Featured Article - September 2012
Short description: The structure of the sensor complex for the TMAO receptor sheds light on bacterial two-component signaling.
Bacteria rely on two-component signaling to sense and respond to changes in their environment. When oxygen becomes scarce, alternate electron acceptors are needed for respiration, and many bacteria rely on trimethylamine-N-oxide (TMAO), a common osmoregulator in marine animals and renal glands. The TMAO reductase (Tor) pathway is upregulated when TMAO binds to a two-component histidine kinase sensor (TorS) and a periplasmic binding protein (TorT) to trigger phospho-relay and downstream transcription.
To better understand this signaling event, Moore and Hendrickson (PSI NYCOMPS) have solved X-ray crystal structures of the TorS extracellular sensor domain (TorSS) and TorT from Vibrio parahaemolyticus in the presence of TMAO (PDB 3O1H), the surrogate ligand isopropanol (PDB 3O1J), and in the absence of ligands (PDB 3O1I) to 2.8–3.1 Å resolution.
The TorSS-TorT complex has a 2:2 stoichiometry with nearly two-fold symmetry. Complexed TorSS protomers contain membrane-proximal and membrane-distal four-helix bundles connected by a coupler, and an intersubunit tower bundle which is contacted by TorT. Similar to other periplasmic binding proteins, TorT includes two α/β domains which form a binding pocket at their interface. It also has a unique C-terminal extension that contacts the distal bundle of one TorSS protomer. Mutational studies in Escherichia coli showed that five of six conserved TorT residues that contact TMAO and residues in a gating loop are critical for function.
Although two-component signaling generates a clear directional output, ligand binding has subtle effects on TorSS-TorT conformation. The authors found that TMAO binding causes a shift from asymmetry to near-symmetry, which propagates along the dyad axis as piston-like translations of the α-helices involved in phospho-relay. The mechanism may generalize, as other histidine kinase sensors are symmetric in the kinase state, while those in the phosophatase state are asymmetric. Adding further support, a chimera comprised of the TorS cytoplasmic domain and Nar nitrate-sensing domain exhibited nitrate-sensitive signaling.
The authors use sedimentation-equilibrium analytical ultracentrifugation and thermodynamic studies to show that TMAO binding is very different between the two sites, and asymmetry is reinforced upon binding to the high-affinity site. This suggests an unusual negative cooperativity that likely requires sufficient TMAO to accumulate for respiration before initiating signaling.
J. O. Moore and W. A. Hendrickson. An asymmetry-to-symmetry switch in signal transmission by the histidine kinase receptor for TMAO. Structure. 20, 729-741 (2012). doi:10.1016/j.str.2012.02.021