ASSIGNMENTS, SECONDARY STRUCTURE, GLOBAL FOLD, AND DYNAMICS OF CHEMOTAXIS-Y PROTEIN USING 3-DIMENSIONAL AND 4-DIMENSIONAL HETERONUCLEAR (C-13,N-15) NMR-SPECTROSCOPY

NMR spectroscopy has been used to study recombinant Escherichia coli C heY, a 128-residue protein involved in regulating bacterial chemotaxis . Heteronuclear three- and four-dimensional (3D and 4D) experiments ha ve provided sequence-specific resonance assignments and quantitation o f short-, medium-, and long-range distance restraints from nuclear Ove rhauser enhancement (NOE) intensities. These distance restraints were further supplemented with measurements of three-bond scalar coupling c onstants to define the local dihedral angles, and with the identificat ion of amide protons undergoing slow solvent exchange from which hydro gen-bonding patterns were identified. The current model structure show s the same global fold of CheY as existing X-ray structures (Volt & Ma tsumura, 1991; Stock et al. 1993) with a (beta/alpha)(5) motif of five parallel beta-strands at the central core surrounded by three alpha-h elices on one face and with two on the opposite side. Heteronuclear N- 15-H-1 relaxation experiments are interpreted to show portions of the protein structure in the Mg2+ binding loop are ill-defined because of slow motion (chemical exchange) on the NMR time scale. Moreover, the p resence of Mg2+ disrupts the salt bridge between the highly conserved Lys-109 and Asp-57, the site of phosphorylation.