Transferred Cross-Correlated Relaxation Complements Transferred NOE: Structure of an IL-4R-Derived Peptide Bound to STAT-6

A new NMR method is proposed which enables the measurement of projection angles in the bound conformation of a weakly binding ligand complexed to its receptor. The method is based on the cross-correlated relaxation mechanism. In analogy to the transferred NOE experiment (trNOE), cross-correlated relaxation can be transferred and measured at the resonances of the free ligand (trCCR), provided the k off rate is within the time scale of the experiment. The concept is validated by the structure determination of an interleukin-4 (IL-4) receptor-derived partially 13 C- and 15 N-labeled phosphotyrosine peptide ligated to STAT-6. Distances have been obtained by trNOE experiments, and the torsion angle Pro(ψ) has been determined using trCCR, measuring either cross-correlated H N −N/H α −C α dipole−dipole relaxation or cross-correlated H α −C α dipole/CO chemical shift anisotropy relaxation. The resulting structure has been described.