In This Issue - Semantic Scholar€¦ · proteins might offer a new way to turn on p53 and kill...
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In This Issue JCB • VOLUME 209 • NUMBER 2 • 2015 184 Text by Mitch Leslie [email protected] TRAMM makes an unscheduled stop during mitosis A membrane trafficking pro- tein moonlights during mitosis, helping to direct the motor protein CENP-E to the kine- tochores, Milev et al.show. The TRAPP protein com- plex controls membrane traf- ficking events such as tether- ing of vesicles to the Golgi apparatus. Milev et al. were surprised to discover that cells lacking the TRAPP component TRAMM (also known as TrappC12) showed mitotic defects. Mitotic chromosomes remained bunched at the spindle poles instead of lining up along the cellular midsection. Although TRAMM remains in the cytoplasm during interphase, the researchers found small amounts of it clinging to chromosomes during metaphase. In cells lacking TRAMM, the outer kinetochores were missing several proteins. The kinetochore levels of CENP-E, which helps strengthen the attachments between chromosomes and microtubules, showed the largest decline in these cells. To confirm that TRAMM helps attract CENP-E to the kineto- chores, the researchers broke the microtubule–chromosome attachments with nocodazole. This treatment usually spurs some kinetochore components, whose levels drop after microtubule at- tachment, to return to the structures. Yet little CENP-E came back to the kinetochores in cells lacking TRAMM. Milev et al. determined that TRAMM is phosphorylated early in mitosis and dephosphorylated before interphase begins, suggesting that its phosphorylation status determines which job it performs. Membrane trafficking shuts down during mitosis, so TRAMM is free to pursue its alternative role. How the protein recruits CENP-E to the kinetochores is unclear. Its interaction with CENP-E appears to be short-lived, but it could team up with other proteins. Milev, M.P., et al. 2015. J. Cell Biol. http://dx.doi.org/10.1083/jcb.201501090. Mitotic proteins take on editorial duties Two mitotic proteins help control RNA splicing during interphase, Wan et al.show. The findings may offer an al- ternative explanation for how some drugs kill cancer cells. Cancer drugs such as taxol halt mitosis. If cells remain in this paused state, DNA dam- age accumulates and p53 often gets switched on, triggering apoptosis. Researchers have assumed that the interruption in mito- sis leads to DNA damage, which activates p53. However, whether taxol-induced tumor regression in vivo is due to mitotic arrest remains unclear. Some proteins that control mitosis also have nonmitotic roles, so cancer drugs could spur cell death by dis- rupting these alternative functions. Wan et al. focused on two proteins, BuGZ and Bub3, that work together to ensure that chromosomes line up properly during mito- sis. BuGZ helps deliver Bub3 to the kinetochores, where Bub3 joins the spindle assembly checkpoint that stops mitosis if chromo- somes aren’t correctly connected to the spindle. Depleting BuGZ turned on p53 and caused cancer cells to commit suicide. However, the researchers found BuGZ also bound to several proteins involved in RNA splicing. This process was perturbed when the researchers depleted BuGZ or Bub3 from control or can- cer cells, causing DNA and RNA to intertwine and form so-called R-loops that promote DNA injury. If the researchers added an en- zyme that removes R-loops, they could reduce the amounts of p53 activation and DNA damage in cells lacking BuGZ or Bub3. The study suggests that targeting the nonmitotic functions of mitotic proteins might offer a new way to turn on p53 and kill cancer cells. Wan, Y., et al. 2015. J. Cell Biol. http://dx.doi.org/10.1083/jcb.201409073. R-loops (green), where DNA and RNA intertwine, are rare in a control cell (left) but prevalent in a cell deficient in BuGZ (right). MLCK stops cells from going full frontal Myosin light chain kinase (MLCK) controls the number of protrusions extended by crawling cells, Lou et al.report. Migrating cells polarize to form a protrusive front end. Previous studies have found that Rho GTPases and mem- brane tension prevent other parts of the cell from forming protrusions. Lou et al. investi- gated the polarization of zebrafish keratocytes, a type of skin cell. Researchers often study migration mechanisms in keratocytes of other fish species, but zebrafish offer several advantages, including a sequenced genome and techniques for gene knockdown. Cells from two-day-old embryos were fan shaped, with one large protrusion at the front end. But Lou et al. noticed that cells from four-day-old embryos often sported multiple protrusions. The difference affected the cells’ movement. Keratocytes from two-day-old embryos crawled swiftly, but cells from four-day- old embryos rotated in place like pinwheels. The researchers found that the activity of MLCK increased in keratocytes from four-day-old embryos. When the researchers inhibited MLCK, these cells extended only a single protrusion. Keratocytes from four-day-old embryos tended to have smaller protrusions than cells from younger embryos. Lou et al. deter- mined that MLCK shrinks these protrusions and shortens their lifetime, thus allowing the cells to generate more of them. MLCK works by inducing myosin to gather in the extensions, causing them to retract. The team found that Rho kinase (ROCK), another molecule that regulates the number of protrusions, played a com- plementary role, mainly affecting myosin accumulation at the cell’s rear end. Lou, S.S., et al. 2015. J. Cell Biol. http://dx.doi.org/10.1083/jcb.201409001. Chromosomes (blue) line up across the middle of a control cell (left), but some chromosomes remain at the spindle poles in a cell lacking TRAMM (right). The mitotic spindle is labeled white, centromeres green. A two-day-old keratocyte (left) shows a single protrusion, whereas a four- day-old keratocyte (right) sports sev- eral protrusions. Cells were stained for F-actin (green), myosin (red), and phospho-tyrosine (blue).
Transcript of In This Issue - Semantic Scholar€¦ · proteins might offer a new way to turn on p53 and kill...
In This Issue
JCB • VOLUME 209 • NUMBER 2 • 2015184
Text by Mitch Leslie
<doi>10.1083/jcb.2092iti1</doi><aid>jcb.2092iti1</aid><au>Mitch Leslie</au><cor>[email protected]</cor><ID>JCB_2092iti1_Fig1.eps</ID><dochead>In This Issue</dochead><doctopic>News</doctopic>TRAMM makes an unscheduled stop during mitosisA membrane traffi cking pro-
tein moonlights during mitosis,
helping to direct the motor
protein CENP-E to the kine-
tochores, <addart type=”rel” doi=”10.1083/jcb.201501090”>Milev et al.</addart> show.
The TRAPP protein com-
plex controls membrane traf-
fi cking events such as tether-
ing of vesicles to the Golgi
apparatus. Milev et al. were
surprised to discover that cells lacking the TRAPP component
TRAMM (also known as TrappC12) showed mitotic defects.
Mitotic chromosomes remained bunched at the spindle poles
instead of lining up along the cellular midsection.
Although TRAMM remains in the cytoplasm during interphase,
the researchers found small amounts of it clinging to chromosomes
during metaphase. In cells lacking TRAMM, the outer kinetochores
were missing several proteins. The kinetochore levels of CENP-E,
which helps strengthen the attachments between chromosomes
and microtubules, showed the largest decline in these cells.
To confi rm that TRAMM helps attract CENP-E to the kineto-
chores, the researchers broke the microtubule–chromosome
attachments with nocodazole. This treatment usually spurs some
kinetochore components, whose levels drop after microtubule at-
tachment, to return to the structures. Yet little CENP-E came back
to the kinetochores in cells lacking TRAMM.
Milev et al. determined that TRAMM is phosphorylated early in
mitosis and dephosphorylated before interphase begins, suggesting
that its phosphorylation status determines which job it performs.
Membrane traffi cking shuts down during mitosis, so TRAMM is
free to pursue its alternative role. How the protein recruits CENP-E
to the kinetochores is unclear. Its interaction with CENP-E appears
to be short-lived, but it could team up with other proteins.
Milev, M.P., et al. 2015. J. Cell Biol. http://dx.doi.org/10.1083/jcb.201501090.
<doi>10.1083/jcb.2092iti2</doi><aid>jcb.2092iti2</aid><au>Mitch Leslie</au><cor>[email protected]</cor><ID>JCB_2092iti2_Fig1.eps</ID><dochead>In This Issue</dochead><doctopic>News</doctopic>Mitotic proteins take on editorial dutiesTwo mitotic proteins help
control RNA splicing during
interphase, <addart type=”rel” doi=”10.1083/jcb.201409073”>Wan et al.</addart> show.
The fi ndings may offer an al-
ternative explanation for how
some drugs kill cancer cells.
Cancer drugs such as taxol
halt mitosis. If cells remain in
this paused state, DNA dam-
age accumulates and p53 often gets switched on, triggering
apoptosis. Researchers have assumed that the interruption in mito-
sis leads to DNA damage, which activates p53. However, whether
taxol-induced tumor regression in vivo is due to mitotic arrest
remains unclear. Some proteins that control mitosis also have
nonmitotic roles, so cancer drugs could spur cell death by dis-
rupting these alternative functions.
Wan et al. focused on two proteins, BuGZ and Bub3, that work
together to ensure that chromosomes line up properly during mito-
sis. BuGZ helps deliver Bub3 to the kinetochores, where Bub3
joins the spindle assembly checkpoint that stops mitosis if chromo-
somes aren’t correctly connected to the spindle. Depleting BuGZ
turned on p53 and caused cancer cells to commit suicide.
However, the researchers found BuGZ also bound to several
proteins involved in RNA splicing. This process was perturbed
when the researchers depleted BuGZ or Bub3 from control or can-
cer cells, causing DNA and RNA to intertwine and form so-called
R-loops that promote DNA injury. If the researchers added an en-
zyme that removes R-loops, they could reduce the amounts of p53
activation and DNA damage in cells lacking BuGZ or Bub3. The
study suggests that targeting the nonmitotic functions of mitotic
proteins might offer a new way to turn on p53 and kill cancer cells.
Wan, Y., et al. 2015. J. Cell Biol. http://dx.doi.org/10.1083/jcb.201409073.
R-loops (green), where DNA and RNA intertwine, are rare in a control cell (left) but prevalent in a cell defi cient in BuGZ (right).
<doi>10.1083/jcb.2092iti3</doi><aid>jcb.2092iti3</aid><au>Mitch Leslie</au><cor>[email protected]</cor><ID>JCB_2092iti3_Fig1.eps</ID><dochead>In This Issue</dochead><doctopic>News</doctopic>MLCK stops cells from going full frontalMyosin light chain kinase
(MLCK) controls the number
of protrusions extended by
crawling cells, <addart type=”rel” doi=”10.1083/jcb.201409001”>Lou et al.</addart> report.
Migrating cells polarize to
form a protrusive front end.
Previous studies have found
that Rho GTPases and mem-
brane tension prevent other
parts of the cell from forming
protrusions. Lou et al. investi-
gated the polarization of zebrafi sh keratocytes, a type of skin cell.
Researchers often study migration mechanisms in keratocytes of
other fi sh species, but zebrafi sh offer several advantages, including
a sequenced genome and techniques for gene knockdown.
Cells from two-day-old embryos were fan shaped, with one
large protrusion at the front end. But Lou et al. noticed that cells
from four-day-old embryos often sported multiple protrusions.
The difference affected the cells’ movement. Keratocytes from
two-day-old embryos crawled swiftly, but cells from four-day-
old embryos rotated in place like pinwheels.
The researchers found that the activity of MLCK increased in
keratocytes from four-day-old embryos. When the researchers
inhibited MLCK, these cells extended only a single protrusion.
Keratocytes from four-day-old embryos tended to have smaller
protrusions than cells from younger embryos. Lou et al. deter-
mined that MLCK shrinks these protrusions and shortens their
lifetime, thus allowing the cells to generate more of them. MLCK
works by inducing myosin to gather in the extensions, causing
them to retract. The team found that Rho kinase (ROCK), another
molecule that regulates the number of protrusions, played a com-
plementary role, mainly affecting myosin accumulation at the
cell’s rear end.
Lou, S.S., et al. 2015. J. Cell Biol. http://dx.doi.org/10.1083/jcb.201409001.
Chromosomes (blue) line up across the middle of a control cell (left), but some chromosomes remain at the spindle poles in a cell lacking TRAMM (right). The mitotic spindle is labeled white, centromeres green.
A two-day-old keratocyte (left) shows a single protrusion, whereas a four-day-old keratocyte (right) sports sev-eral protrusions. Cells were stained for F-actin (green), myosin (red), and phospho-tyrosine (blue).
