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 C-13- and N-15-labeled phosphotyrosine peptide ligated to STAT-6. Distances have been obtained by trNOE experiments, and the torsion angle Pro(psi) has been determined using trCCR, measuring either cross-correlated H-N-N/H-alpha-C-alpha dipole-dipole relaxation or cross-correlated H-alpha-C-alpha dipole/CO chemical shift anisotropy relaxation. The resulting structure has been described.