Journal of Biomolecular NMR

, Volume 36, Issue 2, pp 123–136 | Cite as

Measurement of 15N relaxation in the detergent-solubilized tetrameric KcsA potassium channel

  • Jordan H. Chill
  • John M. Louis
  • James L. Baber
  • Ad Bax


A set of TROSY-HNCO (tHNCO)-based 3D experiments is presented for measuring 15N relaxation parameters in large, membrane-associated proteins, characterized by slow tumbling times and significant spectral overlap. Measurement of backbone 15N R 1, R , 15N–{1H} NOE, and 15N CSA/dipolar cross correlation is demonstrated and applied to study the dynamic behavior of the homotetrameric KcsA potassium channel in SDS micelles under conditions where this channel is in the closed state. The micelle-encapsulated transmembrane domain, KcsATM, exhibits a high degree of order, tumbling as an oblate ellipsoid with a global rotational correlation time, τc = 38 ± 2.5 ns, at 50 °C and a diffusion anisotropy, \({D}_{\parallel}/{D}_{\bot}=0.79\pm 0.05\), corresponding to an aspect ratio a/b  ≥  1.4. The N- and C-terminal intracellular segments of KcsA exhibit considerable internal dynamics (S 2 values in the 0.2–0.45 range), but are distinctly more ordered than what has been observed for unstructured random coils. Relaxation behavior in these domains confirms the position of the C-terminal helix, and indicates that in SDS micelles, this amphiphilic helix does not associate into a stable homotetrameric helical bundle. The relaxation data indicate the absence of elevated backbone dynamics on the ps–ns time scale for the 5-residue selectivity filter, which selects K+ ions to enter the channel.


backbone dynamics cross correlation diffusion anisotropy micelle relaxation interference rotational correlation time selectivity filter 


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We thank Frank Delaglio for software support, Dennis Torchia (NIDCR/NIH) and Chris Miller (Brandeis University) for helpful discussions and Annie Aniana for assistance in sample preparation. J. H. C. acknowledges the support of a long-term EMBO fellowship. This work was supported by the Intramural Research Program of the NIDDK, NIH, and by the Intramural AIDS-Targeted Antiviral Program of the Office of the Director, NIH.

Supplementary material


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Copyright information

© Springer Science+Business Media B.V. 2006

Authors and Affiliations

  • Jordan H. Chill
    • 1
  • John M. Louis
    • 1
  • James L. Baber
    • 1
  • Ad Bax
    • 1
  1. 1.Laboratory of Chemical PhysicsNIDDK, National Institutes of HealthBethesdaUSA

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