Conserved electrostatic fields at the Ras-effector interface measured through vibrational Stark effect spectroscopy explain the difference in tilt angle in the Ras binding domains of Raf and RalGDS

Vibrational Stark effect (VSE) spectroscopy was used to measure the electrostatic fields present at the interface of the human guanosine triphosphatase (GTPase) Ras docked with the Ras binding domain (RBD) of the protein kinase Raf. Nine amino acids located on the surface of Raf were selected for labeling with a nitrile vibrational probe. Eight of the probe locations were situated along the interface of Ras and Raf, and one probe was 2 nm away on the opposite side of Raf. Vibrational frequencies of the nine Raf nitrile probes were compared both in the monomeric, solvated protein and when docked with wild-type (WT) Ras to construct a comprehensive VSE map of the Ras-Raf interface. Molecular dynamics (MD) simulations employing an umbrella sampling strategy were used to generate a Boltzmann-weighted ensemble of nitrile positions in both the monomeric and docked complexes to determine the effect that docking has on probe location and orientation and to aid in the interpretation of VSE results. These results were compared to an identical study that was previously conducted on nine nitrile probes on the RBD of Ral guanidine dissociation stimulator (RalGDS) to make comparisons between the docked complexes formed when either of the two effectors bind to WT Ras. This comparison finds that there are three regions of conserved electrostatic fields that are formed upon docking of WT Ras with both downstream effectors. Conservation of this pattern in the docked complex then results in different binding orientations observed in otherwise structurally similar proteins. This work supports an electrostatic cause of the known binding tilt angle between the Ras-Raf and Ras-RalGDS complexes.