Disordered Motifs and Domains in Cell Control
Tuesday Speaker Abstracts
Nup98 FG-Domains from Diverse Species Spontaneously Phase-separate into Hydrogels
with Exquisite NPC-like Permeability
Dirk Görlich
, H. Broder Schmidt.
MPI f Biophysical Chemistry, Göttingen, Germany.
The permeability barrier of nuclear pore complexes (NPCs) conducts massive transport mediated
by shuttling nuclear transport receptors (NTRs) and, at the same time, suppresses an intermixture
of nuclear and cytoplasmic contents. In Xenopus, it relies foremost on the intrinsically
disordered FG-repeat domain of Nup98. We now analyzed Nup98 FG-domains from
evolutionary distant eukaryotes representing mammals, lancelets, insects, nematodes, fungi,
plants, amoebas, ciliates, and excavates. We observed that dilute aqueous solutions of these FG-
domains spontaneously phase-separate into characteristic "FG-particles" with hydrogel
properties. Phase separation required neither sophisticated experimental procedures nor auxiliary
eukaryotic factors, but occurred already during recombinant FG-domain expression in bacteria.
The Nup98 FG-phases displayed essentially the same permselectivity as authentic NPCs: They
posed effective barriers towards inert macromolecules and yet allowed far larger NTR
⋅
cargo
complexes to enter rapidly. FG-particles even reproduced the known phenomenon that large
cargo-domains inhibit NPC-passage of NTR
⋅
cargo complexes and that cargo-shielding and an
increased NTR: cargo surface-ratio can override this inhibition. Their exquisite sorting
selectivity and intrinsic assembly propensity suggest that Nup98 FG-phases form also in
authentic NPCs and indeed account for the permeability properties of the pore.
Inverse Size Scaling of the Nucleolus by a Concentration-dependent Phase Transition
Stephanie Weber.
Department of Chemical and Bioengineering, Princeton University, USA
Cells must coordinate the size of their structures across a range of length scales as they grow and
divide. Indeed, many organelles, such as the nucleus, mitochondria, mitotic spindle and
centrosome, exhibit size scaling, a phenomenon in which organelle size depends linearly on cell
size. However, the mechanisms of organelle size scaling remain unclear. Here, we show that the
cell size-dependent assembly of the nucleolus, a membrane-less organelle important for cell size
homeostasis, arises from an intracellular phase transition. We find that nucleolar size directly
scales with cell size in early
C. elegans
embryos. Surprisingly, however, when embryo size is
altered, we observe
inverse
scaling: nucleolar size increases in small cells and decreases in large
cells. We demonstrate that this seemingly contradictory result arises from maternal loading of a
fixed number of nucleolar components, which condense into nucleoli only above a threshold
concentration. Such concentration-dependent phase transitions provide a mechanistic link
between organelle size and cell size and may represent a general principle underlying the
functional organization of the cell.
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