Published August 8, 2005
Research Roundup
Retrograde tRNAs
ature tRNAs move between the cytoplasm and the nucleus in S. cerevisiae, report
Akira Takano, Toshiya Endo, and Tohru Yoshihisa (Nagoya University, Japan),
and Hussam Shaheen and Anita Hopper (Pennsylvania State University College
of Medicine, Hershey, PA) in two independent reports.
“The dogma was that tRNAs are transcribed in the nucleus and function in the cytoplasm,
and that is the whole story,” says Hopper. “But the whole tRNA model has become much more
complex, and almost anything one would have thought would be true has turned out to be wrong.”
Previous work showed that, when yeast were starved, tRNAs lacking introns were
more abundant in the nucleus. It was not clear though whether they were processed in the
nucleus and held there, or whether they underwent retrograde transport and returned to
the nucleus from the cytoplasm. With the recent demonstration that yeast tRNAs are spliced
in the cytoplasm, evidence pointed to the transport model.
To check this idea, the groups each took advantage of mutations that prevent nuclear fusion
after mating, generating cells that have two nuclei in a single cytoplasm. When such cells
expressed a tRNA gene from another species—either S. pombe or Dictyostelium—in one nucleus
but not the other, the researchers saw that the spliced exogenous tRNA turned up in both nuclei.
It is not yet clear why processed tRNAs return to the nucleus, but Hopper thinks their
removal from the cytoplasm might limit translation when the amino acid supply is low.
Her group found that tRNA retrograde transport was RAN-dependent under starvation
conditions, whereas Takano et al. saw that retrograde transport was normally RAN-independent.
The difference could indicate a regulatory change that induces retrograde transport during
times of stress. The nuclear presence of tRNAs also brings up the old debates of whether
mature ribosomes can reenter the nucleus and whether translation occurs there as well.
tRNAs (green) move between two nuclei
(blue) in a single cell.
Linking centrosomes and actin
entrosomal changes during cell division are relayed to
the actin cytoskeleton by the centrosomal protein
CP190, report Sasidhar Chodagam, Jordan Raff
(Gurdon Institute, Cambridge, UK), and colleagues.
Although CP190 binds to microtubules and was the first
centrosomal protein identified in flies, researchers have not
detected a microtubule or centrosomal defect in fly CP190
mutants. The catch to those experiments, however, was that
maternal stores of CP190 were available to the mutant embryos
during early development.
The team therefore generated mutant embryos that lack
even the maternal supply. These embryos’ centrosomes seemed
normal, but the dividing nuclei failed to disperse evenly in the
embryo and instead remained crowded at the anterior end.
RAFF/ELSEVIER
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Myosin heavy chain (white) is lacking in CP190 mutants (right).
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This nuclear spreading is known to depend on the contraction
of cortical actin, which occurs in pulses between rapid cycles of
mitosis in the embryo. Actin is contracted by myosin, but, in the
CP190 mutant, myosin was not at the cortex, where it is normally
localized during nuclear spreading.
Constitutively active myosin rescued the defect, but a
CP190 mutant that lacks the microtubule and centrosome binding
domain did not. Thus, CP190 must be able to bind to the centrosome and microtubules to induce proper myosin localization
and activity in response to the nuclear division.
Why is CP190 activity essential in early embryo but not
anywhere else? Raff speculates that, in large cells such as the
young fly embryo, CP190 might be necessary to carry a signal from the replicating centrosomes along microtubules to
cortical myosin and actin.
Perhaps in smaller cells,
such as those found in older
embryos, diffusion suffices
when CP190 is absent.
Reference: Chodagam, S., et al.
2005. Curr. Biol. 15:1308–
1313.
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References: Shaheen, H.H., and A. Hopper. 2005. Proc. Natl. Acad. Sci. USA.
doi:10.1073pnas.0504805102.
Takano, A., et al. 2005. Science. 309:140–142.
HOPPER/NAS
M
Published August 8, 2005
Text by Rabiya S. Tuma
[email protected]
Dynein holds still
ynein, well-known for moving cargo down microtubules, is now also
shown to anchor its cargo at the target site, according to results from
Renald Delanoue and Ilan Davis (University of Edinburgh, UK).
Dynein’s cargoes include the wingless and runt mRNAs, which
concentrate at the apical tip of fly embryos. Delanoue and Davis found that
agents that disrupt microtubule function caused the mRNAs to disperse.
Injection of antibodies against dynein subunits disrupted localization of
both injected exogenous RNAs and endogenous transcripts, indicating that
dynein itself was doing the anchoring.
To distinguish between static anchoring and continuous localization, the
team used a two-step injection design. They injected one batch of labeled RNA,
allowed it to localize, and then injected a second batch along with an ATPase
inhibitor to block dynein motility. Under these conditions, the previously localized RNA stayed put, while the newly injected RNA failed to localize. The team
thus concluded that dynein remains with its cargo and anchors it at the target site.
“We’d like to think the use of a motor for anchoring is a general mechanism both in terms of other cargoes and other motors,” says Davis. “There
could be a whole range of transport and anchoring mechanisms involving
different motors.” He points out that motors are abundant in most cells and
could provide a convenient tether after transport.
p66Shc reduces cytochrome c and generates hydrogen peroxide.
eactive oxygen species (ROS) are
not all accidental and unwanted
byproducts. Marco Giorgio, Enrica
Migliaccio, Pier Giuseppe Pelicci (University
of Milan, Italy), and colleagues report that a
protein called p66Shc purposely siphons electrons from the respiratory chain and uses them
to trigger apoptosis during times of stress.
Cells that lack p66Shc were known to
produce less ROS and be resistant to various
pro-apoptosis stimuli. Giorgio et al. now show
that p66Shc is sufficient to induce mitochondrial
swelling and rupture when added to purified
mitochondria. The protein also induces excess
ROS production, but only when the organelles
are undergoing respiration.
p66Shc takes electrons from the respiratory
protein cytochrome c and uses them to produce
the ROS hydrogen peroxide. p66Shc diverts
only a fraction of the electrons though, and
respiration continues in its presence.
Because hydrogen peroxide can diffuse
through the mitochondria and open holes in
the membrane, p66Shc’s redirection of electrons must somehow be regulated to prevent
unwanted apoptosis. p66Shc increases production of ROS during times of stress, when cellular
damage might be too extensive to repair. But
just how the cell limits p66Shc activity during
healthy times is not yet clear.
R
Reference: Giorgio, M., et al. 2005. Cell.
122:221–233.
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Making ROS for
apoptosis
Reference: Delanoue, R., and I. Davis. 2005. Cell. 122:97–106.
DAVIS/ELSEVIER
GIORGIO/ELSEVIER
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Runt RNA (red) disperses (left to right) after injection of anti-dynein antibody.
Cohesins and breaks in yeast
protein that holds DNA together is also needed to break it apart.
Chad Ellermeier and Gerald Smith (Fred Hutchinson Cancer Research
Center, Seattle, WA) show that the DNA glue cohesin regulates meiotic double-strand break (DSB) formation and recombination in fission yeast.
The cohesins that hold sister chromatids together during meiosis, called
Rec8 and Rec11, are early arrivals on chromosomes during premeiotic
replication. They provide a binding site for Rec10, which is a main component
of linear elements—fission yeast’s version of synaptonemal complexes.
Ellermeier and Smith found that deletion of Rec8 and Rec11 caused
region-specific decreases in DSB formation and recombination. Loss of
Rec10 blocked breakage and recombination throughout the genome. The
team thinks the widespread problems in Rec10 occur because it must be
present to bring in the enzyme that actually clips the DNA, called Rec12.
Rec8 and Rec11, by contrast, are not evenly distributed over the meiotic
chromosomes, and thus their absence only causes intermittent problems.
“It is surprising that cohesins, which hold sister chromatids together,
are so important in recombination,” says Smith, “because cross-overs
occur between homologues.” But the ordered loading process explains the
puzzle. Cohesins are the first to load onto the meiotic chromosomes and
must be there for the rest of the events to follow.
A
Reference: Ellermeier, C., and G.R. Smith. 2005. Proc. Natl. Acad. Sci. USA.
doi:10.1073pnas.0504805102.
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