CHANGES IN THE NMR-DERIVED MOTIONAL PARAMETERS OF THE INSULIN-RECEPTOR-SUBSTRATE-1 PHOSPHOTYROSINE BINDING DOMAIN UPON BINDING TO AN INTERLEUKIN-4 RECEPTOR PHOSPHOPEPTIDE

Proteins recognize ligands by forming specific intermolecular interact ions that often involve solvent exposed residues. Changes in the motio nal properties of these residues upon binding can affect the conformat ional entropy of the system and thus are related to the energetics of binding. The role that dynamics plays in ligand recognition can be inv estigated by comparing the motional properties of a free and ligated p rotein. NMR relaxation studies are well suited for examining changes i n dynamics, especially for motions on a nanosecond to picosecond time scale, Recently, we determined the solution structure of the phosphoty rosine binding (PTB) domain of the insulin receptor substrate (IRS-I) complexed to a tyrosine-phosphorylated peptide derived from the interl eukin 4 (IL-4) receptor [Zhou et al., (1996) Nat. Struct. Biol. 3, 388 -393]. The peptide binds tightly to the protein in a surface exposed p ocket, resulting in the partial burial of many protein residues. Using NMR relaxation studies, the dynamics of the backbone nitrogens of IRS 1 PTB domain were studied in both the free protein and the protein wh en complexed to the IL-4 receptor phosphopeptide. The backbone nitroge ns of many residues that make important contacts to the ligand are mot ionally restricted in the free and complexed protein. Additional resid ues become motionally restricted only after ligand binding, including several residues that do not make any direct contacts with the ligand. These observed changes in the dynamics are compared to structural fea tures of the complex.