CHARACTERIZATION OF UREA-DENATURED STATES OF AN IMMUNOGLOBULIN SUPERFAMILY DOMAIN BY HETERONUCLEAR NMR

The structural and dynamic properties of an immunoglobulin superfamily domain (IgSF), Ig 18', have been characterised by NMR at 285 K, in th e presence of 4.2 M and 6.0 M urea, respectively. Analysis of chemical shift deviations, (3)J(HNH alpha), coupling constants, sequential NOE pattern, and N-15 relaxation data reveals that although the two urea- denatured states are highly disordered, some local turn-like residual structures do exist. Moreover, some distinct differences between the p roperties of the two denatured states are observed. In 4.2 M urea-dena tured Ig 18', regions 80-83 and 86-92 adopt turn-like conformations, f urthermore, region 84-93 is involved in slow exchange processes that o ccur on a micro-to millisecond time-scale. Ln the 6.0 M urea-denatured state, these turn-like conformations are less occupied, and chemical exchange processes in region 84-93 are largely reduced. Ln contrast, r egion 32-36 has persistent turn-like structures in both urea-denatured states. Some correlation between the spectral density function at 0 f requency, J(eff)(0), for the urea-denatured states and the secondary s tructure elements of the folded state have been observed. Except for t he terminal regions, residues corresponding to beta-strands have highe r J(eff)(0) values compared to residues corresponding to loops. The ch aracterisation and comparison of the two urea-denatured states highlig ht residues that possess properties that may be crucial for the initia tion of folding of this domain. (C) 1998 Academic Press Limited.