The transmembrane receptor for aspartate, Tar, in Escherichia coliand Salmonellais representative of a large class of receptors that generates chemotaxis responses by regulating the activity of an associated histidine kinase, CheA. Tar is composed of an Nterminal extracellular ligand-binding domain linked through a transmembrane sequence to a C-terminal signaling domain in the cytoplasm. The isolated cytoplasmic domain of Tar fused to a leucine zipper sequence forms soluble ternary complexes with CheA and an adapter protein, CheW. The isolated complex is biochemically active, has a molecular weight of 1,400,000 Daltons, and includes approximately 28 receptor signaling domains for 2 CheA dimers.
Electron microscopy of the complexes indicates well-defined bundles, presumably composed of numerous receptor filaments surrounding a core of CheA dimers and CheW. CheA also interacts with CheY, the response regulator of the bacterial flagellar motor. Immunoelectron microscopy has provided a general picture of the domain organization of the complexes.