Compartmentalisation via liquid-liquid phase separation in cells
A central organising principle of eukaryotic cells is the compartmentalisation of biochemical reactions by membrane boundaries into organelles. However, not all processes are organised like this. Cells also contain a variety of organelles and compartments such as nucleoli, Cajal bodies, P-granules and nuage that lack a membrane boundary. These membraneless organelles are readily observed with a light microscope, and form by the condensation of macromolecules like protein and RNA into liquid-like droplets. Membraneless organelles are predominantly associated with DNA and RNA biochemistry, and can rapidly assemble and dissolve with changes to the cellular environment or cell cycle.
Our research on model membraneless organelles made of intrinsically disordered regions (IDRs) of proteins shows that the organelle interior is a unique solvent environment, with surprising emergent biochemical properties.
For example, model membraneless organelles can selectively absorb and traffic proteins and structured RNAs, and melt nucleic acid duplexes without the input of ATP, essentially acting as passive helicases.
Our major research aims are to explain how the liquid properties of membraneless organelles provide a general organising principle in cells, and to understand why cells perform certain reactions inside them. To tackle these fundamental biological questions, we take a creative and interdisciplinary approach, using tools and techniques from cell biology, structural biology, polymer theory and bioinformatics. Our research is supported by the world-class Micron Advanced Bioimaging Unit and the superb suit of biophysical instruments within the Department of Biochemistry.
- Nott, T.J., Craggs, T.D. and Baldwin, A.J., 2016. Membraneless organelles can melt nucleic acid duplexes and act as biomolecular filters. Nature Chemistry, 8(6), pp.569-575.
- Nott, T.J., Petsalaki, E., Farber, P., Jervis, D., Fussner, E., Plochowietz, A., Craggs, T.D., Bazett-Jones, D.P., Pawson, T., Forman-Kay, J.D. and Baldwin, A.J., 2015. Phase transition of a disordered nuage protein generates environmentally responsive membraneless organelles. Molecular Cell, 57(5), pp.936-947.
- Chen, C., Nott, T.J., Jin, J. and Pawson, T., 2011. Deciphering arginine methylation: Tudor tells the tale. Nature Reviews Molecular Cell Biology, 12(10), pp.629-642.
Figure 1. Membraneless organelles are liquid droplets with unique solvent properties. Model membraneless organelles made from Ddx4 protein, in which the DEAD box-helicase domain (DEADc) is replaced by yellow fluorescent protein (YFP), are shown in cells and in vitro. Scale bars indicate 10 μm (in cells) and 50 μm (in vitro).
Figure 2. Model membraneless organelles can selectively partition and unwind nucleic acids. The concentration (conc.) of fluorescently-labeled single stranded (ss) or double stranded (ds) nucleic acids with respect to model membraneless organelles are shown in the upper row of images. Short single strands are absorbed very strongly; short duplexes are moderately absorbed; and long duplexes are net-excluded from the organelle droplet. The bottom row of coloured images shows the conformation (conf.) of the nucleic acids inside and outside model organelles. Both short and long double-stranded nucleic acids are significantly destabilised inside the organelle droplet. Scale bar 10 μm.