SCIENCE sciencemag.org
CELL BIOLOGYLiquid but not
contactless
The endoplasmic reticulum
makes molecular contact with
membraneless organelles
By Benoît Kornmann^1 and Karsten Weis^2I
n the past decade, the understanding
of cellular organization has undergone
two major paradigm shifts. On the one
hand, it was demonstrated that mem-
brane-bound compartments exchange
their contents not only through vesicu-
lar transport but also by means of direct
membrane tethering at specific contact sites
( 1 ), revealing a new layer of connectivity
in eukaryotic cells. On the other hand, the
discovery of membraneless organelles, such
as processing bodies (P-bodies) and stress
granules, has revealed that proteins and
RNAs can self-assemble and condense into
liquid-like droplets through weak and mul-
tivalent interactions ( 2 ). This indicates that
the cytosol is not a randomly dispersed soup
of macromolecules but that it is subcompart-
mentalized. On page 527 of this issue, Lee
et al. ( 3 ) bring these two exciting fields to-
gether by showing that a membrane-bound
organelle, the endoplasmic reticulum (ER),
contacts at least two membraneless com-
partments, P-bodies and stress granules, and
influences their behavior.
By imaging live human cells expressing
markers for the ER and for membraneless
organelles, Lee et al. found that P-bodies
and stress granules were in contact with the
ER much more frequently than expected by
chance (see the photo). Membraneless or-
ganelles are biomolecular condensates that
form by coalescence of macromolecules.
P-bodies and stress granules are archetypi-
cal membraneless organelles found in the
cytoplasm of eukaryotic cells. Membrane-
less organelles also occur in the nucleus,
including the nucleolus ( 4 ), a factory for
ribosome assembly, and Cajal bodies that
function in small RNA processing. Interac-
tions within membraneless organelles are
driven by low-affinity, multivalent protein-
protein, RNA-protein, and RNA-RNA in-
teractions. These weak interactions cause(^1) Department of Biochemistry, University of Oxford, Oxford
OX1 3QU, UK.^2 Institute of Biochemistry, ETH Zurich, 8093
Zürich, Switzerland. Email: [email protected]
1D heterostructures offer tangible benefits.
The oxidation or chemical resistance of
carbon nanotubes can be improved by the
protective BN sheath ( 1 , 3 ). A plethora of
opportunities arises when stacking 1D crys-
tals coaxially, such as interesting 1D physics
in heterostructure electronics. The hybrid
shells of 1D heterostructures can be cho-
sen with an electronic band alignment such
that interlayer excitons—electrically neutral,
short-lived particles normally caused by
light—become favored even in the ground
state, reaching Bose-Einstein condensation
into an excitonic superfluid ( 9 ). This enables
scientists to build low-power logic devices or
direct-current electrical transformers.
Furthermore, the strain gradient from the
outer to the inner surface of the tube curva-
ture leads to flexoelectric polarization ( 10 )
and a voltage shift inside the tube, which
changes the offset of electron energy bands
from straddling to staggered. This enables
charge separation under light, which gives
rise to an electrical current at the junction
of the coaxial shells and along the tubes.
New devices such as tunneling diodes and
transistors can be fabricated with hetero-
structures of semiconducting, dielectric, or
metallic coaxial nanotubes.
All of the materials studied by Xiang et al.—
graphitic carbon, MoS 2 , and BN—are widely
used as solid lubricants in their planar 2D
state. The lack of intershell registry between
the disparate materials suggests negligible
friction (superlubricity) ( 11 ). This property
invites the use of 1D heterostructures as na-
noscale bearings ( 12 ), which brings scientists
a step closer to tiny gears and other nano-
machines ( 13 ). However, before the industry
realizes such applications, it must overcome
certain challenges, such as the ability to grow
1D heterostructures in forests (nanotubes
vertically aligned on a substrate), in large
quantities in fluidized bed reactors, or in a
predetermined location on a silicon chip.
The prospect of developing 1D materi-
als of various compositions can motivate a
combinatorial exploration guided by density
functional theory computations or artificial
intelligence models that predict the best syn-
thesis protocols for 1D heterostructures with
attractive properties. This in turn might lead
to a new wave of interest in nanotubes, which
have been overshadowed in the past decade
by 2D materials and their heterostructures.
Flat, planar stacks of 2D materials now can
retain one dimension only, while having an-
other fold onto itself. Even in “Flatland” the
world turns out to be round. j
REFERENCES AND NOTES
- R. Xiang et al., Science 367 , 537 (2020).
- K. S. Novoselov et al., Science 353 , aac9439 (2016).
- L. L. Chen et al., J. Am. Ceram. Soc. 87 , 147 (2004).
- Y. Gogotsi, B. Anasori, ACS Nano 13 , 8491 (2019).
- A. J. Mannix, Z. Zhang, N. P. Guisinger, B. I. Yakobson, M. C.
Hersam, Nat. Nanotechnol. 13 , 444 (2018). - A. N. Enyashin, A. L. Ivanovskii, Comput. Theor. Chem. 989 ,
27 (2012). - R. Tenne, Nat. Nanotechnol. 1 , 103 (2006).
- K. V. Bets et al., Nano Lett. 19 , 2027 (2019).
- S. Gupta et al., arXiv 1908.07513 (16 August 2019).
1 0. T. D u m i t r i c ă et al., Chem. Phys. Lett. 360 , 182 (2002). - Y. Song et al., Nat. Mater. 17 , 894 (2018).
- J. Cumings, A. Zettl, Science 289 , 602 (2000).
- K. E. Drexler, Nanosystems (Wiley, 1992).
ACKNOWLEDGMENTS
B.I.Y. thanks K. Bets (supported by NSF grant CBET-1605848)
for help with computer graphics simulations.
10.1126/science.aba613331 JANUARY 2020 • VOL 367 ISSUE 6477 507A cornucopia of new
one-dimensional materials
is now possible, structured
as nested nanotubes.
Illustrated here is a
heterostructure of a carbon
nanotube, inside two layers
of boron nitride, inside
molybdenum disulfide.Published by AAAS