MAPPING THE NUCLEOTIDE-DEPENDENT CONFORMATIONAL CHANGE OF HUMAN N-RAS P21 IN SOLUTION BY HETERONUCLEAR-EDITED PROTON-OBSERVED NMR METHODS

Heteronuclear-edited proton-detected NMR methods are used to study the nucleotide-dependent conformational change between GDP- and GTP-gammaS-bound forms of human N-ras p21. Amide groups of Asp are used as sensitive probes. When GTP-gammaS is substituted for GDP in cellular N-rasp21, the chemical shifts of resonances Asp-47, -126, -154, and Asn-172, as well as Gly-77 and -151, are not sensitive to nucleotide exchange, whereas Asp-30, -33, -38, -54, -57, -69, -92, -105, and -119 are affected. Distinct chemical shift changes of Asp-33, -38, and -69 indicate that substantial structural changes occur in the effector-binding region and the switch II region. Crystallographic studies of H-ras p21 have indicated that the conformational differences are confined to residues 32-38 and 60-76. Our observations indicate that the nucleotide-dependent structural transitions of the protein in solution may not be identical to those in the crystal. They suggest that the peptide beyond Glu-76 participates in a conformational switch, and possibly is involved in effector function. We propose that the region roughly from Asp-92 to-105, and the region of guanine base-binding motif(s), e.g., 116NKXD, are candidate sites recognized by either a GDP/GTP release factor or a GTPase-affected protein.