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. Trimer or tetramer. 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.