HAMP domain structural determinants for signalling and sensory adaptation in Tsr, the E. coli serine chemoreceptor

Motile Escherichia coli cells track chemical gradients with high sensitivity over wide concentration ranges [recently reviewed in (Hazelbauer et al., 2008; Hazelbauer & Lai, 2010)]. Stimulus detection, amplification, and integration occur in an arrayed network of signaling complexes that contain transmembrane chemoreceptors (methyl-accepting chemotaxis proteins or MCPs), the signaling histidine kinase CheA, and CheW, which couples CheA activity to chemoreceptor control. In the absence of chemoattractant gradients, MCPs activate CheA, promoting frequent episodes of clockwise (CW) flagellar rotation and random changes in swimming direction. Binding of an attractant ligand to the periplasmic sensing domain of a receptor molecule down-regulates CheA bound to the cytoplasmic tip of the receptor (Fig. 1), promoting counter-clockwise (CCW) flagellar rotation and forward swimming. Subsequent sensory adaptation restores pre-stimulus behavior through changes in MCP methylation state, catalyzed by a dedicated methyltransferase (CheR) and methylesterase (CheB).