Site-directed rotational resonance solid-state NMR distance measurements probe structure and mechanism in the transmembrane domain of the serine bacterial chemoreceptor

The serine receptor of bacterial chemotaxis is an ideal system in which to investigate the molecular mechanism of transmembrane signaling. Solid-state nuclear magnetic resonance (NMR) techniques such as rotational resonance provide a means for measuring local structure and ligand-induced structural changes in intact membrane proteins bound to native membrane vesicles. A general site-directed biosynthetic C-13 labeling strategy is used to direct the distance measurements to a specific site; the distance is measured between a unique Cys residue and a non-unique, low-abundance residue (Tyr or Phe). A C-13-C-13 internuclear distance measurement from (CO)-C-13(i) to (13)Cbeta(i + 3) at the periplasmic edge of the second membrane-spanning helix (TM2) of 5.1 +/- 0.2 Angstrom is consistent with the predicted alpha-helical structure and thus demonstrates an accurate long-distance rotational resonance measurement in the 120 kDa membrane-bound receptor. These measurements require a correction for the rotational resonance exchange between the multiple labels of the non-unique amino acid and the natural-abundance C-13, which is critical to distance measurements in complex systems. A second C-13-C-13 distance measurement between the transmembrane helices provides a high-resolution measurement of tertiary structure in the transmembrane region. The measured 5.0-5.3 Angstrom distance in the presence and absence of ligand is consistent with structural models for the transmembrane region and a proposed signaling mechanism in which ligand binding induces a 1.6 Angstrom translation of TM2. This approach can be used for additional measurements of the structure of the transmembrane region and to determine whether the ligand-induced motion is indeed propagated through the transmembrane helices.