Signaling-Related Mobility Changes in Bacterial Chemotaxis Receptors Revealed by Solid-State NMR.

Bacteria employ remarkable membrane-bound nanoarrays to sense their environment and direct their swimming. Arrays consist of chemotaxis receptor trimers of dimers that are bridged at their membrane-distal tips by rings of two cytoplasmic proteins, a kinase CheA and a coupling protein CheW. It is not clear how ligand binding to the periplasmic domain of the receptor deactivates the CheA kinase bound to the cytoplasmic tip ∼300 Å away, but the mechanism is thought to involve changes in dynamics within the cytoplasmic domain. To test these proposals, we applied solid-state NMR mobility-filtered experiments to functional complexes of the receptor cytoplasmic fragment (U-13C,15N-CF), CheA, and CheW. Assembly of these proteins into native-like, homogeneous arrays is mediated by either vesicle binding or molecular crowding agents, and paramagnetic relaxation enhancement is used to overcome sensitivity challenges in these large complexes. INEPT spectra reveal that a significant fraction of the receptor is dynamic on the nanosecond or shorter time scale, and these dynamics change with signaling state. The mobile regions are identified through a combination of biochemical and NMR approaches (protein truncations and unique chemical shifts). The INEPT spectra are consistent with an asymmetric mobility in the methylation region (N-helix mobility ≫ C-helix mobility) and reveal an increase in the mobility of the N-helix in the kinase-off state. This finding identifies functionally relevant dynamics in the receptor, and suggests that this N-helix segment plays a key role in propagating the signal.