Tanja Mittag (St. Jude Childrens Research Hospital), Anne Bremer (St. Jude Childrens Research Hospital), Mina Farag (Washington University in St Louis), Wade Borcherds (St. Jude Childrens Research Hospital), Rohit Pappu (Washington University in St Louis)
Presenting Author St. Jude Childrens Research Hospital
Phase transitions underlie cellular compartmentalization and mediate fundamental biological processes. How they are encoded in the protein sequence is therefore important. Here, we use biophysical experiments, theory, and simulations to generate a conceptual stickers-and-spacers framework to understand phase behavior of intrinsically disordered prion-like low-complexity domains (PLCDs) of RNA-binding proteins. Stickers form non-covalent inter- and intramolecular crosslinks, whereas spacers enable or suppress the formation of these crosslinks. We have previously shown that aromatic residues are the stickers in the PLCD of hnRNPA1. Here, we demonstrate that tyrosine is a stronger sticker than phenylalanine and account for interactions of charged residues. Negatively charged residues are solubilizing spacers. Arginines act as stickers through pairwise interactions with aromatic residues, while lysines weaken sticker-sticker interactions. Low or high values of the net charge per residue weaken phase separation via mean-field electrostatic effects, while a net charge per residue close to zero minimizes solubility and is optimal for phase separation. We further characterize the function of spacer residues, particularly that of the two most frequent spacer types glycine and serine, to ask whether serine acts as a weak sticker via its side chain. Instead, we find that increasing serine contents decreases the driving force for phase separation in agreement with the higher effective solvation volume of serine vs glycine. Our analytical and coarse-grained models accurately predict PLCD phase behavior.
Support or Funding Information
This work was supported by the US National Institutes of Health (grant R01NS121114 to R.V.P. and T.M.), the St Jude Collaborative Research Consortium on Membraneless Organelles in Health and Disease (to T.M. and R.V.P.) and the American Lebanese Syrian Associated Charities (to T.M.).
We reveal the hidden sequence complexities for a family of prion-like low-complexity domains (PLCDs) and determine their influence on phase behavior by multidisciplinary studies. Our results yield a set of rules governing the sequence-encoded phase behavior of such PLCDs, and physicochemical rationalizations for their underlying sequence composition.
