RNA-binding protein Csx1 is phosphorylated by LAMMER kinase, Lkh1, in response to oxidative stress in Schizosaccharomyces pombe

Recent studies have shown that global gene expression during oxidative stress in Schizosaccharomyces pombe is regulated by stress-induced activation and binding of Csx1 to atf1(+) mRNA. However, the kinase responsible for the activation of Csx1 has not been identified. Here, we describe, for the first time, that Csx1 is phosphorylated by S. pombe LAMMER kinase, Lkh1, under oxidative conditions and that the stress-activated binding of the Csx1 to the atf1(+) mRNA was also affected by Lkh1 and Spc1. These data indicate that concerted actions of Spc1 and Lkh1 are required for the activation of Csx1 during oxidative condition in the fission yeast S. pombe.     

Isolation of a mammalian homologue of a fission yeast differentiation regulator

In the fission yeast Schizosaccharomyces pombe the nrd1(+) gene encoding an RNA binding protein negatively regulates the onset of differentiation. Its biological role is to block differentiation by repressing a subset of the Ste11-regulated genes essential for conjugation and meiosis until the cells reach a critical level of nutrient starvation. By using the phenotypic suppression of the S. pombe temperature-sensitive pat1 mutant that commits lethal haploid meiosis at the restrictive temperature, we have cloned ROD1, a functional homologue of nrd1(+), from rat and human cDNA libraries. Like nrd1(+), ROD1 encodes a protein with four repeats of typical RNA binding domains, though its amino acid homology to Nrd1 is limited. When expressed in the fission yeast, ROD1 behaves in a way that is functionally similar to nrd1(+), being able to repress Ste11-regulated genes and to inhibit conjugation upon overexpression. ROD1 is predominantly expressed in hematopoietic cells or organs of adult and embryonic rat. Like nrd1(+) for fission yeast differentiation, overexpressed ROD1 effectively blocks both 12-O-tetradecanoyl phorbol-13-acetate-induced megakaryocytic and sodium butyrate-induced erythroid differentiation of the K562 human leukemia cells without affecting their proliferative ability. These results suggest a role for ROD1 in differentiation control in mammalian cells. We discuss the possibility that a differentiation control system found in the fission yeast might well be conserved in more complex organisms, including mammals.     

A novel gene, msa1, inhibits sexual differentiation in Schizosaccharomyces pombe

Sexual differentiation in the fission yeast Schizosaccharomyces pombe is triggered by nutrient starvation or by the presence of mating pheromones. We identified a novel gene, msa1, which encodes a 533-aa putative RNA-binding protein that inhibits sexual differentiation. Disruption of the msa1 gene caused cells to hypersporulate. Intracellular levels of msa1 RNA and Msa1 protein diminished after several hours of nitrogen starvation. Genetic analysis suggested that the function of msa1 is independent of the cAMP pathway and stress-responsive pathway. Deletion of the ras1 gene in diploid cells inhibited sporulation and in haploid cells decreased expression of mating-pheromone-induced genes such as mei2, mam2, ste11, and rep1; simultaneous deletion of msa1 reversed both phenotypes. Overexpression of msa1 decreased activated Ras1(Val17)-induced expression of mam2. Phenotypic hypersporulation was similar between cells with deletion of only rad24 and both msa1 and rad24, but simultaneous deletion of msa1 and msa2/nrd1 additively increased hypersporulation. Therefore, we suggest that the primary function of Msa1 is to negatively regulate sexual differentiation by controlling the expression of Ste11-regulated genes, possibly through the pheromone-signaling pathway.     

Identification of Ste4 as a potential regulator of Byr2 in the sexual response pathway of Schizosaccharomyces pombe.

A conserved MAP kinase cascade is central to signal transduction in both simple and complex eukaryotes. In the yeast Schizosaccharomyces pombe, Byr2, a homolog of mammalian MAPK/ERK kinase kinase and Saccharomyces cerevisiae STE11, is required for pheromone-induced sexual differentiation. A screen for S. pombe proteins that interact with Byr2 in a two-hybrid system led to the isolation of Ste4, a protein that is known to be required for sexual function. Ste4 binds to the regulatory region of Byr2. This binding site is separable from the binding site for Ras1. Both Ste4 and Ras1 act upstream of Byr2 and act at least partially independently. Ste4 contains a leucine zipper and is capable of homotypic interaction. Ste4 has regions of homology with STE50, an S. cerevisiae protein required for sexual differentiation that we show can bind to STE11.     

Multiple regulatory domains on the Byr2 protein kinase.

Byr2 protein kinase, a homolog of mammalian mitogen-activated protein kinase/extracellular signal-regulated kinase kinase (MEKK) and Saccharomyces cerevisiae STE11, is required for pheromone-induced sexual differentiation in the fission yeast Schizosaccharomyces pombe. Byr2 functions downstream of Ste4, Ras1, and the membrane-associated receptor-coupled heterotrimeric G-protein alpha subunit, Gpa1. Byr2 has a distinctive N-terminal kinase regulatory domain and a characteristic C-terminal kinase catalytic domain. Ste4 and Ras1 interact with the regulatory domain of Byr2 directly. Here, we define the domains of Byr2 that bind Ste4 and Ras1 and show that the Byr2 regulatory domain binds to the catalytic domain in the two-hybrid system. Using Byr2 mutants, we demonstrate that these direct physical interactions are all required for proper signaling. In particular, the physical association between Byr2 regulatory and catalytic domains appears to result in autoinhibition, the loss of which results in kinase activation. Furthermore, we provide evidence that Shk1, the S. pombe homolog of the STE20 protein kinase, can directly antagonize the Byr2 intramolecular interaction, possibly by phosphorylating Byr2.     

组蛋白H3K4甲基转移酶复合物亚基Ash2参与裂殖酵母产孢

在氮源缺乏及信息素存在的条件下,裂殖酵母(Schizosaccharomyces pombe)进行减数分裂并完成产孢。在此过程中,信息素介导的MAPK(Mitogen-activated protein kinases)信号通路调控减数分裂相关基因的表达。Spk1是MAPK通路的核心成员,通过蛋白磷酸化的方式激活转录因子Ste11,从而激活mei2⁺、mam2⁺和map3⁺等减数分裂相关基因的表达。尽管组蛋白H3K4甲基化参与基因转录激活、染色质重塑等诸多生物学过程,但其在裂殖酵母产孢过程中的作用并不清楚。文章通过序列比对,发现裂殖酵母Ash2作为H3K4甲基转移酶复合物COMPASS的亚基具有两个保守的结构域,定位于细胞核内参与H3K4的甲基化修饰。ash2⁺的缺失引起裂殖酵母在氮源缺乏时产孢过程的延迟及产孢率下降。ChIP、定量PCR分析结果显示,ash2⁺的缺失降低了spk1⁺编码区H3K4的二甲基化水平,造成spk1⁺mRNA水平的明显下调。在ash2Δ细胞中,虽然ste11⁺的转录水平没有变化,但Ste11的靶基因mei2⁺、mam2⁺和map3⁺的转录水平明显下降。在裂殖酵母中,组蛋白H3K4甲基转移酶复合物COMPASS的亚基Ash2通过调控二甲基化水平修饰从而调节MAPK信号通路,参与裂殖酵母的有性生殖,为建立表观遗传修饰与减数分裂之间的联系提供了新的线索。