12.2018 | THE SCIENTISTable to control the passage of molecules. To
enter or leave a membrane-encapsulated
organelle, a molecule must traverse its
lipid bilayer. Typically, this occurs via
pores that serve as selective barriers, only
permitting the passage of certain molecu-
lar species. Without either a surrounding
physical barrier or pores, membraneless
organelles control the transit of molecules
using fundamentally different processes.
Whether a molecule will be absorbed
depends on how soluble it is inside the
membraneless organelle. In other words,
is it more attracted to the environment
created by the polymers that constitute
the droplet interior or to the surround-
ing solvent? Anyone can easily observe
these principles in action using just three
ingredients. In a glass of oil and water,
an added drop of food coloring will fall
through the oil and diffuse into the water
due to its different comparative density
and solubility in each of the two layers.
Given that membraneless organelles in
cells consist of many more than three
ingredients, predicting the solubility of a
given molecule is a formidable task.
To gain insight in this area, many
researchers are now reconstituting sim-
plified membraneless organelles in the
lab from their components, and testing
the extent to which other biomolecules
are absorbed or excluded. Even these
simplified systems exhibit complex pat-
terns of partitioning for individual pro-
teins and nucleic acids.
For example, recent work by myself
(T.N.) and Baldwin investigated which
oligonucleotide conformations are
absorbed or excluded by our model
Ddx4-protein droplets. This work
revealed that the extent to which a
synthetic nucleic acid is absorbed or
excluded depends on a combination
of its length and whether it is a flexi-
ble, single-stranded chain with exposed
bases or a rigid double helix.^5 Addi-
tionally, these droplets preferentially
absorb compact RNAs bearing stem-
loop structures.Due to proteins’ variety of sizes,
shapes, and surface chemistries, the
rules governing their partitioning are
far more complex. The partitioning
properties of proteins and nucleic acids
can even affect each other. For example,
a protein that is highly absorbed by a
model membraneless organelle droplet
can import with it a nucleic acid that, on
its own, would be excluded.
Amassing a specific collection of
molecules, membraneless organelles
can themselves behave as microreac-
tors, with surprising emergent biochem-
ical properties. For instance, our study
found that the interior of Ddx4-based
model membraneless organelles can
unwind the normally very stable DNA
double helix in the absence of conven-
tional enzymatic activity or the input
of energy. (See illustration on page 33.)
Membraneless organelles can therefore
be thought of as specialized filtration
devices that, by virtue of their nature as
unique solvent environments, can pro-
foundly affect the structure and/or sta-
bility of the molecules they absorb.
In the last few years, the concept that
cells use liquid-liquid phase separation as
a basic means of internal compartmen-talization has generated a lot of excite-
ment in the research community. Part of
the reason why this idea has taken hold
may be because we are all familiar with
the phase separation of liquids in our
everyday lives. Given that the cytoplasm
and nucleoplasm of cells are themselves
complex liquids, it may not be surprising
that phase separation can occur in this
environment. In fact, due to the nature of
biological polymers, such dynamics may
be inevitable.
The ideas are simple, but the con-
cept of intracellular liquid-liquid phase
separation as a fundamental organiz-
ing principle is powerful. It offers a
new perspective on the nature of bio-
logical matter and provides a unify-
ing conceptual framework in which to
consider many different membraneless
organelles that researchers had previ-
ously seen as distinct. Viewing the cell
through this new lens enables research-
ers to ask fresh questions and gain novel
insight into the mechanisms of several
cellular activities. While there are far
more questions than answers regarding
intracellular phase separation, now that
scientists know these dynamics exist
within the cell, it’s hard to imagine life
without them. gMichael Crabtree is the Todd-Bird Junior
Research Fellow in Biochemistry at
New College, Oxford, and a postdoctoral
research associate in cell biologist Tim
Nott’s lab at the University of Oxford.References- E.B. Wilson, “The structure of protoplasm,”
Science, 10:33–45, 1899. - T.J. Nott et al., “Phase transition of a disordered
nuage protein generates environmentally
responsive membraneless organelles,” Mol Cell,
57:P936–47, 2015. - M. Feric et al., “Coexisting liquid phases
underlie nucleolar subcompartments,” Cell,
165:P1686–97, 2016. - J.R. Wheeler et al., “Distinct stages in stress
granule assembly and disassembly,” eLife,
5:e18413, 2016. - T.J. Nott et al., “Membraneless organelles
can melt nucleic acid duplexes and act as
biomolecular filters,” Nat Chem, 8:569–75,
2016.
As details of the influ ence of phase
separation on cellular features emerge,
the biological community will come to
see the cell in a new light.