National Institute for Basic Biology

Cell Biology

LABORATORY OF CELL RESPONSES

Director GeneralSpecially Appointed Associate Professor

Postdoctoral Fellow:

OTSUBO, Yoko
SHICHINO, Yuichi
SAKAGAMI, Mari

YAMAMOTO, Masayuki

Secretary:

YAMASHITA, Akira

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, to research the mechanism by whichcells switch from mitosis, the kind of cell division that
divides cells equally to create two identical cells, to meiosis,
which is essential for bringing forth genetically diverse
progeny.

I. Signaling pathways that regulate the onset ofsexual differentiation

We have been trying to elucidate how fission yeast cells
switch their mode of cell cycle from mitotic to meiotic.
We focus on a highly conserved kinase, namely Target ofrapamycin (TOR) kinase, which plays key roles in the recognition of nutrition and the onset of sexual differentiationin fission yeast. TOR kinase forms two types of complexes,
namely TORC1 and TORC2. TORC1 contains Tor2 as its
catalytic subunit and is essential to suppress sexual differentiation in the presence of nitrogen. TORC2 contains Tor1
and, in contrast to TORC1, is required for onset of sexual differentiation under nitrogen starvation (Figure 1).
Temperature-sensitive tor2 mutants initiate sexual differentiation on rich medium at the restrictive temperature. To gain
insights into the TORC1 signaling pathway, we have isolated
mutants that initiate sexual differentiation ectopically undernutrient-rich conditions. We are currently characterizingthese mutants.
Nitrogen source

TORC1
Tor2

TORC2
Tor1

sexual differentiation
Figure 1. 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.

16II. The molecular mechanisms that establish
the meiosis-specific transcription profile

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 fission
yeast. Understanding precise mechanisms of this control willshed light on the regulation of timely gene expression duringmeiosis.
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 2).
We have demonstrated that Mmi1 prevents untimely translation of meiotic transcripts by regulating their localization.
Multilayered suppression of meiotic genes by Mmi1 is vitalfor mitotic growth.

Mei2 dot

nuclearexosomeMmi1

Mei2meiRNA

DSR tsipmeio

nscr
tic traFigure 2. 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.

in vivo

During meiotic prophase in fission yeast, the nucleus
migrates back and forth between two poles of the cell. Thisoscillatory nuclear movement is called ‘horse-tail’ movement
due to its characteristic shape and motion (Figure 3). Horsetail nuclear movement enhances pairing of homologous chromosomes and facilitates meiotic recombination. Horse-tailmovement is driven by cytoplasmic dynein, which forms ahuge minus-end-directed microtubule motor complex, anddynactin, which is a protein complex that is required for mostdynein-mediated cellular activities. Cytoplasmic dynein thatis anchored to the cell cortex generates a pulling force on themicrotubule emanating from the leading edge of the nucleus.
This dynein-mediated pulling is the major contributor tohorse-tail movement. Cortical anchoring of dynein is crucial
for the generation of horse-tail movement.
We identified novel subunits of dynactin, and showedthat dynein-related cortical factor, Num1, cooperates withdynactin to establish dynein anchoring at the cell cortex.
We are now studying how the oscillatory movement of thenucleus is accomplished by combining computational simulations with live cell imaging (Figure 4).

(µm)

(min)

(µm)

in silicoIII. Regulation of nuclear oscillation driven
by cytoplasmic dynein during meiotic
prophase(min)

Figure 4. Tracks of the leading edge of the nucleus during horse-tailnuclear movement in a living cell and in a simulation model. Coloredlines in the model indicate tracks of microtubule plus ends.

Publication List:〔Review articles〕
A., Shichino, Y., and Yamamoto, M. (2016). The long non• Yamashita,
coding RNA world in yeasts. Biochim. Biophys. Acta Gene Regul.
Mech. 1859, 147-154.

A., Sakuno, T., Watanabe, Y., and Yamamoto, M. (2016).
• Yamashita,Analysis of Schizosaccharomyces pombe Meiosis. Fission Yeast, ALaboratory Manual, 251-259.

0 min

2 min

A., Sakuno, T., Watanabe, Y., and Yamamoto, M. (2016).
• Yamashita,
Live imaging of chromosome segregation during meiosis in the fission••

yeast Schizosaccharomyces pombe. Fission Yeast, A LaboratoryManual, 260-263.
Yamashita, A., Sakuno, T., Watanabe, Y., and Yamamoto, M.
(2016). Synchronous induction of meiosis in the fission yeast
Schizosaccharomyces pombe. Fission Yeast, A Laboratory Manual,264-267.
Yamashita, A., Sakuno, T., Watanabe, Y., and Yamamoto, M. (2016). A
simple method to induce meiosis and sporulation semisynchronously
in the fission yeast Schizosaccharomyces pombe. Fission Yeast, A
Laboratory Manual, 268-270.

6 min

8 min

Figure 3. Horse-tail nuclear movement during meiotic prophase in S.
pombe. Time-lapse images of nuclear membrane (Cut11, magenta) andmicrotubules (Atb2, green) in the wild-type strain. The cellular contour isshown by the dotted line.

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