title: Nonrandom structure in the urea-unfolded Escherichia coli outer membrane protein X (OmpX).
abstract: On the basis of sequence-specific resonance assignments for the complete polypeptide backbone and most of the amino acid side chains by heteronuclear nuclear magnetic resonance (NMR) spectroscopy, the urea-unfolded form of the outer membrane protein X (OmpX) from Escherichia coli has been structurally characterized. (1)H-(1)H nuclear Overhauser effects (NOEs), dispersion of the chemical shifts, amide proton chemical shift temperature coefficients, amide proton exchange rates, and (15)N[(1)H]-NOEs show that OmpX in 8 M urea at pH 6.5 is globally unfolded, but adopts local nonrandom conformations in the polypeptide segments of residues 73-82 and 137-145. For these two regions, numerous medium-range and longer-range NOEs were observed, which were used as the input for structure calculations of these polypeptide segments with the program DYANA. The segment 73-82 forms a quite regular helical structure, with only loosely constrained amino acid side chains. In the segment 137-145, the tryptophan residue 140 forms the core of a small hydrophobic cluster. Both nonrandom structures are present with an abundance of about 25% of the protein molecules. The sequence-specific NMR assignment and the physicochemical characterization of urea-denatured OmpX presented in this paper are currently used as a platform for investigations of the folding mechanism of this integral membrane protein.
authors.affiliation: Institut für Molekularbiologie und Biophysik, Eidgenössische Technische Hochschule Zürich, CH-8093 Zürich, Switzerland.
authors.forename: Hakim
authors.initials: H
authors.lastname: Tafer
authors.forename: Sebastian
authors.initials: S
authors.lastname: Hiller
authors.forename: Christian
authors.initials: C
authors.lastname: Hilty
authors.forename: César
authors.initials: C
authors.lastname: Fernández
authors.forename: Kurt
authors.initials: K
authors.lastname: Wüthrich
mesh_terms: D000595:Amino Acid Sequence; D001425:Bacterial Outer Membrane Proteins / Q000737:chemistry*; D003198:Computer Simulation; D029968:Escherichia coli Proteins / Q000737:chemistry*; D006867:Hydrolases; D008958:Models, Molecular; D008969:Molecular Sequence Data; D019906:Nuclear Magnetic Resonance, Biomolecular; D011487:Protein Conformation; D011489:Protein Denaturation; D017510:Protein Folding; D012996:Solutions; D013816:Thermodynamics; D014508:Urea / Q000737:chemistry*
chemical_list: D001425:Bacterial Outer Membrane Proteins; D029968:Escherichia coli Proteins; D012996:Solutions; C482905:OmpX protein, E coli; D014508:Urea; D006867:Hydrolases