SOS1 interacts with Grb2 through regions that induce closed nSH3 conformations.

Grb2 is an adaptor protein connecting the epidermal growth factor receptor and the downstream Son of sevenless 1 (SOS1), a Ras-specific guanine nucleotide exchange factor (RasGEF), which exchanges GDP by GTP. Grb2 contains three SH domains: N-terminal SH3 (nSH3), SH2, and C-terminal SH3 (cSH3). The C-terminal proline-rich (PR) domain of SOS1 regulates nSH3 open/closed conformations. Earlier, several nSH3 binding motifs were identified in the PR domain. More recently, we characterized by nuclear magnetic resonance and replica exchange simulations possible cSH3 binding regions. Among them, we discovered a cSH3-specific binding region. However, how PR binding at these sites regulates the nSH3/cSH3 conformation has been unclear. Here, we explore the nSH3/cSH3 interaction with linked and truncated PR segments using molecular dynamics simulations. Our 248 μs simulations include 620 distinct trajectories, each 400 ns. We construct the effective free energy landscape to validate the nSH3/cSH3 binding sites. The nSH3/cSH3-SOS1 peptide complex models indicate that strong peptide binders attract the flexible nSH3 n-Src loop, inducing a closed conformation of nSH3; by contrast, the cSH3 conformation remains unchanged. Inhibitors that disrupt the Ras-SOS1 interaction have been designed; the conformational details uncovered here may assist in the design of polypeptides inhibiting Grb2-SOS1 interaction, thus SOS1 recruitment to the membrane where Ras resides.