Protein-Peptide Binding Energetics under Crowded Conditions.

Nearly all biological processes, including strictly regulated protein-protein interactions fundamental in cell signaling, occur inside living cells where the concentration of macromolecules can exceed 300 g/L. One such interaction is between a 7 kDa SH3 domain and a 25 kDa intrinsically disordered region of Son of Sevenless (SOS). Despite its key role in the mitogen-activated protein kinase signaling pathway of all eukaryotes, most biophysical characterizations of this complex are performed in dilute buffered solutions where cosolute concentrations rarely exceed 10 g/L. Here, we investigate the effects of proteins, sugars, and urea, at high g/L concentrations, on the kinetics and equilibrium thermodynamics of binding between SH3 and two SOS-derived peptides using