The role of backbone motions in ligand binding to the c-Src SH3 domain

The Src homology 3 (SH3) domain of pp60 c-src (Src) plays dual roles in signal transduction, through stabilizing the repressed form of the Src kinase and through mediating the formation of activated signaling complexes. Transition of the Src SH3 domain between a variety of binding partners during progression through the cell cycle requires adjustment of a delicate free energy balance. Although numerous structural and functional studies of SH3 have provided an in-depth understanding of structural determinants for binding, the origins of binding energy in SH3-ligand interactions are not fully understood. Considering only the protein-ligand interface, the observed favorable change in standard enthalpy (Δ H = −9.1 kcal/mol) and unfavorable change in standard entropy ( TΔ S = −2.7 kcal/mol) upon binding the proline-rich ligand RLP2 (RALPPLPRY) are inconsistent with the predominantly hydrophobic interaction surface. To investigate possible origins of ligand binding energy, backbone dynamics of free and RLP2-bound SH3 were performed via 15N NMR relaxation and hydrogen-deuterium (H/ 2H) exchange measurements. On the ps-ns time scale, assuming uncorrelated motions, ligand binding results in a significant reduction in backbone entropy (−1.5(±0.6) kcal/mol). Binding also suppresses motions on the μs-ms time scale, which may additionally contribute to an unfavorable change in entropy. A large increase in protection from H/ 2H exchange is observed upon ligand binding, providing evidence for entropy loss due to motions on longer time scales, and supporting the notion that stabilization of pre-existing conformations within a native state ensemble is a fundamental paradigm for ligand binding. Observed changes in motion on all three time scales occur at locations both near and remote from the protein-ligand interface. The propagation of ligand binding interactions across the SH3 domain has potential consequences in target selection through altering both free energy and geometry in intact Src, and suggests that looking beyond the protein-ligand interface is essential in understanding ligand binding energetics.