrant morphological phenotype distinct from that of g"4 cells. The g"4 phenotype can be partially rescued by the presence of wild-type or Gjfi^^ cells in chimeric organisms, suggesting that G"4-expressing cells produce an intercellular signal that is essential for multicellular development.

In this paper, we show that the Go subunit Ga4 couples to pterin receptors and Identifies a gnaing pathway that is esentil for multicellular development in Dicdyosteium. GA is developmentally regulated, is essential for proper morphogenesis and spore production, and functions cell nonautonomous. We show that Ga4 is coupled to receptors (aFAR) that activate chemotaxis and adenylyl and guanylyl cyclases in response to folate during the early stages of development and to a late class of folate receptors (.8FAR) that