A coarse-grained model for disordered proteins under crowded conditions

Macromolecular crowding may strongly affect the dynamics and function of proteins, with intrinsically disordered proteins being particularly sensitive to their crowded environment. To understand the influences of crowding on chain compaction and phase separation behaviour of disordered proteins, both experiments with synthetic crowders--like polyethylene glycol (PEG) and ficoll--and theoretical models and molecular simulation approaches have been applied. Here, we developed a residue-based coarse-grained model for PEG that is compatible with the protein CALVADOS model. To achieve this, we optimised model parameters by comparing simulations with experimental data on single-chain PEG and on PEG-induced compaction of disordered proteins. With our model we show how titrations of PEG can be used to quantify phase separation propensities of proteins that are not prone to phase separate strongly. We illustrate this for both variants of the low-complexity domain of hnRNPA1, and for wild-type and a redesigned variant of -Synuclein. Notably, we observe that the PEG crowding response changes between charge patterning variants of -Synuclein, which is not the case for the variants that vary the number the of aromatic residues in the low-complexity domain of hnRNPA1. We expect that our model will be useful for the interpretation of crowding experiments with disordered proteins, and we envisage it to be a starting point for in-silico explorations of proteins with weak propensities to phase separate.