The_Scientist_-_December_2018

12.2018 | THE SCIENTIST 31

are functional: they carry out specific bio-

chemical functions. And whereas the liquid-

liquid phase separation in a vinaigrette

occurs because oil molecules and water mol-

ecules repel each other, phase separation

inside cells results from the properties of

particular biological polymers—large mole-

cules made up of many subunits (monomers)

strung together in a chain.

To understand how polymers phase separate

into liquid droplets, it is helpful to imagine the

molecular interactions that the polymer chain

can make—with itself, with copies of itself, and

with the much smaller molecules surrounding

it, the solvent. In a biological context, the solvent

is almost always water. For phase separation to

occur, interactions between polymer chains

must be favored over interactions between the

polymer and the solvent. This allows clusters of

Membraneless compartment Location Typical size (μm) Typical number per cell Known function

Nucleolus Nucleus 0.2–3.5 1–4 Ribosome biogenesis

Nuclear speckle Nucleus 0.5–2 20–50

Gene expression regulation,

splicing factor storage, pre-mRNA

processing and metabolism

Nuclear stress body Nucleus 1–2 2–6

Gene expression regulation

upon stress

Histone locus body Nucleus 0.2–1.2 2–4 Pre-mRNA processing

Cajal body Nucleus 0.2–1 1–10

snRNP maturation regulation

and trafficking

PML nuclear body Nucleus 0.1–1 10–30

Transcriptional regulation,

protein storage

Paraspeckle Nucleus 0.2–1 2–20 Gene expression regulation

Stress granule Cytoplasm 0.1–0.3 1–30

Storage of translationally stalled

mRNA and translation machinery

P-body Cytoplasm 0.1–0.3 4–20 mRNA processing and decay

Germ granule, also known as

P-granule

Cytoplasm 0.1–0.3 1–30

mRNA translation in germ cells,

transposon degradation

MEMBRANELESS MENAGERIE

Approximately 20 membraneless organelles are known, as well as several more membraneless compartments.

© KIMBERLY BATTISTA