Study Reveals Structure in Cell Condensates

Every cell contains millions of protein molecules, some of which have the ability to phase-separate to form non-membrane-bound compartments, called biomolecular condensates. It had long been assumed that there was no further structure underlying these condensates, only solution-soluble proteins, but a group of researchers in Missouri, United Kingdom, and Germany has found that there is, in fact, relevant structure underlying condensates.

The team found proteins formed dynamic structures at concentrations too low to form condensates. “Because of specific interactions, you can make lots of small ‘clusters,’” says Rohit Pappu, the Gene K. Beare Distinguished Professor of biomedical engineering in the McKelvey School of Engineering at Washington University in St. Louis. “The clusters have structures, and the structures encode function.”

The team used in vitro biophysical studies to characterize subsaturated solutions of phase-separating RNA-binding proteins with intrinsically disordered prion-like domains and RNA-binding domains. Surprisingly, and in direct contradiction to expectations from classical nucleation theory, they found that subsaturated solutions are characterized by the presence of heterogeneous distributions of clusters. The distributions of cluster sizes, which are dominated by small species, shift continuously toward larger sizes as protein concentrations increase and approach the saturation concentration.

As a result, many of the clusters encompass tens to hundreds of molecules, while less than 1% of the solutions are mesoscale species that are several hundred nanometers in diameter. The team also found that cluster formation and phase separation can be decoupled using solutes as well as specific sets of mutations.

The findings suggest an “interplay” between networks of sequence-specific and solubility-determining interactions that, respectively, govern cluster formation in subsaturated solutions and the saturation concentrations above which phase separation occurs.

The study, published recently in Proceedings of the National Academy of Sciences, also included researchers from University of Cambridge, Heinrich Heine University Düsseldorf and Technische Universität Dresden.