(A) A core signaling unit (CSU) is composed of six receptor dimers, 1 CheA dimer and 2 CheW monomers (W1 and W1′). Two extra CheW (W2 and W2′) exist in the array forming the CheW ring, but they are not essential for the CSU activity. For the side view of the CSU (bottom left), from top to bottom there are ligand binding domain (LBD), membrane, HAMP (histidine kinase, adenylyl cyclases, methyl-binding proteins and phosphatases) domain, Methylation helix (MH) bundle, glycine hinge (GH), kinase Control (KC). (B) An extended array of 3 × 3 unit cells. The unit cell is enclosed by black lines. Chemoreceptor in red, CheA in blue and CheW in green.