The regulatory (R) region of the cystic fibrosis transmembrane conductance regulator (CFTR) is intrinsically disordered and must be phosphorylated at multiple sites for full CFTR channel activity, with no one specific phosphorylation site required. In addition, nucleotide binding and hydrolysis at the nucleotide-binding domains (NBDs) of CFTR are required for channel gating. We report NMR studies in the absence and presence of NBD1 that provide structural details for the isolated R region and its interaction with NBD1 at residue-level resolution. Several sites in the R region with measured fractional helical propensity mediate interactions with NBD1. Phosphorylation reduces the helicity of many R-region sites and reduces their NBD1 interactions. This evidence for a dynamic complex with NBD1 that transiently engages different sites of the R region suggests a structural explanation for the dependence of CFTR activity on multiple PKA phosphorylation sites.The CFTR chloride channel, the protein mutated in cystic fibrosis, is a member of the ATPbinding cassette (ABC) superfamily of proteins 1 . Like other members of the superfamily, CFTR has two membrane-spanning domains (MSD1 and MSD2) and two nucleotidebinding domains (NBD1 and NBD2). Intracellular regions between the transmembrane segments probably adopt helical structures that extend from the MSDs 2 . Unique to CFTR is the cytoplasmic intrinsically disordered 3,4 R region, of approximately 200 residues, which we refer to as a region rather than as a domain to reflect its lack of a stable, folded globular structure.
NMR relaxation data on disordered proteins can provide insight into both structural and dynamic properties of these molecules. Because of chemical shift degeneracy in correlation spectra, detailed site-specific analyses of side chain dynamics have not been possible. Here, we present new experiments for the measurement of side chain dynamics in methyl-containing residues in unfolded protein states. The pulse schemes are similar to recently proposed methods for measuring deuterium spin relaxation rates in (13)CH(2)D methyl groups in folded proteins.(1) However, because resolution in (1)H-(13)C correlation maps of unfolded proteins is limiting, relaxation data are recorded as a series of (1)H-(15)N spectra. The methodology is illustrated with an application to the study of side chain dynamics in delta131delta, a large disordered fragment of staphylococcal nuclease containing residues 1-3 and 13-140 of the wide-type protein. A good correlation between the order parameters of the symmetry axes of the methyl groups and the backbone (1)H-(15)N bond vectors of the same residue is observed. Simulations establish that such a correlation is only possible if the unfolded state is comprised of an ensemble of structures which are not equiprobable. A motional model, which combines wobbling-in-a-cone and Gaussian axial fluctuations, is proposed to estimate chi(1) torsion angle fluctuations, sigma(chi)()1, of Val and Thr residues on the basis of the backbone and side chain order parameters. Values of sigma(chi)()1 are approximately 10 degrees larger than what has previously been observed in folded proteins. Of interest, the value of sigma(chi)()1 for Val 104 is considerably smaller than for other Val or Thr residues, suggesting that it may be part of a hydrophobic cluster. Notably large (15)N transverse relaxation rates are observed in this region. To our knowledge, this is the first time that side chain dynamics in an unfolded state have been studied in detail by NMR.
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