Involved in the transmission of sensory signals from the chemoreceptors to the flagellar motors. CheA is autophosphorylated; it can transfer its phosphate group to either CheB or CheY. ATP + protein L-histidine = ADP + protein N-phospho-L-histidine. An in vitro complex of CheW/CheA(L)/CheA(S) in a 1:1:1 ratio increases the autophosphorylation of CheA and is required for the binding of CheY, the phosphorylation substrate. This complex accounts for 10% of the total number of molecules. May have three functional domains: one for interaction with CheB and CheY, a second for regulating phosphorylation and controlling the stability of the protein, and a third for receiving input signals regulating CheA activity.