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Nuclear import receptor inhibits phase separation of FUS through binding to multiple sites

(CellPress, 2018)

Cell compartmentalization is essential for cellular homeostasis. Cytoplasmic compartments can be delineated by molecular concentrates, generated by the physical mechanism of liquid-liquid phase separation (LLPS). One example of compartments generated by molecular condensation is the one involving the protein FUS (RNA binding protein fused in sarcoma). FUS is involved in multiple cellular processes and its cytoplasmic localization in compartments is important for its function. In this article, Yoshizawa and colleagues uncover the molecular mechanism controlling FUS LLPS and cellular compartmentalization.
FUS liquid-liquid phase separation (LLPS) is a temperature dependent process. Authors were able to shift temperature from 10°C to 44°C, at 2°C increment steps, while live imaging of the FUS condensation process. This experiment,  performed with spinning disk confocal microscopy coupled to the CherryTemp temperature control system, contributed to the deciphering of the mechanism regulating FUS LLPS.

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Summary | Liquid-liquid phase separation (LLPS) is believed to underlie formation of biomolecular condensates, cellular compartments that concentrate macromolecules without surrounding membranes. Physical mechanisms that control condensate formation/dissolution are poorly understood. The RNA-binding protein fused in sarcoma (FUS) undergoes LLPS in vitro and associates with condensates in cells. We show that the importin karyopherin-β2/transportin-1 inhibits LLPS of FUS. This activity depends on tight binding of karyopherin-β2 to the C-terminal proline-tyrosine nuclear localization signal (PY-NLS) of FUS. Nuclear magnetic resonance (NMR) analyses reveal weak interactions of karyopherin-β2 with sequence elements and structural domains distributed throughout the entirety of FUS. Biochemical analyses demonstrate that most of these same regions also contribute to LLPS of FUS. The data lead to a model where high-affinity binding of karyopherin-β2 to the FUS PY-NLS tethers the proteins together, allowing multiple, distributed weak intermolecular contacts to disrupt FUS self-association, blocking LLPS. Karyopherin-β2 may act analogously to control condensates in diverse cellular contexts.

LEARN MORE | Takuya Yoshizawa et al. Cell, 2018

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