LABORATORY OF CELL RESPONSES

Director GeneralSpecially Appointed Associate Professor

YAMAMOTO, Masayuki

YAMASHITA, Akira

Postdoctoral Fellow:

OTSUBO, Yoko
SHICHINO, Yuichi
NAKADE, Atsuko
SAKAGAMI, MariTechnical Assistant:
Secretary:

Cells sense the environment around them, for example theamount of nutrients and hormones, as well as the temperatureand pressure, and decide what kind of activities to undertakeusing this information. Germ cells, which produce sperm andeggs, begin halving their number of chromosomes during a
special kind of cell division called meiosis, in response tothe ambient conditions. In our laboratory, we use the fission
yeast Schizosaccharomyces pombe, the simplest organismthat performs meiosis (Figure 1), to research the mechanism
by which cells switch from mitosis, the kind of cell divisionthat divides cells equally to create two identical cells, tomeiosis, which is essential for bringing forth geneticallydiverse progeny.

ly under nutrient-rich conditions. In most mutants identified,
TORC1 activity is downregulated and the mutated genes areinvolved in tRNA expression or modification. We are currently characterizing these mutants.

Figure 2. The two TOR complex pathways in S. pombe. TORC1, containing Tor2, and TORC2, containing Tor1, regulate sexual differentiation
oppositely. TORC1 suppresses sexual differentiation in the presence ofample nitrogen.

II. The molecular mechanisms that establish
the meiosis-specific gene expression profile

Figure 1. Life cycle of the fission yeast S. pombe. S. pombe cells proliferate by mitotic growth under nutrient-rich conditions. When starvedof nutrients, especially nitrogen, S. pombe cells arrest the mitotic cellcycle and haploid cells conjugate with cells of the opposite mating type.
Resulting diploid zygotes undergo meiosis and produce spores.

I. Signaling pathways that regulate the onset of
sexual differentiation
We have been trying to elucidate how S. pombe cells switch
their mode of cell cycle from mitotic to meiotic. We focus
on a highly conserved kinase, namely Target of rapamycin(TOR) kinase, which plays key roles in the recognition ofnutrition and the onset of sexual differentiation in S. pombe.
TOR kinase forms two types of complexes, namely TORC1and TORC2. TORC1 contains Tor2 as its catalytic subunit
and is essential to suppress sexual differentiation in thepresence of nitrogen. TORC2 contains Tor1 and, in contrast
to TORC1, is required for onset of sexual differentiation
under nitrogen starvation (Figure 2).
Temperature-sensitive tor2 mutants initiate sexual differentiation even on rich medium at the restrictive temperature.
To gain insights into the TORC1 signaling pathway, we haveisolated mutants that initiate sexual differentiation ectopical-Expression of hundreds of genes are upregulated duringmeiosis. We have shown that specific control of the stabilityof meiotic transcripts, which is orchestrated by the interplay
between RNA-binding proteins and a long non-coding RNA,
contributes to the meiosis-specific gene expression in thefission yeast S. pombe. Understanding precise mechanismsof this control will shed light on the regulation of timely gene
expression during meiosis.
A YTH-family RNA-binding protein Mmi1 plays a crucialrole in the selective elimination system of meiosis-specifictranscripts during the mitotic cell cycle. Mmi1 recognizes aregion termed DSR (Determinant of Selective Removal) in
meiotic transcripts, which is enriched with repeats of hexanucleotide motifs. Meiotic transcripts bound to Mmi1 aredegraded by the RNA-degradation nuclear exosome machinery. Mmi1 also induces formation of facultative heterochromatin at a subset of its target genes.
During meiosis, a meiosis-specific nuclear body, called
Mei2 dot, blocks the Mmi1-mediated elimination system.
The Mei2 dot is composed of the RNA-binding protein Mei2
and a long non-coding RNA species termed meiRNA. Mei2
physically interacts with meiRNA and forms the dot structure
at the chromosomal sme2 locus, which encodes meiRNA.
The Mei2 dot lures Mmi1 through numerous copies of theDSR motif on meiRNA and inhibits its function, so thatmeiotic transcripts harboring DSR are stably expressed(Figure 3).

21National Institute for Basic Biology

Cell Biology

Figure 3. Selective elimination of meiosis-specific transcripts by theMmi1/DSR system. Mmi1 binds to DSR in meiotic transcripts andinduces their degradation by the nuclear exosome during the mitotic cellcycle. In meiotic cells, the Mei2 dot, composed of Mei2 and meiRNA,
sequesters and inhibits Mmi1, so that DSR-harboring meiotic transcripts
escape from Mmi1-mediated selective elimination.

We have shown that Mmi1 regulates the termination oftranscription of its target genes. Mmi1-mediated termination of an upstream non-coding RNA ensures the expressionof downstream genes, one of which encodes a mitogenactivated protein kinase kinase kinase (MAPKKK) essential
for the initiation of sexual differentiation. We have also
demonstrated that Mmi1 prevents untimely expression ofmeiotic proteins by tethering their mRNAs to nuclear foci.
Multilayered suppression of meiotic genes by Mmi1 is vitalfor mitotic growth.

Publication List:〔Original paper〕
L., Shichino, Y., Dangin, M., Thierry-Mieg, N.,• Touat-Todeschini,Gliquin, B., Hiriart, E., Sachidanandam, R., Lambert, E., Brettschneider,J., Reuter, M., Kadlec, J., Pillai R., Yamashita, A., Yamamoto., M.,and Verdel, A. (2017). Selective termination of lncRNA transcription
promotes epigenetic silencing and cell differentiation. EMBO J. 36,2626-2641.

〔Review article〕
Y., Yamamoto, M., and Yamashita, A. (2017). TORC1• Otsubo,
dependent phosphorylation targets in fission yeast. Biomolecules 7, 50.

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