HAMP: The CPU Domain of Bacterial Chemoreceptors

The transmembrane chemoreceptors that mediate chemotactic behaviors in E. coli contain a HAMP domain at the cytoplasmic face of the membrane that governs their input-output signaling transactions. The four-helix HAMP bundle receives stimulus signals from the periplasmic chemoeffector-binding domain via a five-residue control cable connection to a transmembrane helix (TM2). HAMP in turn, through its structural interactions with an adjoining four-helix methylation (MH) bundle, modulates the activity of CheA, a cytoplasmic histidine autokinase bound at the membrane-distal tip of the receptor molecule.To investigate the mechanism of HAMP signaling in Tsr, the E. coli serine chemoreceptor, my lab has characterized the serine sensitivities and response cooperativities of a large collection of mutant receptors that have amino acid replacements in the TM2 - control cable - HAMP - MH bundle region, using an in vivo FRET-based assay of CheA kinase activity.Signaling by wild-type Tsr follows a two-state model of shifts between kinase-activating and kinase-deactivating outputs. Both states correspond to ensembles of mutationally distinct HAMP conformations. A variety of HAMP structural lesions, including ablation of the entire domain, shift receptor output toward the kinase-on state, indicating that the signaling role of HAMP is not to activate CheA, but rather to down-regulate kinase activity in response to chemoattractant ligands. Stimulus signals from TM2 and the control cable probably trigger output responses by modulating the packing stability of HAMP: A loosely packed HAMP bundle allows kinase activity; a tightly packed HAMP bundle deactivates CheA. These signaling shifts occur through an opposing structural interplay of packing stability in the HAMP and MH bundles. Loosely packed methylation helices produce kinase-off output and serve as substrates for subsequent receptor modifications that enhance MH packing during the sensory adaptation phase of an attractant response.