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 to theambient conditions. In our laboratory we use the fission yeast
Schizosaccharomyces pombe, the simplest organism that
performs meiosis, to research the mechanism by which cells
switch from mitosis, the kind of cell division that divides
cells equally to create two identical cells, to meiosis, whichis 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.

8II. 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 shown, in collaboration with a group at theUniversity of Cambridge, that a conserved multifunctional
protein complex Ccr4/Not is recruited by Mmi1 to its target
transcripts and plays an essential role for heterochromatin
formation in the Mmi1-dependent pathway (Cotobal et al.,2015).

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 cell
cycle. 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.

Num1

x0 min

Arp1
Mug5
Jnm1

corte

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. Horsetail movement is driven by cytoplasmic dynein, which forms
a huge minus-end-directed microtubule motor complex.
Cytoplasmic dynein that is anchored to the cell cortex generates a pulling force on the microtubule emanating from theleading edge of the nucleus. This dynein-mediated pullingis the major contributor to horse-tail movement. Corticalanchoring of dynein is crucial for the generation of horse-tailmovement.

MicrotubuleDynein

CellIII. R egulation of nuclear oscillation driven
by cytoplasmic dynein during meiotic
prophase

Num1

Figure 4. Cortical anchoring of cytoplasmic dynein in S. pombe. Dynein
is captured by Num1 at the cell cortex. Dynactin subunits, Arp1, Mug5
and Jnm1 assemble with dynein at the cell cortex and activate dynein.
Dynactin and Num1 cooperate to establish dynein anchoring and enabledynein to generate microtubule-pulling force.

Publication List:〔Original papers〕

2 min

C., Rodríguez-López, M., Duncan, C., Hasan, A., Yamashita,• Cotobal,
A., Yamamoto, M., Bähler, J., and Mata, J. (2015). Role of Ccr4-•

Not complex in heterochromatin formation at meiotic genes andsubtelomeres in fission yeast. Epigenet. Chromatin 8, 28.
Fujita, I., Yamashita, A., and Yamamoto, M. (2015). Dynactin andNum1 cooperate to establish the cortical anchoring of cytoplasmic
dynein in S. pombe. J. Cell Sci. 128, 1555-1567.

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.

We identified three subunits of dynactin, a protein complex
that is required for most dynein-mediated cellular activities (Fujita et al., 2015). The three subunits, namely Arp1,Mug5 and Jnm1, transiently colocalized with dynein at thecell cortex and were essential for the cortical anchoring ofdynein. We also found that another dynein-related corticalfactor, Num1, cooperates with dynactin to establish dynein
anchoring at the cell cortex (Figure 4).

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