Involved in the transmission of sensory signals from the chemoreceptors to the flagellar motors. CheA is autophosphorylated; it can transfer its phosphate group to CheB, CheY or CheV. ATP + protein L-histidine = ADP + protein N-phospho-L-histidine. It is not known whether CheA is actually membrane bound although CheA has two regions at the C-terminal end with the potential to be transmembrane regions.