At high concentrations of PEG, we find that mCherry molecules associate to form large aggregates. By carrying out experiments with a bulkier crowder, dextran, we show that the effects of crowder-protein interactions depend on the crowder's chemical features and its bulkiness.

We find that PEG-wrapping around the side chains of lysine residues is prevalent, occurring in at least 20% of all crystal structures containing PEG, which corresponds to approximately 0.4% of all structures in the PDB.

We observe a specific, stable PEG-protein interaction in which PEG molecules wrap around the side chains of surface-exposed lysine residues.

We provide a detailed view of the influence of PEG crowding on the structural equilibrium and the function of a widely used fluorescent protein, mCherry. PEG interacts with the protein non-isotropically, and it is not well-described as a classical excluded volume crowder.