Disordered Motifs and Domains in Cell Control - October 11-15, 2014

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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